Dog still tests positive after heartworm treatment

Ken writes:

I adopted my dog from a shelter 2 years ago. She was estimated to be 1 year old at the time. She is a mix of Chihuahua and Terrier. She weighs approx. 11 pounds and is 12 inches tall. She tested positive for heartworms but I elected to adopt her anyway.

I had her treated with Immiticide. She was then put on Heartguard and am giving it to her monthly per my Vets instructions. After 2 years she is still testing positive for heartworms (adults) but has been negative for the babies for over a year. The vet has said that she has not previously seen a dog still testing positive for adult heartworms after two years.

The vet has discussed two choices: 1) Second round of treatment of Immiticide, or 2) Continue giving the Heartguard monthly and hope that the adult worms present will die. The vet was not clear on what was the better alternative.

I ask, “What would you do if it were your dog?" and the response was "I guess I would treat it with the Immiticide", but she wasn’t overly convincing.

Overall, Sadie is in good health. She is not experiencing any symptoms consistent with advanced heartworm disease. I am not certain I want to subject Sadie to this procedure unless it is the better alternative. I have read your web pages on this topic and appreciate your thoroughness. I would be interested in your opinion in this matter.

Hello, Ken,

 Generally speaking, the best person to advise you is the doctor who is seeing your dog.

 That being said, there are two scenarios that come to mind.

 First, we have to remember that the Heartgard (and other monthly preventives) work to kill the lifestage of the parasite that exists from mosquito bite up to about five or six weeks afterward.

 Second, it takes around six months from the time of the mosquito bite for the adult worms to be developed in the heart. It is only at this stage that they can be detected with the blood test. This is also the stage when the Immiticide works.

 SO, there is roughly a four-month window where the parasites have matured past the point where the monthly preventive can kill them, but they haven't reached the point where they can be detected with a blood test, or killed with Immiticide.

 If your dog was treated with Immiticide in October, for instance, and started on Heartgard at that time, baby heartworms acquired in June, July, August and September will go ahead and mature, and be showing up in the next spring's blood test.  They are too mature for the preventive, and not mature enough for the Immiticide to get them.

 The Immiticide may have killed all the adult worms present at time of treatment, but it didn't keep the partially mature ones from growing up. Thus you have a positive blood test again, even though you were faithful with your preventive.

 The other scenario is the dog where the worms just don't all die with the treatment. Some dogs (rare) will never get a totally clean blood test.

 My bias would generally be to re-treat with Immiticide in early spring (April is not too late, and don't stop your preventive medicine). Any previous parasite exposure will then be in the life-cycle stage where they can be killed. 

 In the case of a dog who had already undergone Immiticide treatment, and started on monthly preventive right away, we would expect very few worms to be present, and we would expect a good result from the treatment.

I hope this is helpful to you.

128 thoughts on “Dog still tests positive after heartworm treatment

  1. Sandy Youngblood says:

    Our 5 yr old German shep mix was diagnosed in late Aug/Sept with heartworms. My husband said he forgot their prevention for 3-4 months. Fritz took heartguard for a month along with a month of doxycycline and then had the first immiticide injection. In all this time he’s never displayed any symptoms.We’re approaching the time for the final round of immiticide and I’m considering switching to the soft kill method at this point. I believe he was minimally infected and the money and the restricted exercise are making me think we may be able to get by – since he’s already had the one shot.
    Do you have any opinion on that?
    I’ve been searching for anyone else who may have done this but I can’t find a thing.
    I would so appreciate your comments on this.

  2. Doc says:

    Hello, Sandy,

    A single injection of Immiticide kills the weakest worms. Statistically, this is believed to be about 50% of them, primarily male worms, very young worms, or very old worms.

    Two injections kills in the high ninety-percent (from 94 to 100%) of the worms.

    The reason that we give the single injection is to give the dog the opportunity to deal with a smaller number of dead worms at a time.

    You kill a few, they clog up the smaller arteries, and the white blood cells take several weeks to clean them out.

    Once they have been taken care of, you come back and kill the rest, and the dog recuperates from that over the next five weeks.

    You could (and we used to do so routinely) just give the two injections to start with. If there are lots of worms, this is much harder for the dog to deal with.

    Also, with three total injections (the program that is considered standard now) versus two, it pushes you more toward 100% worm kill.

    With the single injection that your dog has received, it is very likely that there are some worms remaining that you could remove with the two injections.

    It is possible that there were only a few and they are all gone. It is also possible that there were only a few, and NONE of them died with a single injection.

    “Slow kill” means that your dog is a time bomb for years until the worms die at an unpredictable time. With the standard treatment, you know the time frame, and can restrict the dog’s activity, and keep it under close observation. You’ve narrowed the window to weeks instead of years.

    • Christine Hogan says:

      My chin-pin had the 30 days of doxy and it has been 2 weeks since the first immiticide injection. I plan to complete the entire ‘fast kill’ regimen and paid in advance.

      I previously commented on a blog about what is the best heartworm preventative (I advised to focus on the active ingredient). I just read your response to my statement that included the efficacy of several heartworm preventatives. I transferred from my phone to my laptop and was not able to locate that specific post to reply. Your response concerned me a great deal. You detailed in your response how several forms of heart preventative have failed (unless there were missed doses not reported); however, you wrote how you used 2 different heartworm preventatives 15 days apart and it still failed. I did read another of your responses about the Proheart 12 that you feel works because it is strictly for heartworms and nothing else. I have to say that my niece used it for her dog and it also failed. This brings me full circle to the ‘efficacy’ of heart wormer treatments.

      It appears to me that there is more than one form/strain (forgive my lack of knowledge on the use of the correct verbiage to use) heartworm and/or the improper use of the heart worm treatment –possibly the ‘slow kill’ method (used widely in shelters) is producing heartworms that are immune to the popular ivermectin, as well as. other kill treatments. Mosquitos are just transferring the immune microfilaria, if they are female, that produce more immune microfilaria.

      Immunity can happen when the microfilaria are weakened but not killed due to either delays in the heartworm preventative, or if the host is not given enough or vomits the medication unknowingly. Also, dogs often gain weight between their annual vet visit, yet the same 6 month or 1 yr supply based on the beginning weight is still given. This would even be the case with puppies that very quickly become large dogs within in 3 to 6 months but owners have a 1 yr supply of medication.

      I purchased K9 Advantix II that suppose to repel mosquitos. Crossing my fingers that Tyg is heartworm free at the end of this treatment and doesn’t get bitten by another blood sucker!

      You mentioned Advantage Multi with the highest efficacy. Moxidectin is the same as ivermectin, yes? Imidacloprid kills only after the mosquito bites/ingests (seemingly already transferring the microfilaria or dies it kill the microfilaria also? With everything said, isn’t this proving that keeping the mosquitos from biting/transferring is the only actual REAL heartworm preventative.

      • Doc says:

        Hello, Christine,
        That article was written when we were going through three years of problems with apparent lack of efficacy. In other regions, they are still dealing with this. In my practice, it was a real problem from 2006 through 2009, but I’m not seeing it now.

        Moxidectin is NOT ivermectin, though it is a related compound. The monthly use of Advantage Multi has been shown to achieve a fairly consistent level of drug in the dog’s body. When you use ProHeart 12 (also moxidectin), the blood level is highest at first, then gradually less and less over the year (though it would usually be enough to prevent infection).

        I am currently participating in a study that is doing genetic analysis on the microfilariae, looking for genetic markers for resistance to the heartworm preventives. My participation is limited to providing blood samples to the researchers, but people are definitely studying this.
        The permethrin in Advantix II does have some repellent effects, so adding it to your program could have benefits.

  3. Conrad Galvez says:

    my three yr old female lab tested positive for heartworm so they gave tri heart plus a pill she would take once a month but the whole tratment runs around 1800.00 dollars and we dont know if it would cure the problem or work a little for her lifetime so they prescribed also minocyline to start i have the prescription but dont know if i should start and maybe get lucky with just taking those two meds and cross my fingers i cant believe this is happening

  4. Doc says:

    Hello, Conrad,

    You don’t say if your dog has been taking preventive medicine regularly, and has just tested positive anyway (perhaps a missed dose, or just less than 100% effectiveness, even with no missed doses).

    Minocycline (like doxycycline) inhibits a micro-organism called Wohlbachia that benefits the heartworm. Knocking that organism down makes the heartworms smaller and weaker, easier to kill and dissolve. You take it for a month, and the effects last for several months.

    It is a preliminary to killing the worms with the Immiticide injections. There are several other posts on the blog about the heartworm treatment procedure.

    The oral stuff is preventive medicine to keep the dog from getting any more heartworms than it already has. If (and this is by no means certain) long term use of the preventive actually did kill the worms, it is a process that would take years.

    Without knowing your dog’s situation, examining the dog, possibly taking chest X-rays, I really cannot give you specific advice on your best course.

    It is obvious that you have questions about the recommended treatment plan. Ask the veterinarian who examined your dog and recommended the treatment. Tell them you need more explanation. That’s their job, and I’ll bet that they will be glad to talk with you.

  5. Georgia says:

    I am looking at a 1-1/2-year-old Chorkie at a shelter that has been treated for heartworms.

    This is what they said of the treatment:
    “He was treated to get rid of the baby heartworms and to start killing the adults. The Heartguard monthly preventative in his case will do double duty of preventing new ones and wearing down the existing ones. This treatment is much easier on the dogs that the full treatment which takes 2 -3 months and requires keeping the dog very calm and still.”

    The only standard of care I can find is that the dog receives 1 shot (which I think is what the Chorkie has gotten) and then 30 days later, a shot, wait 24 hours, then another shot.

    The shelter is saying one shot and just use Heartguard to get rid of the rest. Is that new information that I just am not finding on the internet?

    I’m very nervous about that approach. I would do the Heartguard regardless, so that’s not a problem. It just seems like he is getting 1/3 of the treatment and then giving the Heartguard with the hope that it does the job.

    Would you get this Chorkie under these circumstances?

  6. Doc says:

    Hello, Georgia,

    I would presume that the shelter folks are believers in the “slow kill” method. This is somewhere between waiting for the worms to die of old age and actually having them die a little sooner than that with the Heartgard.

    At 18 months of age, I would guess that the dog has very few worms. Keeping him on Heartgard should certainly be done.

    My bias would be to go ahead and do the remaining injections on schedule.

    Such a young dog would probably do fine.

    You would really need to have the dog examined by your veterinarian and follow his/her recommendations.

  7. Conrad Galvez says:

    ya going back like i was sayin about my three yr lab she was not on any type of prevenitive med like heartgard or others out there so i am going to talk to the vet and c if there is any other meds she can take to at least live a long life she is not showin no symptoms yet but im crossin my fingers and ill proably have to pay that 1800 dollars in payments my brown lab just loves to play all the time i cant believe she will die off well ill c when i talk to the vet in a couple days when shes come back to work Thanks

  8. Conrad Galvez says:

    forgot to mention my Lab is on Tri Heart Plus monthly tablet for heartworm since this Dec 2014 when she went in for anual checkup and they told me she tested positive for heartworm so wee stayed with that but i went to a different vet for second opinon and c if she would test positive with them yup she did i was hopin she would be negative but i lost and besides that minocycline she took for a month well that ended Feb 1 2015 but my pup is lookin good u wouldnt think she would have this problem

  9. Doc says:

    Hello, Conrad,
    If she didn’t take any preventive medicine three years, I’m not too surprised she acquired the heartworms.

    I don’t know how bad the mosquitoes are where you live, but in my part of the country, even one year without medication would result in heartworm infection.

    Stay on your Triheart so that you don’t get any more worms than you already have, as then you might have to do the big-buck treatment all over again.

  10. jane says:

    We had our 89lb rescue lab treated for heart worms 6 mo ago. He was determined to be stage 1 with a sonogram. The vet said since he was stage 1 he would only need 2 back to back injections. she then put him on heart guard and he has been on it faithfully every month. we just went back for his re test and he still tested positive. Not sure what to do at this point. Any suggestions would be appreciated. The vet suggests re treat with 2 more injections or continue to use the heart guard and re test sgain in 6 mo. Worried and disappointed.

  11. Doc says:

    Hello, Jane,

    If he was treated 6 months ago, that would be in October.

    Mosquito exposure has its peak months in July and August. The preventive medications “reach back” to kill whatever was put in by mosquitoes in the past 5 to 6 weeks (six weeks maximum).

    Therefore, if you started on Heartgard on the first of October, you had no coverage for May, June, July and the first half of August.

    The baby heartworms that entered the body in June would not have been susceptible to the Immiticide until December (six months after the mosquito bites).

    So, your October treatment killed the adult worms present at that time, but not the developing worms that didn’t reach maturity until December, January, February and March.

    In other words, last summer’s exposure has now matured and you have adult worms present again.

    If you just stay on Heartgard, you shouldn’t get any more than you have right now. Those will, however, take years to die.

    Continuing on the Heartgard, and doing another 2-injection treatment should get you clear and enable you to stay that way.

    Your dog is unlikely to experience much difficulty with this next round of treatment, but the person best equipped to advise you is the doctor seeing your dog.

    Contact your veterinarian and share your concerns and questions with him/her.

  12. Nikki Wilson says:

    Our dog was just diagnosed with heartworm about a month ago. She was showing no symptoms at all! We took her in to get her tested because we had heard you could put them on a heartworm preventive that would obviously prevent heartworm infection but also help prevent against tape worms (which she kept getting). We live in Moab, UT where there is no reported cases of heartworm this year and only eleven last year. We did get Kimber from a shelter in Durango, CO when she was only two months old and we have no clue where she came from before there. She just turned three the first of June. She is extremely active, we go on a run everyday and she has twenty acres to enjoy with our other two dogs. We were not expecting for her heartworm test to come back positive. We had the first test done with our regular vet and were not impressed with the lack of knowledge and sympathy of the disease. We choose to go to another vet in town and had two more tests done. She tested negative for the microfilarie and positive for the adult heartworms or as they called it the “antigen” test. The vet said it was possible that there was only a female worm burden or the worms may have been sterile. This I never really understood, but I was some what relieved that they were not reproducing! We started Kimber on heartguard and Doxycycline. She was on the Doxy for two weeks before the Immiticide treatment. After the first shot Kimber was lethargic for about three days.They of course said this was normal especially since they sedated her as well. We finished out the doxy and currently have her on a steroid. She still shows no signs of being infected, no coughing or gagging. She did stop eating regularly and would not eat her dry dog food. I started making her homemade dog food and she got her appetite back pretty quick. So she is drinking, eating and eliminating normally. She has now been going through the treatment for two weeks. She is extremely mad at us for keeping her restricted and would run and play if we let her. She is definitely affected by all the pills we have been giving her and i’m sure the immiticide is not helping either. My heart breaks every time she begs to go outside or when she sees the other dogs running and playing. We have twenty acres for her to enjoy and she is not used to being completely confined.
    I guess I am having some second thoughts about putting her through this treatment. Do you see this a lot where dogs have no symptoms and no side effects from the treatment? Can the worms be killed with the first immiticide injection? We are wanting to test her after the first injection just to see if she comes back negative. The whole process is extremely exhausting for my husband and I as well as our fur baby! I am just looking for a little more information on dogs with no symptoms. I am finding a lot of information based on dogs with a large worm burden and really no information on dogs with very small worm burdens. Any advice or information you can give me would be greatly appreciated!
    Thank you!
    Nikki Wilson

  13. Doc says:

    Hello, Nikki,

    I understand your feelings. The good thing is that you should have a good long-term outcome with this.

    Any reaction from the Immiticide should be long-gone now.

    You are restricting the exercise so that the dead worms don’t create more of a problem in the smaller pulmonary arteries before they are dissolved.

    The antigen test is for a protein from the female heartworm reproductive tract. If there are fewer than 4 female worms, it is possible to get a false negative test.

    Thus, testing a few weeks after a single Immiticide injection may not mean a whole lot.

    My bias would be to go ahead and complete the treatment with the second round of two injections. I believe this is what is in the dog’s best interests in the long run.

  14. Rae flontek says:

    I adopted an adorable chihuahua from a rescue. She had been in 3 different puppy mills, then when she could not have more puppies she was given up as bait for dog fighting. They left her in the woods to die after she had been attacked, but was luckily found by a girl and given to the rescue.
    She was treated a year ago for heartworm, but now tests positive again. The vet who treated her said sometimes they can test false positive up to 18 months after treatment and suggests I leave treatment for now. My vet wants to treat her now. I hate to put her through more trauma unless really necessary. What is your advice. She has no symptoms.

  15. Doc says:

    Hello, Rae,
    I would suspect that the dog had been exposed to heartworm through the mosquitoes in the months before the heartworm treatment. It would be six months after that exposure before the worms would develop to a stage that could detected or be susceptible to the Immiticide treatment.

    While you could see some holdover heartworm protein in the blood causing a false positive test for several months, I don’t think it likely that is the case at 18 months.

    She probably doesn’t have many worms, but when she is in good health and doing well, my bias would be to treat her to remove those.

    Certainly you should keep giving her preventive medicine each month.

    The veterinarian who is actually seeing your dog is the best person to advise you. I can only give general information and opinion.

  16. lynn says:

    my 14 year old American Bulldog was just diagnosed with heartworms. The vet put him on Heartgard and doxycycline 430 days. When does the heart guard start killing the baby larva.also he still a very active dog. We usually walk about 1 mile a day…. my vet did not seem concerned about this amount of exercise. What do you think?

  17. Doc says:

    Hello, Lynn,

    What do I think? I think the doctor actually seeing your dog is your best source of information.

    That being said, the doxycycline is probably doing more to clear the microfilariae (baby heartworms) than the Heartgard. The Heartgard works overnight and is then gone from the body, so it’s already done whatever it’s going to do.

    The doxycycline will usually sterilize the females (temporarily), as well as weakening them and shrinking their physical size.

    If the dog had a negative test last year, and has been taking preventive medicine, he would have very few heartworms, so they are unlikely to cause a problem with mild to moderate exercise.

  18. Ashley says:

    I rescued a Chow Chow/Spitz mix in April from a rescue that had received him from a kill shelter. The rescue did not have a lot of information for me on his past history but they guessed his age was about two years old. He was tested for heartworms for in May when I took him for a check up, he tested negative. He was on a preventative that the rescue gave us called valuheart (a generic brand from a town and country store) that I continued after we rescued him and on Nov 11 he tested positive on what they called “snap” test but the blood work came back negative. So they did another round of testing, taking more blood and doing an extensive test which came back positive. He is on an antibiotic and his first round of heartgard (he has a six month supply). Next week we go in for the first injection but I am having second thoughts because I feel like the first round of testing should have shown a positive on both blood work and the “snap” test. He has no symptoms, he plays, he runs, his appetite is regular, he drinks water regularly, and he has no cough/gag (unless he gets excited and pulls too hard on his leash). I just want some advice on what to do. It concerns me to pump my dog with chemicals for what seems like a couple of worms.

  19. Doc says:

    Hello, Ashley,

    I get what you are saying about not wanting to do an unnecessary treatment.

    Here’s the thing: the tests are not quantitative. They are pretty much yes or no. You can have a dog that has MANY adult heartworms, and yet tests consistently negative for the baby heartworms in the blood. I have done post-mortem exams on several such dogs.

    If there are fewer than four females, the test for heartworm protein in the blood (the snap test and others like it) can give a false negative. It tells you absolutely nothing about the number of males, and positive is just positive – it doesn’t tell you how many females are present.

    When I am concerned about numbers of worms, I am looking at the dog’s mosquito exposure history. Where was he living during the summer? Was he inside or outside? How many mosquito seasons?

    A chest X-ray won’t quantify the number of worms, but it will show if there is already any heart damage, or damage to the pulmonary arteries (lung arteries where the heartworms live).

    A young, athletic dog will experience some damage to his arteries even with just a few worms present, as the blood flow of vigorous activity whips them around inside the vessel.

    As far as “pumping him with chemicals”, the month of antibiotics is the biggest part of that. The Immiticide isn’t much drug. Other than some soreness at the injection site, it is unlikely to bother the dog at all. It takes the worms several days to die.

    Breaking the treatment process down into these various stages gives the dog’s body the least amount of stress in dealing with the dead worms clogging his arteries. The doxycycline (or minocycline, another antibiotic) weakens the worms, and makes them physically smaller. They die more easily, and they don’t clog up as much artery. The single injection kills the weaker worms, so all the dead worms don’t hit him at the same time. When those have been dissolved over the next few weeks, the two injections kill the rest. Again, not so many dead worms to deal with at one time.

    Do stay on the Heartgard throughout the process.

  20. Ashley says:

    A chest xray was done but the vet didn’t seem concerned about what she saw.
    I’ve had him since April, he lives mostly inside while I am at work, he gets to be outside in the afternoons but not more than 1.5 hours (that was until he was diagnosed). We do live in Central Texas but I didn’t think his exposure was severe (this summer of 2015). I don’t know much about his life prior to us but the rescue had a negative and I had a negative in May when he was on preventative.
    I’m wondering if I do the antibiotic, and one immiticide treatment if that will kill off what worms are affecting him.

  21. Doc says:

    Hello, Ashley,

    You really need to do the complete protocol. A single injection will almost certainly leave worms present. We used to just give them two injections, no antibiotics, no single injection preliminary. Lots more complications that way. Three injections gives the best chance of 100% kill.

    That being said, there are some dogs where you cannot clear every worm. My personal opinion on these patients is to just keep giving preventive medicine, rather than repeating the whole rigamarole.

    Treating as thoroughly as possible makes sense for your dog. He is young and strong, and has a long life ahead of him. It would take years for those worms to die on their own, and you wouldn’t be restricting his activity when it happened.

    This is better.

  22. Connie Wheeler says:

    We have had our dog since he was 10 months old along with 3 others. We have them on Heartguard every month. He is now 3 and in June he tested positive for heartworms. He was treated with back to back injections over two days. Confined for 6 weeks and today we had him retested for his 6 month check up. Sadly he tested positive again for baby heartworms. Our vet was unsure how she wanted to move forward but decided to give him some immiticide liquid form and retest in 2 months and continue to give him Heartguard which we have done every month since getting him. Do you think this will work or do you think we will have to do the injections again? Thanks

  23. Doc says:

    Hello, Connie,

    Did he test positive for adult heartworm antigen (the protein from the female worms that you would see on the little tests that look like a pregnancy test)?

    Ordinarily, one would expect the monthly Heartgard to have killed the circulating microfilariae (the microscopic baby heartworms), but it may not have.

    You might have removed the adults, but not ever really killed their circulating babies.

    I suspect the oral liquid your dog was given was a high dose of Ivermectin (the active ingredient in Heartgard). Immiticide is only available as an injection.

    To clear the bloodstream of baby heartworms, we often use this form of the drug, and the dose of ivermectin is about 400 times the dose that would be in the monthly Heartgard.

    I would certainly continue to give him the Heartgard while you are waiting for the retest.

    The two injections of Immiticide, 24 hour apart, will clear most of the adult worms, but some can survive. Using the 3-dose protocol (one dose, followed one month later by two doses) has a higher percentage of kill in the research studies.

    It is possible that some of the adults survived, but it would be a small number. It is also possible that you have some new infection because your dog doesn’t do well with the Heartgard. Some dogs just don’t get the protection they need; apparently they don’t absorb the drug as well as other dogs or something (we’re not entirely sure what the problem is exactly).

    When we have patients who have repeated infections after being cleared, we have had the best luck with switching them to Advantage Multi as their new preventive after treatment.

    If the baby heartworms are gone on the retest, and the antigen test is negative, then great. If they are still there after the big dose of ivermectin, then you probably have a live mommy and daddy adult in there. You might get a negative antigen test, even so, as with fewer than four adult female worms, we can get false negative antigen tests.

    Every case is different, and your best source of information is the doctor seeing your dog. I really cannot recommend whether you should re-treat the dog, or just continue with preventive, or switch to a different preventive. Without seeing your dog and knowing the case, I just cannot make a recommendation.

  24. Leah Crotsley says:

    I was at the vet today with my dog. Three years in a row her heart worm test came back pos. then they run it again and it’s fine. Has anybody else had this happen. She takes heartguard on the 28th every month. I have never missed a dose since I got her 6 years ago. She was 9 weeks old when I got her.

  25. Doc says:

    Hello, Leah,

    If they are running an antigen test for the adult heartworm protein in the blood, very low levels of antigen can give false negatives. So, if you have fewer than 4 adult female worms, you can get a positive test on some occasions, and a negative test on others. After three years, it is also possible that the worms finally died.

    Sending a blood sample to an outside reference laboratory is the way to double check this. They have more sensitive testing methods.

  26. Anne says:

    We have a young rescue pug going through treatment right now. He has had the 3 shot protocol. He tested + for baby heartworms prior to the treatment. We just got his results back from his post treatment and the baby heartworms and it was – . Can we read anything into this? Does this mean there is a good chance the overall treatment was successful? He won’t be tested for adult heartworms until Nov.

  27. Doc says:

    Hello, Anne,

    It is not uncommon to test negative for the baby heartworms if you have been taking preventive medicine along with the treatment. It doesn’t tell us a whole lot at this point. I think your odds of success are very great, though.

  28. maryellen says:

    My 6 year old westie had a positive heartworm snap test 1 year ago. They sent the blood out for a second test and it came back negative. One year later, she now tested positive on the snap and also on the test they sent out. Subsequently they tested for babies and found none. Yesterday I took her for an ultrasound and the doc saw no heartworms. Her oxygen levels were low after exercise (88-89)for several tests so the ultrasound vet recommended still treating her for heartworms. She has no symptoms but mysteriously her bark has changed over the last two months with no explanation. Neither vet thinks this is related but my friend thinks she should be check for laragyl paralisis. Im baffeled by this and don’t know what to do. Shes on her 5th day of doxy and Im afraid to do the treatment anyway (of coarse)I have not seen any mention of the ultrasound results. I thought this was a good way to either confirm or negate the diagnosis. I now this is a lot to digest but I’m not sure where to turn.

  29. Doc says:

    Hello, Maryellen,

    With very small worm burdens, the tests can give false negative results, even at the reference laboratories. I suspect there were a very small number of worms last year, and the same this year.

    The ultrasound may reveal the presence of heartworms, but can easily miss them in small numbers.

    With such a small number of worms, the odds of her having complications with the treatment are very low, roughly on a par with having the same problems if left untreated. I think you have more potential for gain than loss.

    I cannot speak to the barking issue, though partial laryngeal paralysis is possible. My grandmother developed paralysis of one side of the larynx and was speaking in a whisper for the last two years of her life.

    A C-T scan or MRI might show something putting pressure on the nerve supply (if that is actually what is going on).

    I cannot imagine how the heartworms situation could relate to the barking situation.

  30. maryellen says:

    thank you for your quick response. Could you please clarify the comments 1) roughly on par with the problems if left untreated and 2)I think you have more potential for gain than loss. Since I want to be 100% positive before I treat her are there any other tests I can do? Do you think it might be a good idea to wait a month or two and then have the blood work retested? Thanks in advance.

  31. Doc says:

    1. I think that both are unlikely to cause problems, but both could.

    2. Getting rid of the worms is a plus that lasts the life of your dog. Chances of having complications with the treatment are remote, given your circumstances. Possible, but remote.

    3. I don’t think that there are any other tests that will give you more information than you now have.

    4. I doubt there will be any change in the bloodwork that would be helpful. You have an extremely small worm burden, so you can get false negative tests. If it turns up positive again, it’s the same. If it turns up negative next time, you can’t really trust it, based on past results.

  32. Jessica says:

    I rescued a puppy from the south who was brought up to Wisconsin. The rescue records show she was on a heart worm preventative and when I got her home I started her on heart guard every month. At her one year check up she tested positive for microfilaria and heartwom. The rescue paid for her treatment (antibiotics, the two injections over two day) During this time she took a monthly preventative still (a different one because she is a hearing breed) We just finished our 30 days of crate rest, but I felt she was still not doing well in the heat and tiring easily after short walks. My vet finally agreed to do a blood test to put my mind at ease, even though she said it would tell us if she was really negative yet. My dog is still microfilaria positive. My vet had me double check the dosage of the preventative, which should work for a dog up to 55lbs and my dog is only 34 lbs. she said she has never seen this before, and my dog should be negative because she took a preventative two days before the test. Any thoughts on what is going on or how to proceed?

  33. Lisa says:

    We just adopted an approx. 10 mo old dog from a shelter last week. Took her to our vet for exam. Gave blood sample & that test came back positive for adult heartworms. Had us give another blood sample to do another test to check for babies (same lab as first test) That test came back negative so they took more blood & sent it to another lab to verify. This test came back negative for both adults & babies?? We live in an area with hardly any cases of heartworm so concerned our vet may not have a lot of experience with heartworms. Vet is supposed to call us on Mon. After she consults with someone. Should we be worried? Does our puppy have heartworms or not? Also puppy is on panacur c for guardia. Could this be affecting the heartworm test? She wasnt on panacur for the first two, but had been on it for two days when the 3rd test was done.

  34. Doc says:

    Hello, Jessica,

    My recommendation would be to continue the monthly preventive year-round and re-test 4 months after the two injections.

    One dose of preventive frequently does not clear the microfilariae, even if there are no fertile adults left to produce new ones.

    It is also possible that you didn’t have a complete kill, but my advice is the same at this point.

    Continue regular monthly preventive medicine and re-test.

  35. Doc says:

    Hello, Lisa,

    The Panacur won’t affect the heartworm test. Really small worm burdens can give inconsistent results, false negatives, then positives, then false again.

    I would follow your veterinarian’s recommendations. My advice (bearing in mind that I have not seen the dog or test results) would be to continue monthly preventive and re-test in six months.

  36. Lisa says:

    Thanks Doc! Third blood test came back negative from outside lab. Dr. Had one more blood sample drawn & sent to same lab as first test (which was the one that came back positive for adults) and it came back negitive so Dr. said absolutly no adult heartworms but like you she said to start on heartguard & test again in 6 mo. So grateful she is heartworm free. Fingers crossed she will stay that way. Thanks again for your advise.

  37. Kathleen McLeod says:

    My currently 5 year old rescue Chihuahua tested positive for heartworm when I adopted her 18 months ago. She had no.symptoms. An echocardiogram showed such a high burden that the vet and cardiologist advised the 3 injection melarsomine protocol after only 30 days of Advantage Multi and one round of doxycycline. (She continues on monthly Advantage Multi, no lapses.) It was explained that she might still test positive after treatment because all microfilarae might not be dead when treatment commenced. Six months later she was negative for microfilarae but antigen positive. An echocardiogram showed greatly reduced burden of adult worms but still a substantial number. Melarsomine protocol was repeated. Six months after second protocol, she is still antigen positive, microfilarae negative. Vet has few ideas–maybe melarsomine resistance? Long time to clear antigen? He will get a Knotts test performed to see if microfilarae might be there previously undetected. Have you seen any cases like this? She has never had any symptoms and seems in excellent health. Is she in danger from adult worms that are likely sterile but may take a long time to die?

  38. Doc says:

    Hello, Kathleen,

    Some dogs will never totally clear all the worms, so says the expert from the American Heartworm Society (not me). If the echo does not show a significant worm burden, I would be tending to just stay on preventive medicine, rather than a third, 3-dose Immiticide regimen.

    Some day the worms still present will die of old age, and they may move and cause a problem, but you will probably be okay.

    Just my feeling on the matter. Since I haven’t seen your dog or the imaging, I’d say it’s best to rely on the advice of your veterinarian who has.

  39. Michelle Daniel says:

    I adopted another dog last Feb.(2016). The rescue group had her tested for HW in Dec.’15 by doing a smear test as well and it was negative, then when I adopted her in Feb’16 she was tested again and it was negative. I have had her on Heartgard every month, religiously. I just took her in ( a month early too) to get her annual testing and vaccines updated. She tested positive. How can this be? She was negative in two prior testings and now 11 months later she is positive even while never missing a dose of preventative. I am sick about this. We adopt all our dogs. One a shelter treated with the slow kill method and we have had her 12 years and she is great. The other we did the aggressive, which is just so scary. We are getting her retested at another vet today to make sure, but how could this happen?

  40. Michelle Daniel says:

    And now an update from my question this morning. We took her to our old Vet that is some distance from our home. They ran a smear test in their office and it was negative. WHAT IS HAPPENING? Who is right? What do we do now. One vet is saying positive, one says negative.

  41. Doc says:

    Hello, Michelle,

    If a dog has a very small worm burden, you can get conflicting results. If there are fewer than four adult female worms, you can get a false negative antigen test. The antigen is a protein from the female heartworm’s reproductive tract. Male worms are not detected on this test at all.

    I’m not sure what you mean by a “smear test”. When we look for the microfilariae (microscopic baby heartworms), you can sometimes see those in a drop of blood if there are lot of them. If there aren’t many, they can be missed without a concentrating exam, like the modified Knott’s test. This takes 1.0 cc of blood and puts any microfilariae in the whole sample into one drop, so you are less likely to miss them.

    Unless you have both males and females, there won’t be any babies in the blood.

    Many dogs taking heartworm preventive will show no babies in the blood, even if there are adults of both sexes present.

    I have had patients where we sent the results to a teaching hospital reference laboratory and the results were still equivocal, and they recommended a retest in six months.

    A likely explanation is that the dog just has a very small number of worms present, possibly they’ve been present all along and the first test was the false negative.

    At any rate, you don’t have very many worms present.

    I cannot make recommendations for a pet I haven’t seen. However, it is likely that simply continuing this dog on preventive will work out okay, based on what you have told me.

  42. Michelle Daniel says:

    Thank you for the clarification. We are just so confused as to what to do. My old vet said the test they did today (a re-test in their office) was the antigen test? Looked for the babies and the females and both were negative. The Vet that did it last weekend sent the blood off to an outside facility and they are the ones that said she is positive. They want to start her on Doxycycline for 30 days and then the 3 Immiticide injections. I asked about just keeping her on preventative and she lectured me on the damages the adult worms are doing to her heart and lungs and they never do the slow kill. I asked if she was a strong positive and she said that was like saying someone was a little pregnant. I am so confused. My original vet of over 25 years has used the term ‘strong positive’ himself. Yet this (very young) Vet acted like that was idiotic and not a thing. I just do not want to subject her to this very aggressive poison if she truly doesn’t need it. Is there something else we can do to see 100% if she has worms? Should I request from my Vet that showed negative if they can send out a sample to a lab? Thank you for all your help.

  43. Doc says:

    Hello, Michelle,
    I think that generally speaking, you cannot really estimate severity of infection from the results of the test. It’s either positive or negative. Except when it’s equivocal, looking like neither one definitely. In those cases, we believe there are very small numbers of worms.

    The reference labs can usually detect smaller number of worms than the tests we use in our clinics.

    You are a little mistaken in calling the Immiticide a “very aggressive poison”. The problems with treatment are related to dead worms clogging up the arteries. It is rare to have anything worse than some temporary soreness at the injection site with Immiticide. Lots of dogs don’t even have that.
    The problem with “slow kill” is that you don’t know when the worms are going to die of old age, years from now. It is when they die and move from a big artery downstream to a small one that trouble occurs.

    When you are going through the treatment process with Immiticide, you know when that is going to occur, and can restrict the dog’s activity to reduce risk of complications.

    It won’t hurt to re-test, but if your dog has a really low number of worms, the results may continue to go back and forth.

    The upside of that is that dogs with really low numbers of worms rarely have any complications with the treatment.

  44. Michelle Daniel says:

    Update: My old Vet ran 4 more antigen tests in house and they all came back negative. They sent off to a lab and had a special heated test done and sure enough, it was positive. There are studies now proving that if a dog that is positive (un-knowingly) and is on preventative, that it can show a False Negative as the drug in the preventative bonds the blood and will not show the true results. The only way is to have the plasma heated and that forces the separation in the blood. So this is what the specialist recommended and so we did it and this is where we are now, she is positive. So we will be starting her on the Immiticide soon.

  45. Doc says:

    Hello, Michelle,

    The heating is supposed to break down the complexes of heartworm antigen/antibody, to release free antigen. In other words, the body is trying to get rid of this stuff. Antibodies are protein molecules that bind to something that is “not the body”, in this case the protein molecules from the female heartworm’s reproductive tract. Once the two are bound together, there’s nothing left to react in the antigen tests we run in the clinic.

    Heating the sample is supposed to free up antigen (if it’s there). If you have to do this to detect the presence of worms, it is very likely that the worm burden is really small.

    That’s good, as the bottom line here is that fewer worms equals fewer chances for side-effects with the treatment.

  46. Vic Rice says:

    We adopted Sweetie about 2 months ago, she was rescued from a puppy mill (we are trying to help her get over trust issues), she was previously treated for heartworms, we have given her the Heartguard religiously… Anyway, she started coughing the other day, we took her to the vet. He indicated an enlarged heart and blood vessel (signs of HW damage). She tested positive for adult heartworms, but not babies. Her heartworm treatment was 8 months ago. I am leaning toward waiting on retreating as I have read that they will sometime be positive for some time. Especially with her heart damage already noted, I do not want to put her through the treatment if I do not need to.. All advise and comments are welcome!

  47. Doc says:

    Hello, Vic,
    The latest reading I’ve done suggests that adult heartworm protein can persist for up to 7 months after the treatment. What about 8 months? Hard to say.

    We know that there are some dogs from whom you can never completely eliminate every single worm.

    The tests are not very reliable as far as estimating the quantities of worms present.

    The chest X-ray doesn’t really tell you whether that damage is new, old, or ongoing. If you had X-rays from before the treatment 8 months ago, it would be helpful to compare the two for changes.

    Treating her for heartworms again shouldn’t really have any direct effect on her heart. It is the arteries in the lungs that suffer when the worms die and move downstream.

    Sometimes a dog in this situation will benefit from a course of corticosteroid therapy if the problem is old damage in the arteries. Relieving inflammation opens up the vessels, and the dog’s arteries and lungs function better, and the cough may stop (at least for a time).

    Without knowing everything about the dog’s history and actually seeing the dog and the X-rays, I really cannot give you specific advice.

    • Kalie says:

      Hey Doc,
      Question you might not be able to answer. I adopted my 2 months ago from a rescue, knowing that she was HR+, but knowing nothing about treatments. Rescue made me sign a contract stating that I would not do fast-kill. Knowing nothing, I signed it… already loved the dog, thought it was no big deal. Moved with dog out of state and took her to the vet. Told him the dog had finished doxy and was to start slow kill. Vet took tests, reconfirmed HR, anemia and urine crystals. Definitively recommended fast kill… dog is young and healthy, no s/s as of yet. Trusting my vet, dog got first dose today… (traumatic by the way, struggled, needle bent, took two tries, sedative didn’t work…rough time). Now I’m regretting, afraid the rescue will find out and forcefully remove her or sue me if she has a bad outcome. Have you ever heard of anything like this?

      • Doc says:

        Hello, Kalie, I’m sorry to be so late replying, but we had a revamp of our website and the comments were hidden until today. I cannot imagine the rescue’s rationale behind trying to prohibit “fast kill”, as this is the standard of care recommended by the American Heartworm Society. “Slow kill” can be appropriate for some dogs, but certainly not for all. I’m no attorney, but I cannot imagine this contract being enforceable. It imposes a lower standard of care than is customary, and assumes that you have the technical expertise to evaluate the dog’s medical condition.

        I would speak with your veterinarian (if they haven’t already recommended this) about doing some type of sedation prior to the dog’s next visit. There are a variety of “chill protocols” to get the dog calmed down before he gets all jacked up by a combination of previous bad experiences and pain. If a dog is already freaking out, sedatives tend to not work so well. You want to get started at home before he ever leaves the house.

  48. Michelle Daniel says:

    We are starting her on Doxycycline now. I was wondering what your thoughts were on doing the Immiticide vs. slow kill ? Since we are looking at the strong possibility that she has minimal worms after needing the heated testing done to get a + result after 4 neg. in-house tests. The rescue I adopted her from suggested to just do the slow kill and stated that is all they have done with their personal dogs and rescues and all their dogs in their program are doing excellent many years later. I have done the Immiticide with my other adopted dog and it was a bit scary as she was so hyper. Now this most recent dog is 1,000x’s more hyper & we are concerned about trying to keep her calm. She is young and healthy, and very active. Our oldest dog we adopted at 2 yo and she was slow kill treated by the shelter, she tested neg. after 14 months, and is now almost 14 yo & shows no signs of her age. My vet is suggesting either, that it was our choice but warned of the damage the worms are doing the entire time. We are so confused and just want to get all the information we can. Thanks you for all your help.

  49. Doc says:

    Hello, Michelle,
    My personal philosophy is to get them killed with the Immiticide so that I’m not waiting for a reaction to worm death for 3 years. If I know when they are dying, I can monitor the dog, and I can medicate with prednisone to minimize inflammation. Then I’m done with it.

  50. Nikkie Hartmann says:

    Hi there, I just came across your blog and hope you’re still monitoring this thread!

    Before I adopted him, my dog was treated for heartworm on 4/24 and 4/25 – Ivermectin and Immiticide treatment (1.4 ml IM on 4/24/17) and repeat dose of Immiticide (1.4 ml IM on 4/25). He started monthly heartworm preventative after and has had two doses since. His new vet tested him for adult and microfilariae on 6/11 and both came back positive. He had a follow-up echocardiogram because of a heart murmur showing worms still present in the heart. The vet recommended retreating ASAP. Does this seem appropriate to do just two months after initial treatment? I worry the treatment has not had time to completely kill off all the worms, and his body may still be breaking down those that died. I don’t want to overload his system and am inclined to wait the additional 4 months and retest at 6 months as the guidelines recommend. Thanks!

  51. Doc says:

    Hello, Nikkie,

    Sorry to be late replying. I always feel that the best doctor to advise you is the one seeing your dog.

    Having said that, if you keep the dog on monthly preventive medication, he shouldn’t get any additional worm burden while you are waiting to try the re-test in four months.

    However, if the worms are interfering with heart function, you might be making a mistake to wait that long.

    You might talk with your veterinarian about monitoring him for a few more weeks and repeating the echo.

  52. Anne says:


    I’d appreciate your perspective on an alternative approach to treating heartworm in a young otherwise healthy dog.

    Aside from cost — why not remove the adult worms via some sort of cardiac catheterization procedure — entering through the jugular I presume — and then use the adulticide on the remaining worms that cannot be fished out. It seems that there would be less risk of worms migrating to bad places if you got the bulk of the worms out before killing them with adulticide.

    If this approach is at all feasible, I would want to take my dog (4 yo 50 lb lab/pitbull mix) to a center that does a lot of these procedures so that they had the skill/experience that comes with lots of cases.

    Could you suggest such a center? I assume they would be more in the south since there is a greater prevalence of heartworm in the south.

    Thank you in advance for your opinion!

  53. Doc says:

    Hello, Anne,
    Sorry to be so late replying, but I have been out of town.

    What you suggest sounds great, but is not feasible. The worms don’t really get into the right side of the heart much until the pulmonary (lung arteries are filled with them, all their branches).

    When heartworm retrieval forceps are used as a salvage procedure, they are used primarily to remove worms from the vena cava outside the heart (accessed via the jugular vein), and perhaps into the right atrium. Heartworms are not usually located here, but when they are, the dog gets dramatically ill very quickly with “post caval syndrome”. Sometimes you can remove enough worms to get them out of this crisis and then the dog still must undergo the regular treatment procedure to remove the bulk of the worms that are inaccessible to the forceps.

    To get into the pulmonary arteries, you would have to maneuver all the way through the heart making numerous turns and trips back and forth. This would be impossible without using dye and a fluoroscope, and would take hours if you did have such equipment.

    The procedure you suggest sounds good, but just cannot be done with today’s technology.

  54. Alex Christianson says:

    I help run a canine rescue and have a question regarding one of our rescues. He was diagnosed with HW in March- received doxi for a month then was given 1 shot on April 26 and then another shot April 28th down south. He arrived up North and we had him retested in June and he was still positive for HW and they want to start the treatment all over again- 1 shot then 30 days later 2nd and next day 3rd. We are concerned about doing another round so close to the last partial round- is that ok to do? Also, isn’t it normal to have a HW+ test 1 month after treatment- isn’t 6months a better indication. Any clarification would be wonderful.
    Thank you

  55. Doc says:

    Hello, Alex,

    Generally speaking, the protocol is to start on Heartgrad, give 4 weeks of doxycycline,rest four weeks, give one injection of Melarsomine (Immiticide, Diroban), wait one month (during which I give prednisone for inflammation in the arteries as the worms break up and move), then give two injections of melarsomine, 24 hours apart.

    Giving the two injections closer than 24 hours is considered to be more toxic, and more than 24 hours apart is considered to be less effective.

    Even if all the worms are dead, the adult heartworm protein can persist in circulation for four months.

    I would continue on preventive, retest in September. If he is still positive, I would just do the two injections 24 hours apart, not a full 3-shot series.

  56. Marian Burgess says:

    Hello, I live in the UK and have recently adopted and re homed a Romanian rescue dog who has heartworm. In the UK we don’t have heartworm so vets know very little about it. My vet is treating my dog but this is her first time of treating any dog with this condition. Would you be happy to confirm that we are going in the right direction for treatment as it doesn’t appear to be along your suggestions and recommendation

  57. Doc says:

    Hello, Marian,

    While I could compare the treatment being prescribed with what we consider to be standard in the USA, I cannot really speak to the specific treatment of a dog that I have never examined.

    I would be happy to help in any way that I can.

  58. Barb says:

    Hello, My 3 yo lab/hound mix is still testing positive after 1 year. Her yearly heartworm test in Dec 2016 was positive. She takes Heartgard, I missed one month in 2016. She had her first Immiticide injection on Dec 22. The second injection was on Jan 19 and the third on Jan 20. On Jan 22/23 she developed some odd symptoms (a sac/hanging skin on her chest and belly and then her eye was swollen almost shut with bumps all over her head). They’re thinking she had a strange reaction to the Immiticide. On Feb 17 she went back and had a dose of an oral med (not sure of the name). We retested her in September and it was positive. She was put on Doxycycline for 1 month, thinking this might help weaken any remaining worms. Retest was done in December which again was positive. She was on monthly Heartgard throughout the treatment. We are a little concerned about doing a repeat Immiticide treatment due to the weird reactions she had. I would love your opinion or thoughts. Thanks!

  59. Doc says:

    Hello, Barb,

    I have heard some heartworm experts say that some very small percentage of dogs can never be completely cleared of the worms, despite the full treatment.

    Given the difficulty that your dog apparently had with the last treatment, would be inclined to just keep her on heartworm preventive medicine.

    The best person to advise you is the doctor actually seeing your dog. There are lots of factors that I cannot be aware of in a long-distance relationship.

    You might also talk to your doctor about switching to a different preventive medicine. In dogs where Heartgard seemed to have a lack of efficacy, I have switched the patients to Advantage Multi and done better.

  60. Mary Ellen Trusheim says:

    Hello. We recently adopted a 1-1/2 to 2 year-old dog from a rescue. He tested positive for heartworm and received Immiticide shots on January 27 and 28. The rescue says that he needs to go back for a third shot of Immiticide on February 27. The treatment has been hard on him, and even now at three weeks out he is still rather lethargic.

    From what I have read, the older treatment protocol was two shots 24 hours apart and then no more. I have also read about the newer recommended 3-shot protocol (with one month of doxycycline treatment prior to starting), but unfortunately this protocol was not followed. What I am confused about is why the dog needs to go back for a third shot since the older protocol was followed? Thank you.

  61. Doc says:

    Hello, Mary Ellen,

    I must admit that I don’t see the rationale on following two injections with one a month later. That is not standard, so I don’t know what your doctor’s rationale is. You should just ask them.

    I also would wait until the dog feels fully recovered from the first go-round before I would give any more Immiticide, regardless of the rationale.

  62. Mary Ellen Trusheim says:

    Thank you for your response. I was mistaken about the third shot. It is an elevated dose of Ivermectin, not another dose of Immiticide. Even still, we are going to skip it and just start Heartgard since our dog has had such a hard time with the heartworm treatment.

  63. Doc says:

    Thank for the clarification. Unless you have one of the herding breeds like Collie, Border Collie, Australian Shepherd, Sheltie, Blue Heeler, or similar, you are very unlikely to have any problems with the higher dose of Ivermectin used to clear the bloodstream of microfilariae (microscopic baby heartworms). However, staying on Heartgard will probably clear the microfilariae eventually.

  64. Sabrina says:

    We just adopted a Weimaraner from the south who finished slow kill treatment of heartworms on 3/26. We took her to the vet yesterday and she tested ve+ for baby worms. We don’t have much of an ecosystem for them in Chicago, so I’m assuming they came from her former home. They told us to continue Heart guard which I understand. But does this mean it’s a new infestation possibly? Or is this sequela from precious infestation. There were no adults.

  65. Doc says:

    Hello, Sabrina,

    Without knowing what you gave as “slow kill” and for how long, I can’t evaluate that much.

    If a dog has been through what should be an effective treatment regimen and still shows evidence of heartworms, it can be either of the situations you describe. That is, you might not have cleared all the worms, or the animal could have been infected and the worms have been at a stage of development that the treatment was not effective against. Thus, they continued to mature and now you find evidence of them.

    In any event, we know that the test for adult heartworm protein can give us a false negative if there are fewer than four adult female worms present. If you are seeing microfilaria (baby heartworms) at this point, then you probably have at least one mommy and daddy in there.

    The good news is that if the test for adult heartworm protein is negative, you have very few adult worms present. I would certainly continue the preventive medicine so that you don’t get any more developing in the future.

    Without a more complete history and actually seeing your dog, I really cannot give you any more specific advice.

  66. Lorraine says:

    Last May I adopted a chihuahua from a rescue, she was HW positive. She was given the 2 injections, then came to me a week later. She has been on pregnisone till Ferbruary with a slight cough. Now she is taking Temnal P, I can’t seem to wean her off or she coughs.
    X-ray in sept shows an slightly enlarged heart.
    In November she tested neg for HW.
    I just had her tested again and she’s HW +
    I can’t understand how this happened, since she been on iverheart preventive.
    She is now on doxy and scheduled to have the repeat 2 shots to clear the heartworm in June
    Am I doing the right thing to have shots repeated?

  67. Doc says:

    Hello, Lorraine,

    Since the dog is still having these coughing problems, I would probably go ahead with the re-treatment. There shouldn’t be many worms present at this point, so she should do well.

    It is possible that even with a few worms, there is inflammation in the lung arteries that is causing the cough whenever she’s off the corticosteroids. So, getting rid of the worms is good.

    Do continue the preventive so that she doesn’t get any more worms than she already has.

  68. Lorraine says:

    Thank you for your response.
    Should I change Heartworm preventive since it didn’t seem to work?
    It seems like the iverheart plus didn’t work?
    What would be a better choice?

  69. Doc says:

    You should discuss this with your veterinarian. I cannot prescribe for patients I have not examined. I would certainly consider switching preventives. In patients with recurring infections, we have changed to Advantage Multi with good results.

  70. Phil says:

    Hello there. I rescued a a 9 pound chihuahua terrier mix a little over a year ago in Atlanta who was HW positive. I brought him back to California with me and immediately started treatment with the heart guard and doxy then immiticide 3 injection treatment. 6 months later he was still positive and did one more immiticide injection.

    i went to the vet today and he tested STRONG POSITIVE. The vet said she is going to talk to a cardiologist but she recommends doing the 3 injection treatment again. meanwhile he’s been on monthly heart guard the whole time. ugh. i’m so upset. He seems so healthy.

    Should I get a second opinion? Try another Vet hospital? Or put my dog through another treatment. It’s so hard on him. He’s probably a little over 3 years old now.

  71. Doc says:

    Hello, Phil,

    This is a tough call. While the injections may have been pretty uncomfortable, after 3 injections, there shouldn’t be many heartworms left.

    Therefore, if you go through the cycle again, I wouldn’t expect much in the way of complications from the dead worms.

    I have been told by heartworm experts that there are some dogs that you will never totally clear and get a totally clean test.

    I advise you to see what the specialist tells your veterinarian and work with them.

  72. sharyl Harrison says:

    We just adopted a 2 yr old lab/retriever rescue dog,

    only to find out that she has heartworms. After some research we found she has many of the symptoms. I understand the treatment, buti’m more concerned about her emotional state. Obviously we haven’t had time to get to know her yet, & from her previous environment isn’t too trusting of people and now this. What can we do for her emotionally? What do we give her instead of hard, dry dog Food? We. Are so worrried about her and not the least bit happy that the “pound” hdn’t even tested the her to treat this disease. We want a Normal life with our wonderful girl & don’t know what to do. She is seeing a vet early Mon morning, the soonest we could get in. Any information you can give us would be wonderful.

  73. Doc says:

    Hello, Sharyl,

    Sorry that I was off line over the weekend and couldn’t give you any info before your visit. Having said that, the best doctor to advise you is the one seeing your dog.

    Most dogs are quite happy with dry food. It is fine nutritionally, and is certainly more economical and simple to feed than canned foods. Raw foods are not recommended.

    Stay with a trusted name brand, avoid cost-cutters and house brands.

    Feeding people food makes it more difficult to feed a balanced diet. Try to keep people food to less than 10% of the diet if you give her treats of that nature.

    Emotionally, I would say to try to be consistent, don’t mix signals for her. It can’t be okay to jump on you when you have on your overalls, and not when you have on your best dress. This is confusing to the dog.

    After she has had a chance to get accustomed to her new home, it would be great to find an obedience class. This is quality time that you spend with the dog. The dog learns constructive behaviors, and learns that these please you.

    At 2 years of age, it would be unusual to have enough heartworms to cause significant damage, but your veterinarian can evaluate that with physical examination, and possibly chest X-rays.

    Feel free to come back with questions, but start by asking your veterinarian.

  74. Jean says:

    I adopted a rescue dog from Louisiana last August. She was pulled from a kill shelter in March and was HW positive. The rescue group started her on a monthly does of Advantage Multi and gave her a month of doxycycline followed by two shots of Immiticide 24 hours apart.

    I had her tested nine months after her last injection and she tested positive for the adult antigen. No babies. My Vet and I decided to wait a full year from the last injection and test her again. She tested positive again and a confirmatory panel was also positive. Clearly, she needs to be retreated.

    I am starting her on a month of Doxycycline, but my question is regarding the number of shots of Immiticide following the doxycycline. The American Heartworm Society Guidelines say if the dog is still positive to give a follow-up treatment of two injections of Immiticide. Is this the recommended number of follow-up injections even if her initial treatment was only two, and not the protocol three, injections?

    I would love to only give her the two injections 24 hours apart because she is very active and the restricted activity will be difficult for her. That being said, I want to do what is best for my dog. My vet consulted with two different cardiologists and each had a different answer to this question.

    My dog is a 4 year old, 13 pound rat terrier. Her blood work all came back normal and we did x-rays that my
    Vet said looked normal and non-concerning. She exhibits no signs or symptoms of HW disease.

    Your thoughts on this would be greatly appreciated.

  75. Doc says:

    Obviously, I haven’t seen your dog, so can only give you general advice.

    The research shows low 90% kill with two injections, high 90% kill with three injections. Some dogs will never be totally cleared.

    If it were me in this situation, I’d just do the two injections. You’ve already had two, so that makes four.

    She probably has very few worms, as she feels good and has normal X-rays. Chances are very good that she will breeze through it this time.

  76. Jean says:

    Thank you so much for your quick response. My gut instinct was to just do the two shots, but I kept coming back to not wanting to put her in harms way by overloading her arteries with dead worms.

  77. Doc says:

    If she has been on the Advantage Multi all this time, she shouldn’t have any new worms, just the few that didn’t get killed with the previous treatment. It is very important to continue the preventive meds so you don’t have it all to do over again.

  78. Hannah says:

    My almost 5 year old dog has had two cycles of Immiticide treatment over the last 3 years, and is still showing positive for heartworms. He has been on prevention since he was 6 months old, religiously. I’m now at a loss for what to do, obviously the Immiticide hasn’t worked to this point. Hoping for some encouragement and insight for what comes next…

  79. Doc says:

    Hello, Hannah,

    I have been told by the experts that some dogs can never be completely cleared. These dogs do have most of the worms cleared by the treatment.

    Stay on your preventive. With dogs that have problems, I like to use Advantage Multi. This tends to get a steady state level of moxidectin in the body over a period of several months (versus the high level of Proheart injection, that gradually gets less and less).

    Your dog will probably do okay.

  80. Steve Rider says:

    My female beagle, 7 yr old and adopted 2 yr ago, tested positive for heartworm at her first vet visit. She was treated with an injection and placed on Heartgard. Several months later she was still positive, so the 2-shot treatment was done, followed by restricted activity. Several months later she still tested positive and went through the treatment again. Now, several months later, I’m told she STILL is testing positive. How is this possible? The vet is recommending 30 days on doxy. then the 2-injection treatment again.

  81. Doc says:

    Hello, Steve,

    When a dog is infected with the heartworm larvae, it takes six months before they reach adult-hood.

    They are susceptible to the preventive in the first 5 to 6 weeks after infection. Then the developing worms are not affected by the preventive. They also are not affected by the Immiticide treatment until they are fully mature at six months.

    So, there are “windows” in the development of the parasite when neither type of medication affects them.

    That is one potential explanation.

    Another is that some dogs cannot be completely cleared of the parasite, no matter how many times you treat them.

    The good news there is that there are generally very few worms that survive, and the dog leads a normal quality of life.

    Without having seen and examined your dog, I cannot really make a valid recommendation for you.

    If there were no complicating factors, with this history I would probably just stay on the preventive medication at this point.

  82. Viki J Carson says:

    I have two large dogs. Anatolian Shepard and Great Dane. Both have tested positives for heart worm. I got the Dane as a puppy and she has been on Heartgard all of her life. The other I got from a rescue, with Heartgard meds and vet tested as neg. I never missed a month on giving them the meds. How can this be happening to them?

  83. Doc says:

    There have been some problems with lack of efficacy in the Mississippi delta area. We switched our patients with this problem to Advantage Multi and have had better results with the high risk patients.

    Your veterinarian should be able to pursue the guarantee program to pay for your dog’s treatment. After they have been cleared, you might talk with him/her about changing to a different preventive medication.

  84. Sophie Holeman says:


    I adopted my dog a couple years ago who was heartworm positive. He had all the symptoms (gagging, heavy panting after barely any strenuous activity). He went through the whole treatment (doxy, the 3 injections) and is on monthly preventative. His last injection was imid-July, and when he was clear to exercise again in October, all of his symptoms went away! It was incredible, absolutely no gagging, he wasn’t panting abnormally after exercise anymore.

    We went in for the baby heartworm Test and it was negative (great). Our vet said we can’t test for the adult heartworms until one year after treatment. A few months ago I noticed the exact same symptoms are back. I have brought him into my vet to tell her about the symptoms. He would sometimes even wake up from sleeping gagging, or even when he had just been laying around the house.

    Is it possible the worms are back? I don’t think he has had the adult heartworm test as they told us to wait a year. I know something is wrong as I have seen firsthand what he was like for a short period of several months (no gagging, and no heavy panting). I feel like my vet is not listening to me and keeps telling me it is not heartworm but something deep down just tells me it is. It is the EXACT same symptoms he used to have. They have done xrays again and confirmed his heart and lungs looked fine. Tested for bloat, and he was fine.

  85. Rachel Price-Rayner says:

    We have an approximate five year old rescue beagle who tested negative for Microfilaria (the rescue didn’t run the antigen test) when we adopted her. She started Heartguard Plus the day we got her and continued. Four months later, after a near fainting incident and increased coughing, she was diagnosed with adult heartworm from an antigen test at an ER vet. Her chest x-ray looked normal. We were in the process of moving across country to a non heartworm state (WA) but obviously continued to give her monthly preventative. Her new vet immediately started her on the three injection protocol that was administered correctly and six months later she was antigen negative (Oct). She has never tested positive for microfilaria. We switched her to Advantage Multi at that time as she has larval leak hookworm infection and this is the only preventative and treatment method that works. She still had a cough, which we and our vet attributed to her larval leak hookworm condition, but after a near fainting incident last week I took her back to the vet for another heartworm test. Her test came back yesterday antigen positive and her x-ray didn’t show signs of heart damage. It seems you are saying that her low worm burden may have shown a false negative and may not have been eradicated fully and perhaps we should do two more melarsamine shots 24 hours apart? My vet is not advising me at this moment and suggests I contact my old vets from LA and VA as they are more versed in heartworm conditions (her bloodwork is normal). I have put in a call to a cardiologist at WSU for advice, but found this board. Any ideas as she is symptomatic, but x-rays are normal?

  86. Rachel Price Rayner says:

    Sorry, just to add to my other comment, should my beagle commence with a 30 day course of Doxy before her two further Melarsomine shots 24 hours apart if that is the recommended protocol going forward?

  87. Doc says:

    Hello, Sophie,

    I can appreciate your viewpoint, with the outward clinical signs being so similar. However, when we look at your dog’s treatment history, it appears impossible that he could have acquired enough new heartworm infection to produce clinical signs of disease.

    Since his heart and lungs look fine on the X-rays, that also goes against it being heartworm-related, at least as far as the problems we see with the lung arteries being clogged with worms (or heart enlargement due to weakened heart muscle).

    It really does sound like there is some other problem that hasn’t yet been diagnosed.

    I understand your feeling that you aren’t being listened to. Sometimes we (doctors) think that we are communicating more effectively than we really are.

    It does sound to me like you need more diagnostic testing here. It just doesn’t sound like you could be having a recurrence of the heartworms under these circumstances.

  88. Doc says:

    Hello, Rachel,

    Some dogs will never fully clear. I personally would not be going through more melarsomine injections if the dog was on preventive since early last summer and copmleted three injections by August.

    If she didn’t start preventive before the mosquito season, then you may in fact have a new infestation, but this depends on the timing of when the preventive was started and when the melarsomine injections were given.

    In either case, one wouldn’t expect much of a worm burden. Since the radiographs are normal, my next step with a dog in this situation would be to work up for other heart disease. An echocardiogram (heart ultrasound) and electrocardiogram (ECG) would be my next steps at this point.

  89. Kathleen says:

    My dog I fostered and then adopted and she had already started the “slow kill treatment” with doxy for a month and the multi advantage gel monthly x 10 months. I have heard she may test positive for antigens for 6 months post treatment but with slow kill was worried with how long she is infected and tested her after completing the 10 months of multiadvantage. She was negative for the microfilaria but positive for the adult antigen. They gave the option of continuing 3 months of multi advantage. What would you recommend? My vet is a home vet so we have not done chest xray but no cough and heart sounds good. Does the advantage multi also work as preventative? I just started her on month 11 of multi advantage but deciding on possibly imiticide ? Just unsure if positive antigen could be testing dead protein or a false positive within 6 months post treatment?

    • Doc says:

      Hello, Kathleen,

      The study I have seen on the doxycycline/Advantage Multi treatment suggested the worms would be cleared within 6 months. However, this cannot be guaranteed. If the dog is very young (so no big heartworm load and no damage yet), and has no clinical signs, I’d feel okay with continuing the Advantage Multi. I’d then retest in 3 months. If still positive, then you still have heartworms present. I suspect you still have heartworms present now.

      If the dog were older, I’d plan on going ahead with the Immiticide.

      My recommendations can only be general, as I have not examined your dog. You really need to discuss your concerns with the doctor treating your dog.

  90. Jay says:

    Hi, I read your post with gread interest and really wanna ask one thing. How less effective is 2 immiticide shots given 1 week apart?
    My dog has been diagnosed with heartworm and treated with immiticide with 2 dose plan. The progress has been like this :
    1. 2weeks on medication
    2. one immiticide shot
    3. one week on medication
    4. another immiticide shot
    5. medications (doing this right now)
    6. on the day 7th of the step 4 above(another shot), the doctor said he will give me instructions afterwards.
    My concern is why the shots were one week apart. Any material say 2 shots should be 24 hours apart. Actually on the day of the first shot, the doctor was telling me to bring the dog again for another shot tomorrow but i only knew 3 dose plan was the standard and asked him with my concern, and he said then bring him back one week later. Now I’m concerned that maybe this was safer but the worms may not be cleared all. Just, how much less effective is the shots given 1 week apart? And if my dog should be treated again later, can the worms inside be resistant to the shots and may not be killed forever?

    • Doc says:

      Hello, Jay,

      In reading on Veterinary Information Network, what I find is that the studies on efficacy of the Immiticide only show proven efficacy when the two injections are given 24 hours apart. There really isn’t any hard data about what will happen if you give the injections 7 days apart. Some folks feel that if you don’t give them within that 24 hour window, then it would be the same efficacy as a single injection (which kills the weaker smaller worms, but not the total worm burden). It hasn’t really been studied, from what I can tell.

      At this point, I’d let it play out, and if the dog isn’t cleared, do the two-injection protocol 24 hours apart.

      This situation is very unlikely to cause resistance to build up in the worms. The Immiticide will still be as effective as it ever was going to be.

  91. Jay says:

    Thank you for the answer doctor!
    Then, once this treatment is done and the worm test is done someday,
    is it possible that my dog will show false negative because less than the detectable number of worms are alive(few but still alive)? I found out the heartworm test is not 100% correct and only shows positive only when more than 2~5 of the female worms are present.
    Also is it possible that since the initial shots, the survived worms have hidden deep inside somewhere in my dog’s body and the more immiticide shots later may not have effects on them?

    • Doc says:

      Hello, Jay,

      It is true that the antigen test can give a false negative when there are fewer than four adult female worms present. However, if there are that few worms, the worm burden is unlikely to cause the dog any medical problems.

      The worms are located in the pulmonary arteries, and if there are lots of them, inside the right side of the heart, as well. There have been very rare reports of the worms developing in other locations, like the muscle of the diaphragm or the anterior chamber of the eye. These are bizarre and rare.

      I don’t know how much deeper you could get than your heart and lungs, frankly.

      They are not known to inhabit other body tissues, except for rare and freakish occurrences.

      There are thought to be some patients where you cannot clear 100% of the worms, but these dogs have such a low worm burden that they don’t suffer from them.

  92. Molly says:

    Hi Doc,

    My dog was about 9 months old when I rescued her about a year and a half ago. She tested negative for heartworm at her first checkup when we got her and has been on a heartworm preventative since then (we’ve never missed a dose). But when we went in for her 1 year checkup she tested positive. Since then we’ve gone through the full three injections of heartworm treatment.

    We just went in for her checkup 4wks post treatment and she still tested positive for the antigen although the vet said there weren’t any microfilaria in her blood. The vet said we will re-test her in October (1 year after diagnosis) and hope that she tests negative then but that we should continue to restrict her activity until then.

    I’m wondering how long post-treatment is it normal for a dog to test positive for the antigen? Would there be any value in us testing her again before October? Is it the normal protocol to restrict activity until the antigen test comes back negative? I’m pretty crushed to hear that I have to keep her restricted for 8 more months. She is a young, active dog so it’s been pretty tough.

    • Doc says:

      Hello, Molly,

      It is very common to still see circulating antigen (adult heartworm protein) at this point, even if the adults are all dead and dissolved. We usually don’t test until 14 weeks following treatment.

      I haven’t seen your dog, so I can’t really disagree with your veterinarian’s recommendations. Generally speaking, I feel safe returning the dog to normal activity (gradually) six weeks after the third injection, but again, I haven’t seen your dog.

  93. Kelly says:

    Hi, I rescued my dog last October and he was heartworm positive. I immediately started him on preventative via my vet’s instructions and it seems like the protocol he’s following is very different from the standard. He gave my dog the first shot immediately after seeing him and gave me medicine to give him for 30 days, then gave the two other shots 24 hours apart and then gave me medicine for 60 days at our 1 month checkup. We tested him again last month after 60 days and he was still positive so he gave him another 2 rounds 24hrs apart and gave me medicine again. I’m supposed to take him to get tested again in a week and I’m concerned that if he tests positive again, is it safe for him to get another 2 rounds of shots? My vet seemed trustworthy but now I’m doubting if it’s ok or not. It just seems like a lot of shots and from what I read, you would recommend testing at 6 months if the dog is still testing positive

    • Doc says:

      Hello, Kelly,
      I can’t really speak to your dog’s condition, since I haven’t seen him. The drug melarsomine that we use is not very toxic for the dog. I wouldn’t be worried about repeating it.

  94. Kelly says:

    Hello! So glad to have found this thread, as I live in New York where heartworm is less common, and even excellent vets know less about the condition.

    I adopted my 5-yo dog River from Louisiana about a year ago (last March) and he was heartworm positive. He went through a month of doxy, followed by a dose of adulticide, and then two more doses back to back a month later. He has a bad reaction to the dying heartworms resulting in lots of lung inflammation and was on steroids for months, poor guy. But he made it through, and four months later antigen and microfilaria both came back NEGATIVE.

    Unfortunately two months after his negative test, he started coughing a lot and tested positive for heartworm AGAIN. Test was confirmed positive antigen but NEGATIVE microfilaria. His echocardiogram shows a heartworm infection that is more than mild (not just a worm or two) with worms in his pulmonary arteries, and changes to his lungs on x rays. No pulmonary hypertension, and he never lapsed preventatives.

    I love my vet, but even a specialist cardiologist is stumped about how he could possibly be re-infected so soon, or how he could have so many heartworms left after thorough treatment. Because heartworms are less common here we’re having trouble accessing knowledgeable care. River is the sweetest dog, and any help at all in finding someone to help get to the bottom of this would be greatly appreciated.

    • Doc says:

      Hello, Kelly,
      That situation is unprecedented in my experience. There is a point in the heartworm life cycle between about 6 weeks post-infection and six months post-infection where the preventives have no effect on the developing worm and they aren’t adults that can be treated with the Immiticide.

      Thus, you might have a few heartworms develop from the most recent mosquito exposure prior to getting the dog, but you wouldn’t expect it to be very many – one season in the mosquitoes doesn’t usually result in the acquisition of very many worms (which is apparently what has happened here).

      I don’t have any great ideas here. I would be going through the process again, as unattractive as that proposition is.

  95. Mary says:

    Hi Dr. Mobley, I’d welcome your advice on the following: is there a reason for keeping a dog exercise-restricted after testing negative for both antigen and microfilaria? I’m in Massachusetts, I adopted a hw+, 55lb, ~8 year old cattle dog from Texas in January 2021. The rescue treated her in Dec 2020 with heartguard and doxycycline as a “slow kill” approach before she arrived and provided 30 more days of doxy in Feb 2021. She’s been getting her heartguard monthly since December now. She tested negative for antigen and microfilaria this month, May 2021. My vet wants to retest her in 6 months to confirm she is cleared, but said to keep her exercise restricted for those 6 months. I pressed on this and they said only bathroom breaks and 5-10 minute walks at most, no other exercise. I want to ask the vet for more details but also want more info before the conversation. I didn’t find anything to support this in the American HW Society literature. If you could let me know your thoughts I’d appreciate it so much.

    • Doc says:

      Hello, Mary,

      I’m not sure exactly what the reasoning is on this, but I haven’t seen the dog.

      Generally speaking, if it were my case, and we were worried (as you can have false negative tests with very small worm burdens), I’d get chest X-rays. If they looked normal, I wouldn’t be too worried about exercise.

      The other thing you could do would be to get the blood tested at a commercial lab with heat treatment. This will detect some types of false negative tests.

      If the dog wasn’t symptomatic before treatment ( no shortness of breath, no coughing, no fainting spells, no problem with exercise) and chest X-rays look good now, I think the risk would be very small for exercise to produce problems.

      Again, I haven’t seen the dog, don’t know the situation, so I can only speak generally. The doctor seeing your pet is your best source of advice.

  96. Kelly says:

    Hello! Thank you for your response above re: my dog River. I’m coming back here with clarified information — after speaking with the cardiologist, it actually does seem like his worm burden is quite low. Just a few worms. The vet thinks he had a false negative in March, and two true positive tests in April. So likely a few worms survived the treatment. In this case, they’re giving me the choice to either re-treat him with a two-dose melarsomine protocol, or to continue giving him Heartgard Plus every month until the infection clears on its own. He is a 5 year old playful, active dog but his lungs do have damage from his allergic reaction to the last treatment. Do you think repeating is still the right move?

    • Doc says:

      Hello, Kelly,
      With only the information that I have, I would repeat the two-dose treatment. It is unlikely to cause the dog much trouble with so few worms still present.

  97. Leslie says:

    Hello Dr
    Our dog is about 14. We’ve had her 4 years. She has received heartworm meds every month faithfully from us as well as her previous owner.
    However, yesterday she came up with a H+ test.
    A second test will be done this week in hopes that this first one is a false positive.
    Have you ever experienced this? Any thoughts about why this would happen if it is actually positive? Is it safe to treat her at this age? Her CBC and senior panel are all fine. Any recommendations would be appreciated.

    • Doc says:

      Hello, Leslie,
      We do occasionally see apparent lack of efficacy. Sometimes the dog has managed to puke up a dose when nobody saw it, sometimes things just don’t work 100%, as we would wish them to.

      With a 14 years old dog who feels okay, and who would certainly have a low worm burden (regular preventive medicine), it would be hard for me to recommend the adulticidal heartworm treatment. While she would probably tolerate it okay, the issue is whether it would either improve her quality of life, or prolong her life.

      Your regular veterinarian is your best source of advice, as he/she has actually seen your dog. That being said, I would feel that the adulticidal treatment would be of limited benefit in a dog of this age under these circumstances.

  98. Harri says:

    My dog is a rescue from the streets, so when I took him in for evaluation, he was positive for heart worm. The vet estimates he is about 3 yrs old, 25 pound mix (possibly Cairn/Shihzu). He was treated using current recommendations and is now over 2 weeks out of his third shot. He is on Prednisone tapering dosages. He is not by nature very yappy or excitable, however he will bark at seeing dogs, etc. out the window. When I leave him alone, I block access to any windows where he might see anything that would excite him. I do walk him, less frequently now, since the steroid tapering has decreased the thirst/drinking/urinating needs. However, I was and am taking him out 4-6 x a day. He has to navigate stairs to get in and out, and he does get a little excited when he sees other dogs, but generally he doesn’t go wild. I’m wondering if I’ve allowed too much activity. He’s very obedient, and really the perfect dog. He does seem to sleep a ton of the time. Am I hurting him with these outings? His last two shots were May5 and 6.

    • Doc says:

      Hello, Harri,
      I think that as long he is either indoors or on a leash when outside, you are not putting undue stress on the dog.

  99. Hannah Howard says:

    Hello and thank you so much for this thread. I hope you may be able to provide some insight to our dog’s heartworm situation.
    We adopted our rescue dog Larry in August 2019 and he tested negative for HW and we started him on his monthly preventative. The vet said he was about a year old. In August 2020 we took him in for his one year checkup and he tested positive for heart worm. We continued his monthly preventative and followed the doxycycline, prednisone, and 3-shot immiticide regiment. He received his last shot in November 2020. We just took him in for his follow up test in June 2021 and he tested positive again for antigens and microfilaria. The vet has never seen this happen and has presented the slow kill option as an alternative to another round of immiticide but does not have a clear preference on which route to take. Larry is showing no symptoms whatsoever, has lots of energy, normal appetite, etc.
    I’m wondering if there’s a chance that he will test negative if we wait a few months and have him re-tested. And then if he still tests positive if we should continue on the slow kill route because we’ve essentially been doing it for almost two years already.
    The vet is great but doesn’t really seem to know what to do and I’m hoping you may have a more concise recommendation. He did okay physically with the immiticide but it was very stressful for him and I don’t want to put him through it again if not absolutely necessary. I appreciate you taking the time to answer all these questions.

    • Doc says:

      Hello, Hannah,
      I think it would be reasonable to use the “moxi-doxy” route at this point: 30 days of doxycycline and switch to Advantage Multi as the heartworm preventive.

      That being said, we are told by the experts that some dogs won’t ever be 100% cleared (though this is unusual). In cases like this, there are usually such a small number of worms surviving that they don’t really cause the dog a problem.

  100. Elle says:

    Thank you for all the wonderful information. I was considering adopting a heartworm positive dog from the spca. Your responses to individuals has helped clarify the treatment/testing/worm life cycle – really appreciate you taking the time!

  101. lin says:

    I recently took my 2-year-old dog to the vet and he came out positive for heart worms. He was around 6-7 months without his heart worm preventative and since I don’t drive because I am not legal yet no one really did anything to help me take my dog to get preventatives. Sadly… but now they ended up giving me something called doxycycline and something to make the symptoms less severe ? it starts with the letter p, but I would need to double check. They didn’t specify if it was baby worms or adult worms, but I was wondering how long would it take for him to completely recover. They did say to come back in a month to “recheck” but would that mean they’ll put the shot? Also, when he’s on doxycycline, should I avoid him running or playing? He’s an extremely active dog and it saddens me to keep him tranquil and inside.

    • Doc says:

      Hello, Lin,
      You don’t need to restrict the dog’s activity while on the doxycycline. Prednisone is a synthetic form of cortisone that helps minimize inflammation in the lung arteries when the dead worms move. I’m not sure why you would be on it now.

      You do need to be giving preventive now so that he doesn’t get any more worms than he already has.

      The usual course is to give doxycycline for one month, wait one month, give a single injection of Immtiticide, one month later two injections (24 hours apart), then six weeks to recover from that (worms die, then get dissolved by the white blood cells).

  102. Bella says:

    Hello, our 3 year old rescue just showed up positive on his annual heartworm test. We adopted him 2 years ago with a negative heartworm test and he was put on Heartgard immediately. His test last year was also negative. We have not missed any doses of Heartgard. How can this happen?

    • Doc says:

      Hello, Bella,

      Sometimes we get false negative tests with very small numbers of worms. Sometimes our dogs get rid of the Heartgard dose without our knowing it. If my dog barfs under a bush one day, I could miss it.

      Less likely would be false positive test. It is always worth a second test to confirm the results of the first. We do in-house testing, but if a dog tests unexpectedly positive, we send blood out to a reference laboratory to confirm.

  103. deb says:

    The most dreaded thing has happened. My dog tested H+ yesterday. He is coming to two years old. He has been regularly on preventive meds (every 35-45 days), but we did miss a dose back in July 2022 where there was a gap of 60 days in between doses. We have resumed the monthly doses since then. My vet said he could even have been infected while at breeders/pet store before we got him (at 12 weeks old). Or he could have been infected during that lapse in Jul 2022. Anyway, vet suggested a repeat test next week to confirm before embarking on treatment. He has not displayed any signs or symptoms except that he does sleep quite alot (has been like this for the past year). His lifestyle is quite sedentary with 2 short walks a day. I am wondering (and hoping) that he is in the early stages where symptoms have not developed yet and the prognosis is good? Will he have a good chance of long, quality life after treatment? I am tormented, thinking of how I had let him be infected 🙁 🙁

    • Doc says:

      Hello, Deb,
      If he has missed very few doses, then you probably have a very small number of worms, and prognosis for treatment and recovery is very good.

      Indeed, in a two-years old dog, if he had been outside in the mosquitoes all summer with NO preventive, most patients would still be in good shape, with no significant damage to the heart and lungs yet (though more worms would be present). In our severely heartworm endemic area with a very long mosquito season (and lots of them), untreated large-breed outside dogs may begin to show clinical signs of heart disease at somewhere between 3 and 5 years of age.

      In your case, while there is always a small chance of severe complications, I would expect a good outcome.

      45 days between doses is pushing it pretty hard. I feel pretty good up to 35 days, so if we strive for the same day each month, we can be a day or two late and not worry.

  104. Judy says:

    Hello Doctor, I so appreciate this blog and I hope you are still answering questions. I have a perplexing situation on my hands. I have a lovely dog that I rescued who came from Louisiana in December 2020. As so often happens, she tested negative for both hw and mf at the shelter vet but at her 6 month re-check she was positive (I found your blog then and learned so much from it!). She had been started on Proheart 6 in the rescue. My Vet and I did the full AHS protocol. My vet gave her Proheart 12 at the same time as she started doxy. We completed the 3 shot protocol. She was negative for MF at the 30 day post injection check and negative for HW in May 2022. All good, right? I switched her from Proheart to Interceptor Plus when we got the negative test and before she was due for another Proheart shot. She was negative again this past February for HW – 10 months past the previous negative test. I thought we were in the clear! About a month ago, she had a strange incident in the early morning when she seemed to have something like a spasm in her upper shoulder or neck. I was barely awake so it was hard to describe but immediately after I could just tell she felt off. She’d normally be jumping up and down telling me to get up and start the day. Instead she lay very still. She seemed alert but stunned and she wasn’t moving. Out of old habits when we were going through hw treatment, I checked her gums. They were very pale! I took her to an ER/urgent care center nearby. Long story short, she recovered quickly, they ruled out just about everything but told me she was positive for hw on a SNAP test. I almost passed out! She got a screeing cardiac ultrasound that looked normal but suggested I get follow up with a cardiologist. That day, I took her to a veterinary hosptial and got xrays and an echocardiogram. The results were mostly good and “normal” – no pulmonary hypertention, and “questionable” mild enlargement of the right side of the heart – but not even clear enough say that. But – there were “few organisms seen in the pulmonary branches” indicating a “low worm burden”. So that seems to confirm that she actually has an active case of HW, sadly. She has almost finished a month of doxy and we switched her to Advantage Multi as well. So…would you repeat the protocol or would it be sensible to do six months of Advantage multi, retest again and if positive then do immiticide? I’m just so confused and honestly can’t understand how this could happen. We live in Eastern Massachusetts near Boston – so ti seems hard to believe the Interceptor failed within the first year but I guess it’s possible. Thoughts? Sorry for such a long post!

    • Doc says:

      Hello, Judy,
      Sorry for the delay, but the blog was hacked and we had to do some house-cleaning.

      The antigen test (what you’d be checking on the SNAP) is for a protein found in the heartworm female reproductive tract. We know that if there are fewer than four adult females, it can give a false negative test. The results also can be inconsistent. It is quite possible that there are no more worms now than when you got the negative tests after her treatment.

      With a very low worm burden, I would feel safe doing the month of doxycycline plus monthly Advantage Multi (so called “slow kill” or “Moxi-Doxy”). My understanding is that it usually takes closer to a year to clear the dog, rather than six months.

      If a dog doesn’t lead an athletic lifestyle, and is your basic house-dog, walking on a leash, maybe running around the yard (versus running for hours while hunting), most tolerate a small worm burden with no difficulty. It would be possible to have even a few worms to migrate and damage a blood vessel, but it rarely happens under those circumstances. Some dogs cannot be completely cleared no matter what you do, but the worm burden is so small that they don’t suffer ill effects from it.

      You need to discuss this with your veterinarian (who actually sees your dog), but I’d have no problem with the Moxi-Doxy in this dog.

      • Judy says:

        Thank you so very much for your reply! My Vet is wonderful and is also comfortable doing Moxi-Doxy in this case. Still, I deeply appreciate your input given how much experience you have treating this condition. It has helped me feel more confident in this approach. Bless you for all you do our pets and thier owners!

  105. Lauren says:

    Hi Doctor,

    I’m writing to you from Los Angeles where our dog Jack has had a complicated heartworm journey. Here is our story: Jack was believed to be heartworm negative at the time of adoption from Texas in October 2020, and then tested positive for heartworm in October 2021 at his annual exam (class 1, asymptomatic without evidence of disease burden with normal chest x-ray and normal cardiac exam and labs). He completed the AHS protocol with doxycycline and 3 melarsomine injections over the following months. He then remained antigen positive when retested 9 months post-treatment in October 2022. We reached out to the AHS at that time and we elected to recheck him 1 year post-treatment. We were thrilled when he tested antigen negative in January 2023. Unfortunately, we just had his annual appointment and he tested antigen positive again. He has remained on Heartgard preventative monthly since adoption in October 2020 (never missed a dose) and has never been symptomatic. He has always been MF negative (at the time of diagnosis and after treatment), which has led us to believe that his preventative has been working. Given that we have been on prevention continuously and that we live in a low transmission area, we are thinking a new infection is unlikely. What do you think is going on here?

    We are repeating an antigen test with another lab which is pending, but we are uncertain of what to do from here.

    If this repeat test comes back positive, would you recommend completing the “treatment failure” course (i.e., 1 month of doxycycline followed by 2 melarsomine injections 24 hours apart)?

    If this repeat test comes back negative, would you presume that his heartworm burden is so low that he has fluctuating positivity on the antigen tests? In this case, would you still recommend another round of treatment?

    We acknowledge that this is likely a rare situation without much data, but we greatly appreciate any thoughts or insights you may have.

    Thank you very much in advance for your time!

    • Doc says:

      Hello, Lauren,
      We know that if there are fewer than 4 adult female worms, it is possible to get false negative antigen tests. These can come and go, positive and negative with a very low worm burden.

      You could go through the melarsomine injections again (I personally wouldn’t bother with the doxycycline again), or not. Some dogs can never be totally cleared, but you can get them to such a low worm burden that they don’t experience any ill effects from the presence of the worms.

      With such a low worm burden, it is unlikely that your dog will experience any noticeable side effects from repeating the treatment (other than soreness at the injection site and the damage to your pocketbook).

      You might consider switching to Advantage Multi, rather than Heartgard. The failure rate in my endemic area has been near zero with Advantage Multi, and somewhat more than that with Heartgard.

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