More Heartworm Questions

After my last post, one reader asked me if I would post the questions and answers.  I replied (pretty much without thinking) that they are posted as comments on the original post.  So how would you know where to look, gentle reader?   I'm going to start posting the ones that have more general interest. I'm taking more time to write some of them than I do to write a regular blog entry.  This one, for instance, popped up as a comment on "Problems with Heartworm Treatment".

The Question:
I have enjoyed reading your thoughtful responses to so many questions. Here is mine asked with much respect.

What are your thoughts on the "slow kill" method? A rescue I foster for
recommends Doxy for a month (although I found a study that suggests 2 weeks
is better) then Ivermectin every 2 weeks for up to a year until a negative result [blood test free of heartworms]
is met. The reasons are primarily to make the treatment less harsh and of
course financial [less dollars].

My second question is what Heartworm treatment is recommended for a lethal white [gene]
Aussie with heartworms. I believe they are in the Collie family and cannot have ivermectin.

My Answer:

The doxycycline is intended to kill a microorganism called Wohlbachia that infests the heartworm itself.  Many researchers believe that a great deal of the reaction we see to the breakup of the dead worms is related to the presence of this organism.  The doxycyline pre-treatment is not a universally adopted practice, but is inexpensive and does no harm (unless the delay allows more heart and artery disease to develop in dogs who have clinical signs of disease, versus just a "positive blood test").

Even when the Wohlbachia is gone, you still have chunks of dead worm to deal with, and they WILL cause inflammation in the pulmonary arteries. The inflammation causes swelling of the blood vessel, which restricts its inner opening.  This makes circulation in the lung tissue poor, and increases the pressure on the compromised artery, and increases the back-pressure the hear must pump against.

This brings us to your question about using long-term ivermectin to kill the worms, and whether this is somehow "less harsh".  Dead worms are dead worms, no matter what kills them.  When they die and drift downstream, it doesn't matter what drug killed them or how long it took. They will still lodge and plug things up and cause inflammation.

With the dramatic increase in the number of dogs who get small numbers of heartworms despite taking monthly ivermectin (Heartgard 30, etc.) I have serious doubts that taking ivermectin every two weeks is going to kill the heartworms.  Previous studies documenting this noted that it took as long as two years for the worms to die with this treatment (with no treatment at all, the worms would die of old age in about four years).  These studies were made before we began to see the apparent failure of the drug to prevent heartworms 100% (in the past 3 years).  Since they are getting new worms while taking the ivermectin, it's hard for me to have confidence in the drug getting rid of the old ones.

Even if we accept that giving ivermectin twice monthly would eventually kill the heartworms (and I am not confident of this), we have to ask if this is really a good thing.  When we give the standard treatment of the Immiticide injections, we know that the worms will die within the next few days.  Thus we are alert for signs of complications. We also are now giving the dogs prednisone after their treatment in order to minimize the inflammation experienced when the dead worms move. 

The bottom line is that we know when the worms will be dying and moving, so we can be on the alert and detect complications rapidly, and deal with them. We know that the dog's activity should be restricted while the dead worms are clogging up the works, and we can do this for the next five or six weeks.

If the worms were going to die at some unspecified time in the indefinite future, how in the dickens are you going to be on constant "red alert" for the next two years?  Are you going to crate the dog for two years?  Give him prednisone for two years?  I submit that you will not be doing any of these things.  Therefore, when the worms DO die, you are more likely to experience complications.

Since complications are related to how many dead worms are breaking up and hitting the smaller pulmonary arteries, it would make sense that the dog would do better if the worms died one at a time over a long period of time.  There is NO evidence to say that this is what happens with the ivermectin (if indeed, anything is happening with the ivermectin).  Even if you could prove that "in six months, the worms began to die one at a time, regular as clockwork, one-tenth of the worms per month for ten months" (and this is absurd), would you crate the dog for ten months, starting six months from now?

The sad fact of the business is that the Immiticide treatment is the only thing that we know is effective.  We can reduce the risk  by giving a half-treatment, and then waiting a month to give the full treatment.  This also makes it more likely that ALL of the worms will be killed.  (Unfortunately, there is no such thing as 100%, "always" or "never" when we're talking about this).

This (Immiticide treatment) would be the appropriate treatment for the Aussie.  While they cannot tolerate high doses of ivermectin, they tolerate the preventive dose in Heartgard 30.  You could certainly use Interceptor as your preventive instead, but it's not going to be effective in ridding the dog of adult heartworms.

It is certainly cheaper to give Ivermectin twice monthly for years than to do the Immiticide treatment.  On the other hand, giving nothing is even cheaper.  Doesn't work, but it's cheap.

Sorry that I don't have a quick fix for you.

Thanks for reading and writing,
Everett Mobley, D.V.M.

69 thoughts on “More Heartworm Questions

  1. Lynda says:

    Dr. Mobley, since you stated this:

    With the dramatic increase in the number of dogs who get small numbers of heartworms despite taking monthly ivermectin(Heartgard 30, etc.)

    Are you recommending something other than Heartgard for your patients? Have you seen an increase in the number of dogs infected while on Interceptor?

    I remember you writing on your blog previously about dogs on preventative becoming infected and I asked my vet at my dog’s yearly checkup and she said she hasn’t heard of that happening in my area. Is this becoming more widespread across the country?

  2. Doc says:

    Hello, Linda,

    This problem appears to be mainly in the Mississippi valley region, south of Jackson, Missouri. The farther south you go, the worse it is. It does not appear to be a problem in other mosquito infested areas, other coastal areas, lake areas, etc.

    The reason for this is unclear, though several explanations have been proposed, most notably more tropical storms in the Gulf, leading to more flooding, leading to more mosquitoes, leading to more heartworms.

    It is difficult to accept this explanation when you live in the Mississippi Delta on reclaimed swampland where rice is farmed. We have always had and continue to have a jillion mosquitoes.

    As to Heartgard versus Interceptor, the percentage of failure rate is about the same with all three drugs that we use: ivermectin (Heartgard-30), milbemycin oxime (Interceptor and Sentinel), and selamectin (Revolution). Other drugs and products on the market are related compounds: avermectins.

    If I found a preventive that was clearly better, you can bet I would be switching everyone to it. Unfortunately, this is not the case. No new drugs in the pipeline, either.

    I’m attending another seminar on this on April 21st, and will post afterward.

    Thanks for reading and writing.

  3. kathleen says:

    I’ll check back to see what you learn at your seminar. I give my three dogs Heartguard. I certainly want to protect them from heartworms and would like to know the best way to do that.

  4. Twila says:

    Have 2 labs both tested positive to heartworms.
    1-lab vet treated, new blood work still positive.
    2- lab pulling more blood work and sending to different lab. Interceptor given first of month always.
    Interceptor gives us year supplies due to dogs being treated and still contacting heartworms.
    Are other people having this problem with dogs.

  5. Jason says:

    Dear Dr. Mobley,

    I found your blog through google and was very impressed. I have a few brief questions and I’d greatly appreciate your thoughts.

    Recently my wife and I found a Pit Bull mixed dog in our neighborhood. We tried like crazy to find the owners, but had no luck. We are not willing to see the dog put to sleep, and thus she is now ours. I took her to our vet yesterday and he told us she had heartworms (Spot Test) and that treatment is extremely likely to be successful (assuming class 1). I knew little about heartworms at the time, and thus asked just simple questions. I’ll see him in two weeks, but after some research, I have many thoughts and few answers.

    We are not wealthy by any means. She is young and has no cough nor breathing troubles after activity. Your article on the “slow kill” treatment was pretty scathing, but in this case does it at least seem reasonable? I was thinking weekly ivermectin with regular doxcycline for 6 months. My hope is that the adult heartworms’ lives will shortened, the extended time span of treatment will only result in limited cardiac damage, and that embolism problems will be minimal.

    Also, some vet sites mention that ivermectin based preventative products are better for dogs w/ heartworms than milbemycin products, due to the milbemycin’s rapid killing of the larvae. Do you agree? Some also mention that ivermectin products are more effective on shortening adult heartworms lives. I’m confused and not sure what is true and what is just internet recklessness. Regardless of whether we treat with immiticide or try the “slow kill,” it seems the dog needs to be on some preventive heartworm pill to make sure new larvae heartworms are forming.

    I’m sure you’re busy, but hopefully you’ll have a chance to offer your assistance. Thank you

  6. Doc says:


    Heartworm disease does not generally affect people in any way, other than how they feel about their dogs.

    The heartworm larvae enter people all the time via mosquito bites, but they rarely live and leave the spot where they enter. There are rare reports of finding an adult heartworm in a person’s abdominal cavity, or some other weird location.

    Sometimes the migrating microscopic baby heartworm will reach the lungs, and the body forms a little knot around it. This can show up on a chest X-ray, leading to a lung biopsy to find out whether you have TB or cancer or what.

    Generally speaking heartworms do not affect people.

  7. Doc says:

    Hello, Dolores,

    You should give the pill as soon as you realize that you missed one. Do not stop giving the pills in a situation like this.

    If the dog has had mosquito exposure during this time, then it may possibly develop heartworms. You will not be able to detect this with a blood test until at least six months after the missed pill.

    So, get back on schedule, keep giving the pills, and get a blood test in six months.

    Thanks for reading and writing.

  8. Roberto says:

    Hi Doc!
    I see you told Dolores to if one if I misses a pill for heartworms..that they should give the pill as soon as you realize that you missed one…now I was wondering once you give the missed pill to your you give him the rest of the pills counting from when you actually gave him the missed pill or do you keep counting form when you were suppose to have given him that pill..hope this makes sense 🙂 ?!

  9. Doc says:

    Hello, Roberto,

    You could start counting 30 days or four weeks or one month from the time that you finally remember to give the pill.

    For most people (like me), if I am late giving it on the first of the month, I probably will do worse trying to do it on the seventh the next month or the fifteen the next month. I’m afraid I would just be worse about forgetting.

    I recommend setting a date that’s easy to remember, like the first, or when you pay the rent, or payday or something.

    The medicine is really safe. Thus, if you were two weeks late one month, giving it on the fifteenth instead of the first, it would be perfectly safe to give it on the first of the next month. Two weeks apart is no problem. Two DAYS apart would be safe, as far as that goes.

    Thanks for reading and writing.

  10. Mychol says:

    Hey Doc

    I was giving my dogs their monthly heartworm medicine. I had already fed my 8 month old puppy, Panda, and when I was taking the heartworm out for my 2 1/2 old dog, Bear, Panda ate it! I was wondering if anything bad would happen to her after eating 2 heartworms.
    Thanks for your time.

  11. Doc says:

    Hello, Mike,

    The medicine is super safe. The active drug ingredient is present in very small amounts. While I wouldn’t recommend it on a routine basis, she could probably eat a handful of monthly doses with no ill effects.

    Not to worry.

  12. Doc says:

    Hello, Karjat Plot,

    I’m not sure I understand your question. Baytril is the trade-name for enrofloxacin, a fluoroquinolone antibiotic.

    When it was first approved, the dosage was recommended to be split twice daily. This is still on the label. However, it now appears that this class of drugs is more effective when the total daily dose is given once daily.

    It is important not to exceed a daily dose of 5mg/kg/day in cats, as this can cause damage to the retina in the eye, resulting in blindness.

    I don’t use Baytril for “every day” infections, reserving it for tougher germs. Over-use of the drug will result in resistant bacteria, and reduced usefulness of the drug.

    I really cannot advise you on dosing your pets, as I have not examined them.

    Thanks for reading and writing.

  13. Anonymous says:

    Surgical removal of the adult heartworms is also a treatment that may be indicated, especially in advanced cases with substantial heart involvement

  14. Doc says:

    This treatment is usually reserved for dogs with post-caval syndrome. This is where the worms locate themselves in the large veins returning blood to the heart, usually the posterior vena cava. This results in a very rapid deterioration of the dog’s condition. Red blood cells are cut like putting them through a sieve. The liver circulation is badly disturbed. They rapidly become anemic and icteric (jaundiced). They are unresponsive (usually) to any palliative medical therapy.

    Inserting a long forceps from the jugular vein in the dog’s neck down to heart level allows attempts at removal of these worms. You are plucking them out blindly (you hope).

    There will still be worms remaining in the heart, and in the pulmonary arteries, but (if successful) you can buy the dog some time for conventional treatment.

    The procedure’s success rate varies considerably between surgeons. Some do well,others have abandoned the procedure. It is certainly not without risk, but these dogs have little alternative.

  15. Ashley says:

    What are the chances that a dog who is on a heartworm preventative and been heartworm free for 4 years (their whole life) can catch heartworms from a dog in the home who has been treated with the “fast kill” for heartworms?

  16. Doc says:

    Hello, Ashley,

    Having the heartworm-infected dog in the home does not really raise the risk of infection for the dog on preventive medicine.

    The overall level of mosquito exposure is a much greater factor. There are plenty of infected and untreated-hosts in the environment elsewhere.

    Sometimes our preventive medicines are not 100% effective, but even “preventive failure” cases have had MOST of the worm infection prevented.

    I really would not be a bit more worried about having the other dog in the house.

    Thanks for reading and writing.

  17. Eva says:

    I took a small rescue dog in to foster a month ago. When I had her spayed, she had severe complications post-op, but stabilized and pulled through with IVs. The vet ran a HW test and she came up positive (and in retrospect, I have noticed some occasional coughing/gagging previously, but nothing else). The surgery has been 2 weeks ago and the dog is doing well and is happy again. I will probably opt for the immiticide/doxycycline treatment, but am very worried about being able to keep the dog quiet, as I have many animals in and around the house running around and this little dog is a terrier mix with a high excitement and activity level. What is also a grave concern, is my working very long hours and thus being gone sometimes 12 hours or more (I live alone and in the country without anyone being able to check on the dog while I’m gone), so if complications arose I most likely would not be around to catch it on time! Taking off work unfortunately is not an option since I am on a very tight budget and can’t afford the loss in income (the dog’s treatment is even being financed through donations). Would it be feasable to keep the dog tranquilized to keep her quiet while going through treatment (split-dose)? Is there anything else I can do to greatly reduce the chance of embolisms, so as not to have to worry myself sick for the 2 month of treatment when I’m at work?

  18. Doc says:

    Hello, Eva,
    There is a product made by Vetriscience that is called Composure, available in liquid or chewy treats. It is a supplement, rather than a drug that really helps calm a lot of dogs. It doesn’t always work wonders, but in some dogs it DOES.

    Low doses of the antihistamine Benadryl are pretty harmless and make most dogs a little drowsy. In more extreme situations, tranquilizers can be used.
    Sometimes we have to just keep the dog in a crate, or in a puppy play-pen.
    If she is in the house, she is not nearly as likely to produce those complications of exercise as she would running outside.
    I would bet that you will do fine.
    Good luck.

  19. Christine says:

    Hi Doc,

    Thank you for such an informative site. It has provided me with alot of comfort.
    I rescued a pound dog 3 weeks ago, HW +. On 5/2 he began Immeticide x 2, 24 hours apart. He was originally asymptomatic, with right heart enlargement thus rated a borderline class 2.
    He is approx 7 yrs old, 30 lbs, lab/basset ?????
    Question: he has had approx 3 episodes of labored breathing lasting about 10 min each on different days. Took him to the Emergency vet first time and it had resolved by the time we arrived so it was chalked up as possible “pain”. He has tolerated the discomfort and I have not treated.(Rimadyl was dispensed) All occurred at rest (no anxiety/activity trigger) No cough, no fever, lungs were clear, gums remained pink. It is now day 7 post treatment. He has been activity restricted to the living room area only, leash to potty only. (He refuses the crate)He eats, drinks, potty’s etc. Unfortunately he has separation anxiety issues when I go to work which scares me to death. Me only household.
    We had an episode this morning- same thing, upon waking. Resolved spontaneously. I am awaiting a return call from my vet.
    I try not to be a “nervous nellie” ( I am an ICU RN )
    I know the worms are dying and shifting, but is this something that I should expect for awhile or do you think steroids should be added– are they added routinely? Do they act preventatively against embolism? Thanks very much for your time and for this site.
    Oh, and again I have contacted my vet..and waiting.


  20. Doc says:

    Hello, Christine,

    We now start prednisone (steroids) at time of treatment. I used to wait to start them until the dog showed signs of distress.

    You and your veterinarian will just have to work together on this. Not knowing all the details, it’s hard for me to advise you.

    Generally speaking, I think I would be starting this dog on steroids now. You can’t give Rimadyl and steroids together, though.

    Steroids minimize the inflammatory reaction in the pulmonary arteries, so that they don’t constrict down around the worm fragments, which would aggravate the restriction of blood flow and increase pressure. The rimadyl won’t really do that.

    Good luck.

  21. Cindy says:

    Hello. I rescued a HW+ dog (14 lbs pom mix) from the Dallas Animal Shelter. He was not up for adoption due to being HW+ and was schedule to be euthanized. I had heard about the shortage of HW treatment meds but also heard about the slow kill method and thought I would give it a try. He just took his last weekly dose of Heartguard. A day after, he started couching/gaging several times a day, then more frequent a few days later. Is this normal? It sounds like a asthma attach.

  22. Doc says:

    Hello, Cindy,

    Without chest X-rays it’s hard to tell if your dog’s coughing problems are related to the presence of heartworms or not. Pomeranians often have trouble with weak airways, even collapse of their trachea (windpipe). This can certainly cause respiratory distress.

    Until Immiticide (the heartworm treatment drug) becomes available again, we are treating these dogs by giving doxycycline for one month, as well as putting them on Heartgard. This is unlikely to kill any heartworms, but should keep them weaker, and smaller, and stop them from producing babies in the blood.

    With the history you have given, I would suggest contacting your veterinarian for an exam (and probably X-rays) to see if there is heart enlargement, pulmonary artery enlargement, lung problems, or airway problems.

    Thanks for reading and writing.

  23. Remi Goldstone says:

    I can’t remember if I gave my chihuhua his heartguard. Should I risk double dosing or wait until next month?

  24. Doc says:

    Hello, Remi,

    Give it now. The medicine only stays in the body for 24 to 36 hours. NO problem with overdosing here.

    Give it.

    Thanks for reading and writing.

  25. pat says:

    Can i start heartguard prior to 30 days after last Trifexis? Dog just tested positive for heartworm after testing neg 6 monthss ago

  26. Doc says:

    Absolutely. The heartworm preventive portion of the Trifexis is long gone (out of the body in 24 to 36 hours).

    The low dose of Ivermectin in Heartgard is compatible with the long-acting flea control in the Trifexis, same as Comfortis (Spinosad).

    Really high doses of Ivermectin, as are sometimes used to treat demodectic mange, are NOT compatible with Spinosad.

    No problem switching to the Heartgard.

  27. pam says:

    Rescue on Trifexis for 2 years just found out she has had them all along Just had first of 3 shots – they want her on Heartguard – please explain Heartguard vs Trifexis and what to treat fleas if she is not on Trifexis. Vet of 2 years never tested her for heartworms

  28. Doc says:

    Hello, Pam,

    Heartgard contains a low dose of ivermectin. The heartworm gurus now believe it is the product least likely to cause a problem in a dog who already has heartworms.

    Trifexis contains spinosad for the fleas, and milbemycin oxime for the heartworms. Some dogs with microfilaria present in the blood (the microscopic baby heartworms) can have a bad reaction with the milbemycin (also used in the product Sentinel).

    My experience has been that dogs already taking the product usually don’t have a problem continuing it. However, yours could be that one that does.

    As to fleas, you could use a topical like Vectra 3-D, or you could take Comfortis, which is spinosad alone, the same flea control that is in Trifexis (same manufacturer, also). Comfortis is compatible with Heartgard; it’s just two medicines instead of a combination pill.

  29. Debbie says:

    I have a 7 year old pit bull. My income is very low as I am on disability.My girl,Abbey was diagnosed with heart worms but I didn’t have the money for the treatment.

    I was on a rescue site and asked for prayer for her. A girl who had worked at a rescue shelter contacted me out of the goodness of her heart! She told me that her facility in Ms was very low funded so they consulted with a local vet who told them to use Ivermectin and they did. They had excellent results.

    This is pure Ivermectin to my understanding as she said I would buy it at my local feed store. She even called that vet(she is in another state now) and told him my dog’s weight, breed,etc and he told her the correct cc to give her.

    I was headed to buy it today and then ran across your page and saw the articles about taking the steroids, even for the slow kill. Can this be bought as well or does it need a prescription? I do not have money. I barely get by but I love my Abbey. She will not be a problem to keep “quiet” because she has always just laid around with me. We take walks and play “rope” and things but she is and always has been kind of sedentary. I am now afraid to start the Ivermectin after reading about needing a steroid as, I haven’t seen anything here about buying it in a liquid pure form as this girl informed me about. Your thoughts please?

    Abbey is highly allergic to fleas! Just going out to potty, they bite her legs and make big sores so I give her Benedryl for that when it gets bad. After reading your articles, I see that this is okay to give with Ivermectin. I guess it would benefit the treatment AND her flea allergy.

    She goes everywhere with me, sleeps on her blanket on my bed and is constantly by my side so I would be very alert to any symptoms.

    BTW, Abbey did have pneumonia, healed very well and fast, but other than that has shown NO signs of the HWs. Thanks~Debbie

  30. Debbie says:

    P.S. I give Abbey a tablespoon daily of diatomaceous earth food grade for overall health. It has made her coat very pretty and her eyes very bright. It did rid her of other parasites as well. I found round worms in her stool and one day a couple weeks ago, she threw up a LONG slender worm..VERY long. My son said it could have been a tape worm. Was the size of vermicelli. Could THAT have been a heart worm??

  31. Doc says:

    Hello, Debbie,

    You can’t throw up a heartworm. They live inside the blood vessels in the lungs. For them to leave the body, you would have to be coughing up blood.

    Tapeworms are flat, and segmented. It’s hard to say what the dog threw up. Horsehair worms are parasites of insects, and occasionally a dog will eat a cricket or something and vomit up the horsehair worm. They are quite long and thin, as you describe. Not significant to the dog’s health.

    “pure ivermectin” I suspect refers to the injectable product for deworming cows. There is no evidence that this will clear the dog of adult heartworms in a reliable manner. People commonly dose it at about 400 times the dose approved in Heartgard or Iverhart. It works as heartworm prevention (provided your dog isn’t sensitive to those doses, like a Collie – it would kill them).

    If I had this situation, I would be keeping the dog on Heartgard, or another approved ivermectin product. This should be safe, and would keep you from getting more heartworms than you already have.

    You cannot buy systemic corticosteroids like prednisone without a prescription. You don’t need them along with the Heartgard.

  32. Jennifer Miranda says:

    We took in an 11 lb Shih tzu that is heartworm positive (vet stated a light case based on blood test). She shows no signs. We are trying to decide which kill method to go with but leaning towards the slow kill with Heartguard. Vet perscribed Heartguard Plus 2x per month. In your opinion are we on the right track? Should we also give Minocyclin?

  33. doc says:

    Hello, Jennifer,
    The best person to advise you is the veterinarian actually seeing your dog and familiar with your case.

    Minocycline given for 3 weeks will cause the heartworms to weaken and physically shrink, and be easier to kill for the next 60 days or so. It also usually stops the production of microfilariae (microscopic baby heartworms).

    “Slow kill” is REALLY slow – up to 5 years before all the worms die. Risk of thromboembolism (worms blocking up an artery and damaging circulation in the lungs) is present until all the worms have died and been dissolved by the white blood cells.

    This is why the American Heartworm Society no longer recommends “slow kill”.

    Again, every case is individual, and your veterinarian has actually seen your dog to evaluate. My comments can be of a general nature only.

  34. Peg Farrell says:

    We have a chihuahua /terrier mix rescued last April, HW +. Vet recommended slow kil because of her size, I think. We gave her doxycycline for 36 weeks-a strict schedule of 6 weeks on, 2weeks off, 4 weeks on, etc. We finished that mid Dec. Concurrently we gave Heartgard twice/month. Still doing that. She has had several episodes of cough/gagging lasting only a few min. 2 times gagging continued for an hour or 2 with small yellows bile vomit.
    Today she had labored breathing, fast shallow breathing, scared with ears peeled back. She even started shaking a little. Vet took an x ray(I think the first ever) said her heart was enlarged and fluid was gathering around her lungs(?). She put her on lasix for 10 days.
    My question is what can we do now? Try the fast kill? She has to get healthier first. What’s her prognosis. No blood results yet

  35. Doc says:

    Hello, Peg,

    For right now I would say that the best thing is to let your veterinarian treat the clinical signs you are seeing. Be sure to give her frequent feedback on the dog’s condition, at least once daily.

    The deal with slow-kill versus fast kill is that you have no idea when the worms will die and shift position. Thus you have no warning when this is going to occur, and you don’t know whether this illness is related to the death and movement of a heartworm, or something else entirely.

    Slow kill or fast kill, when the worm dies, it moves into a smaller blood vessel and the body reacts to the dead worm.

    For right now you just have to play the cards you’ve been dealt, and deal with the the symptoms she has until she gets stable.

  36. Bobbie Elkins says:

    I have a 6 yr old rescued boxer that I’ve had for about 6 months.She is heartworm + We haven’t been able to do treatment as it has taken this long to get her healthy.This month we are dealing with demodex mange.Once we get this under control how long should I wait to begin treatment? And will this throw her back into demodex mange again?

  37. Doc says:

    Hello, Bobbie,

    Most folks are treating Demodex with high doses of ivermectin. This will also act as a preventive against getting more heartworms than you already have.

    The current recommendation from the American Heartworm Society is to begin the treatment with four weeks of doxycycline or minocycline. This drug weakens and shrinks the heartworms by inhibiting the Wohlbachia micro-organism that is beneficial to the heartworm. Then you take four weeks off before you actually begin the Immiticide injections to kill the worms.

    With a dog who has Demodex, they often remain carriers, even when they look clinically normal, and you cannot find mites on a scraping. Under great stress, they may break out with some spots, but do not usually have a generalized case all over their body.

    It is possible that you could have some recurrence of the Demodex if your dog has difficulty with the heartworm treatment. I would not let that stop me from getting the treatment done, though.

    Work with your veterinarian and follow his/her advice.

  38. Rae flontek says:

    P.S. My adorable chihuahua had her severe wounds repaired, and has been on monthly heartgard since I adopted her.

  39. Elizabeth Seal says:

    This has been very helpful. My shelter dog a dog lab/pit X was diagnosed HW+ yesterday. I have 4 other dogs 2 are 15 and 2 are 5 years of age and creating a plan to manage activity is worrying me. We started with twice daily Doxycycline and prednisone and, the plan is split Immiticide. We have stairs and 10 steps to the outside. We are taking 1 step at a time. The dog is young lab/pit mix abt 50 lbs and high energy. How can I monitor his heart rate to know what increases it? he flies to the bed. he lets me carry him to floor. I’m thinking I will put mattress on the floor. He is in his crate as I type. thinking of 120 days of crating is overwhelming me. I also need to travel to see my 92 year old mother in the next 3-4 months. When is the best time to go? Before the first shot?

  40. Doc says:

    Hello, Elizabeth,

    I would go before you do the Immiticide injections.

    You don’t need to crate the dog for this length of time. You don’t let him run loose, you don’t encourage him to do aerobic activity. If he goes nuts outside, then leash walk. Unless he is turning cartwheels on the leash. You just do what keeps him the calmest.

    The less he runs and charges about, the better. He isn’t going to sit still for months. That’s impossible, and undesirable.

    You just make the best compromise you can.

  41. Stacy Armstrong says:

    Our Mastweiller rescue is HW+ she is 93lbs and 2-3? Years old. We have started her on Doxy (200mg BID) for 1 month. I am leaning toward the slow kill method for a variety of reasons. My vet is not happy or familiar with treating any client this way. I am wondering if there are guidelines or recommendations on the invermectin dosing? Here’s my thoughts to game plan:
    1 month of doxy
    Continue monthly Heartguard plus or do we need to do every 2 weeks? (Is it safe with the plus since it has an additional ingredient).
    Should I be giving an anti-inflamatory with it and at what stage?
    Thank you for all your information.

  42. Doc says:

    Hello, Stacy,
    Most veterinarians are not comfortable with the “slow kill” as it can be a “no kill”. There is also no way to predict when the worms will die and shift position (which is what causes the complications). This is when we want to restrict the dog’s exercise and activity, and when we would give anti-inflammatory doses of prednisone.

    Would you give it now, next month, six months from now, two years from now? When? No way to know. Also no way to know when you should be restricting the dog’s exercise to minimize excess stress on clogged pulmonary arteries.

    You can’t confine the dog with no exercise for two years.

    Heartgard Plus has a very mild de-worming drug called Pyrantel in it. It’s extremely safe, so that’s no problem.

    Giving it twice monthly may or may not change the effectiveness. No data in controlled studies is available.

    The standard dose of doxycycline (or minocycline) is 10mg/kg twice daily. That would be about 400 mg twice daily for a 93 pound dog.

    That kind of dose can also cause some digestive upsets . The dose you are giving isn’t likely to do anything good or bad.

    Your best source of advice is a veterinarian who is actually seeing your dog. If you don’t have confidence in your doctor, then look for one that you do have confidence in.

  43. JAM says:

    I’ve read the advice given here and appreciate it. I’m having a hard time with my vet, as she gets annoyed when I ask for practical solutions to keeping my 2-year-old active, excited lab mix calm. She is 40 pounds.

    So here’s the situation. We adopted our dog in September 2016 shortly after she had been rescued from Mississippi (we live in northern Illinois). She was heartworm free when she was tested in September, but, unfortunately, we only gave her a couple of heartworm preventatives that we had left from our previous dog. Since have had dogs for 20+ years with no heartworm cases, we didn’t think much of it. We also didn’t know about the danger. She tested positive this past September and we started her on Intercept, doxy and prednisone. There were no microfilaria present upon the test, so the vet said it’s likely she either has sterile worms or single-sex adults. Our dog has shown no symptoms at all. Currently we are scheduled for the first adulticide on November 8.

    I am terrified about the need to keep her calm. The vet keeps telling me I just need to crate her. She has never been crated, although we keep her in the kitchen when we’re not home. That’s where she sleeps at night, too. Also, the treatment schedule comes over the holidays when our grandchildren — 1, 2-1/2 and 4-1/2 will be with us from out of town. Without being cruel to the dog, I cannot see how we can keep them apart. I asked the vet if we could wait until after the holidays, and she was very negative about that. I’m looking at getting a second opinion but found this forum and thought this would be a good next step.

    In a nutshell, is it OK to wait until January to do the adulticide (split-dose), when it’s cold, snowy and uneventful and thus an overall calmer environment? Should I get an x-ray or echo of the heart to make sure this is an OK decision? We are not wealthy but can afford the treatment and are willing to spend the money to do what’s right for our dog.

    Thanks for any advice. I appreciate it!

  44. Doc says:

    Hello, JAM,
    I’m thinking from reading your note that you acquired the dog in September 2015, one year ago. That would mean a negative test last year and a positive test this year in September.

    Since it takes six months from the mosquito exposure for the baby heartworms to mature to a point that they can be detected, the positive test in September 2016 is the result of exposure in 2015 (that had not progressed far enough to be detected last year).

    If you did not give heartworm preventive in the summer of 2016, then any new baby heartworms received from mosquito exposure during that period will be showing up six months later.

    I really don’t know what kind of mosquito exposure and heartworm re-infection you would have in your geographic area. Here it’s very heavy, as bad as in Mississippi. If I were presented with the dog in my practice area, I would probably wait to start the treatment so that my follow-up two-dose Immiticide treatment (one month after the single dose) would be catching the previous summer’s exposure as well.

    In your case that may not be a problem to deal with.

    I haven’t seen your dog or your situation, so I can only give general advice. You do need to keep an open discussion with your veterinarian, or seek a second opinion with someone who can actually physically evaluate your dog, which I obviously cannot.

    That being said, when we talk about restricted exercise, I never tell people to crate their dog. We want the dog to take it easy, so no aerobic exercise. No running loose, no frisbee games, no playing ball to exhaustion. Walking on a leash, playing inside the house, controlled activity in the yard are all okay. Vigorous hard play is to be avoided. Many dogs go nuts if tied up or crated and that is counter-productive. We are looking for whatever gets the dog as close as possible to being a couch potato for the weeks following the Immiticide injections.

    As far as waiting until after Thanksgiving, I personally wouldn’t have a problem with that. The number of worms present will not increase in that period of time. If there were a lot of worms and heart damage already present, then the sooner the better. If there are not a lot of worms, then it wouldn’t make any appreciable difference.

    A chest X-ray won’t let you see the worms. However, it will show whether or not there are changes in the pulmonary arteries, which is the first thing that gets damaged. If the chest X-ray looked normal in a 2-years old dog, I wouldn’t have a problem with waiting a month or two to continue the treatment. You may have to repeat the doxycycline, as its effect only lasts a little over two months.

  45. Charles Walden says:

    I am giving Trifexus to my dogs. It is expensive and I wonder if there is a cheaper alternative or can it be given bi-monthly?

  46. Doc says:

    Hello, Charles,

    Trifexis is one of the more pricey heartworm preventive medicines, but it also gives the best flea control. It has the same flea control medicine as Comfortis, and costs very little more. If your were using Comfortis as flea control, using Trifexis would be like getting your heartworm preventive almost free.

    If you don’t need such good flea control, Sentinel has the same heartworm preventive and a lesser flea control medicine. It stops the fleas from reproducing, rather than killing them. This is good, as they lay up to 50 eggs per day, which infest your home. These don’t hatch, which is very good. You would still have to do something else to kill fleas on the pet if you have a problem.

    Heartgard has no flea control, nor does Interceptor. Both of these are very economical.

    DO NOT resort to giving the medicine bimonthly. This will NOT be effective in preventing heartworms. It will also deprive you of the guarantee program that would pay for heartworm treatment if the medicine were not 100% effective in preventing heartworm infection.

    If you decide to change medicines, you will need a blood test at the time of the switch, and a second one six months later to keep your guarantee in force.

  47. Shannon Bohl says:

    I’m not sure you are still answering questions but I have a few. I live in South Dakota where we do not see much of heartworm. Some rescues are bringing up dogs from Texas. I am wondering if the hw dog tests negative because it was too soon for the test to detect the adult heartworms, and the dog is put on preventative immediately, how long would the hw dog be a risk to another dog w/o preventative by having a mosquito bite it and then transfer the larva to the unprotected dog? Because if only the beginning larva stages can be transmitted and the preventative kills the larva/microfillariae, how would the mosquito get the larva and incubate/transmit it? And if that is correct, how fast does the preventative kill all the larva/microfiliariae? Thank you in advance for your time and answers!

  48. Doc says:

    Hello, Shannon,

    If the dog tests negative, then there are no microfilariae (microscopic baby heartworms = larvae) for the mosquito to transmit.

    It takes six months from mosquito bite infection with the larvae for them to develop into adult heartworms that can be detected with either the antigen test for adult heartworm protein, or the test for microfilariae in the blood.

    The larvae in the dog’s bloodstream are the newborns, produced in his body by the adult heartworms present in the chest. They never grow up. In order for them to be infectious to other dogs and be able to mature, they MUST cycle through the mosquito. The mosquito is an intermediate host necessary to the life cycle. It is not just a carrier.

    When the mosquito transmits the larvae to a new dog, they stay at the injection site for a little over a month. This is when they are susceptible to being killed by the preventive medicines. By six weeks, they have changed to the next life stage, entered the bloodstream and traveled to the chest where they will mature into foot-long worms.

    The preventive kills them only during that first month at the injection site.

    In an infected dog, it is possible for the preventives to sterilize the adult worms without killing them, and even to kill the babies out of the infected dog’s bloodstream. This gives you a false negative test for the larvae. A dog in this situation would not be a source of infection for other dogs. This does not happen in every dog, however. Some heartworm positive dogs on preventive medicine will continue to test positive and be a source of infection to other dogs.

    When you give a dose of preventive it should kill the susceptible microfilariae in the subcutaneous tissues (where the mosquito infected the dog) within 24 hours. Many of the drugs will be out of the dog’s system within 48 hours.

    You cannot count on the preventive medicines clearing an infected dog of either the babies in the blood or the adult worms. There are a lot of publications on so-called “slow kill”, but the results of giving long-term preventive medicine, with or without doxycycline, are not consistent or dependable. It is no longer advocated by the American Heartworm Society.

  49. John Burgener says:

    I have become involved in a case with a lab a neighbor was fostering and which I was interested in adopting. She tested strongly positive on a snap test and had a microscopic confirmation. The rescue group (SoCo in GA) has the dog at present and are planning to take her to GA for a slow kill treatment.
    Does the increased mosquito population of GA make this counterproductive?

  50. Doc says:

    Hello, John,

    My personal feeling is that the slow kill treatment is often a “no kill” treatment. It is no longer recommended by the American Heartworm Society. However,if the dog stays on monthly heartworm preventive, I don’t think Georgia is any worse than Iowa.

  51. Tori says:

    Hi Doc,

    I’m not sure if you are still responding to this thread but I figured I will try anyway.

    My dog Juno was adopted from Arkansas to Seattle where we live. She was dx’d with HW back in Dec 2018. We’ve gone through all the treatment and retested last month (30 days after the last injection) for microfilaria, she was still positive. We were recommended to give her another dose of heartgard and retest one month after. Well, we retested yesterday and she is still positive for microfilaria.

    The vet does not know what we should do because she doesn’t deal with HW often because Seattle is not heavily infested.

    My questions are:
    -Should we retest for adult HWs now since there’s a chance that there might still be some?
    -Is it normal to retest positive for microfilaria after the full treatment?
    -Should we do a CXR to determine the disease progress?

    Thank you so much in advance! -Tori

  52. Doc says:

    Hello, Tori,

    There are some dogs who cannot be 100% cleared of heartworms by the treatment. These dogs DO usually get rid of most of the worms, though. The small number of worms remaining generally don’t cause a problem for the dog.

    You would think that one worm in your blood vessels would kill you, but with small numbers the blood just flows around them.

    We usually don’t do our final test for adult heartworm protein (the antigen test) until 4 months after the third injection of melarsomine (Immticide or Diroban). Even when all the worms are dead, adult heartworm protein can continue to circulate for quite a while.

    Microfilariae still present can mean that you still have at least one viable adult male and female worm. On the other hand, if all you have done to eliminate the microfilariae is give Heartgard, a larger dose of ivermectin by injection may eliminate babies that the low dose in Heartgard did not.

    Your veterinarian could check on Veterinary Information Network for a dose, or use the dose in Plumb’s that is given for Demodectic mange.

    Chest X-ray will let you see whether you have clear lungs and a normal sized heart. If you want to get an idea how the dog will fare long-term, this would be informative. If the dog shows a lot of scarring in the lungs and/or an enlarged heart, you know that there was a lot of damage before you started the treatment.

    How old is the dog? It usually takes 3 to 5 years (mosquito seasons) in our area to accumulate enough worms to cause heart damage.

  53. Tori says:

    Hi Doc,

    Thanks for the speedy response! I just got news from my vet who consulted a cardiologist regarding Juno. The vet recommended rechecking for antigen 9 months after the final adulticide injection, not 4 months. This doesn’t really make sense to me when she’s still positive for microfilaria and there might be a chance that they would grow up to be adult ones. The vet recommended that we give her advantage multi as scheduled and rechecking 1 month later for microfilaria.
    I guess my questions would be:
    -Do you recommend waiting or go ahead and have one ivermectin injection now to kill em all?
    -Is there a chance for the microfilaria to grow up if they are resistant to the preventatives?

    by the way, she’s about 3.5 years old.

    Thank you so much in advance!!


  54. Doc says:

    Hello, Tori,

    The microfilariae in her blood will not grow up. They have to cycle through a mosquito, where they undergo a change of life-stage, before they can mature.

    The Advantage Multi may kill them. If not, you can still do the ivermectin injection.

  55. Ann Bond says:

    Hi Doc,
    I adopted a 10 year old Jack Russell mix from the local shelter. Did Doxy Predisone and Heartguard pre treatment. Vet put him on Vetmedin for slightly enlarged heart. My pup got his first adult killing injection yesterday and when I picked him up, vet was very busy and when I asked about safety and continuing to give vetmedin. I didn’t get a straight answer I was comfortable with, so I would love to hear from you if Vetmedin paired with first injection (2nd and 3rd given in 30 days) is safe for the dog? I have enjoyed reading your previous answers, but didn’t see anything about Vetmedin. Thank you in advance.

  56. Doc says:

    Hello, Ann,

    An enlarged heart is generally a heart that has stretched because the muscle has gotten weak and flabby. With heartworms, this is due to the constant overwork of forcing the blood through vessels that are partially clogged with worms.

    Vetmedin causes the heart muscle to contract a little more forcefully. It also opens the blood vessels a little bit to reduce the resistance to flow (easing the heart’s workload).

    The experts talk about what a wide margin of safety it has in treating congestive heart failure (weak flabby hearts).

    I am no pharmacologist, but I think it should be very safe to continue the Vetmedin as prescribed. The melarsomine used to kill the heartworms doesn’t have any direct effect on the heart or blood vessels, so there is unlikely to be any interaction.

    If your dog has any signs of difficulty, you should contact your veterinarian. Don’t rely on “Dr. Google”.

    Look for:
    cough, difficult breathing, loss of appetite, fever, coughing up blood (even just a drop), or just generally feeling bad.

  57. Sue says:

    Hi Doc,

    My Australian shepherd is 1.5 years old and he has been on Interceptor for heartworm prevention since we brought him home (he was 12 weeks old). Unfortunately, he tested positive for heartworm during a test last week.
    He tested negative for heartworm in March of this year. It was a shock because we never skip a dose of heartworm prevention. his vet has him on Doxycycline currently. He saw some inflammation around the lungs in x-ray, but he says that he saw nothing under the microscope (not sure what he was talking about) and that he believes that it is a very early case. He also mentioned that he may just have one female worm in there? How would he know this? We are still in the Doxycycline stage of treatment, but I am extremely anxious over having to somehow keep a 1.5 year old intact male Australian shepherd calm. He humps his bed and whatever else he can get. He is a “jumper” too. I know I will have a heck of a time. My vet said I’d have to restrict his activity, but not sure what that means exactly.
    Do you think that it would it be ok to keep him restricted to just 1 room in the house and then take him out on a leash when he has to relieve himself? Also, I was wondering how often do these bad events occur with the dead worms moving around and causing problems? I was told that his prognosis is excellent and that he has a very early case of heartworms, but I am paranoid now that if he jumps down from the couch he’ll dislodge a worm and stroke out. This isn’t helping my anxiety. I would appreciate any advice or just moral support.
    I have enjoyed reading your posts.

    • Doc says:

      Hello, Sue, I am sorry to be so late in replying, but our website was re-vamped and the blog comments went into limbo. If the pup has been on heartworm preventive since acquired, and previously had a negative test, then he could not have very many worms present.

      There are two types of tests done. You check for adult heartworm protein in the blood using an antibody test (looks like a home pregnancy test). This protein is from the adult female reproductive tract. We know that if there are fewer than four adult worms, there can be a false negative test in this. Under the microscope, one can usually see the microscopic baby heartworms (microfilariae). Some dogs have so few worms that the protein test is negative, even though there are babies seen under the microscope. There are also dogs who test positive for protein, and we don’t see any babies in the blood. That’s why both tests are recommended.

      As far as restricted activity goes, it is unlikely that the dog is going to exercise hard enough to cause a problem if he is inside the home. Outdoors, you can walk on a leash as far as you want to go. It is not recommended to let the dog run loose (even in the yard). You want to avoid aerobic exercise that would give a sustained increase of blood pressure. It is possible that a short burst of activity could cause a problem, but it is unlikely. Getting on and off the couch would not be a risky behavior. I would not be crating the dog for all those weeks post-treatment. The restricted activity is recommended for the weeks following the melarsomine (Immiticide or Diroban) injections.

      With large numbers of worms and dogs that have clinical signs of heart disease, there is a high incidence of complications. In your situation, the likelihood of complications is very low.

  58. Alam William says:

    If you only want to protect your pooch from heartworms, go for Nuheart. But if your dog often gets infected with roundworms or hookworms, opt for Heartgard Plus.

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