Are Heartworms getting worse? A Seminar: Part 5 of 6

Immiticide (2) Immiticide Treatment to Kill Adult Worms

In re failure to clear dogs of worms with the Immiticide treatment, there are several considerations that are important, particularly in dogs that have not been receiving preventive medicines.  I have written previously about problems with the treatment.

Immiticide does NOT kill immature adult worms, or “juvenile worms”.  These are small worms, less than one inch long, in the “tissue phase” or the pulmonary circulation (arteries in the lungs). These are worms 60 to 120 days post infection.  They are long past being susceptible to preventive medicines and the Immiticide doesn’t kill them either.  You would need to treat the dog at least four months after the mosquito season (peak July and August) to be sure these juveniles would be old enough to be killed by the medicine.  That would mean treating in January or February in dogs that have not been taking preventive medicine. [This is not as relevant to the dog that has been taking year-round preventive, but has developed a few heartworms in spite of that.]

In dogs who have not been taking year-round preventive, treating at the wrong time guarantees that juvenile heartworms will survive.  The dog WILL have adult heartworms within a few months, despite the treatment.

Effectiveness of different treatment regimens

When we talk about percentages of worms killed, we are talking about the number of worms that were experimentally implanted in test subjects.  We are not talking about the worms in a single dog.  In these tests, some dogs were totally cleared of worms and some were not.  In other words, if ten dogs were infected with ten worms each (100 worms), and ten worms survived, then we say 90% of worms were killed.  This is true whether the ten surviving worms were in one dog, two dogs, or divided equally between the ten dogs (and they weren’t).

Two stage Heartworm treatment:

A single dose of Immiticide kills male worms.  All females survive.  Statistically, infections are 50/50 male/female.  This is why you consider that you are killing “half the worms” with a single injection.

A treatment of two injections (24 hours apart) kills 90% of worms.  This has been the “standard” treatment.  This will totally clear some dogs (if they don’t have tough worms) and will leave a few worms present in others.  

Giving the two doses of Immiticide more than 24 hours apart decreases
its effectiveness.  Giving the doses less than 24 hours apart increases
the effectiveness, but also the toxicity of the drug.

Three injections (a single injection, followed four weeks later by the two injections 24 hours apart) kill 98% of worms.  In the past, this has been used primarily for dogs where larger worm burdens were suspected.  [The dog only had to deal with the breakup of half the dead worms at a time – less stress on his system.]  This will totally clear most dogs, but some dogs will still have worms present.

Two injections followed two months later by two more injections kills an even higher percentage of worms, but you cannot ever guarantee 100% kill.  This regimen is not in widespread use (but has been studied in the laboratory setting).

In the past, I have been treating dogs who are apparent "preventive failure cases" with two injections, 24 hours apart, based on the near certainty that they have very low worm burdens.  I am currently re-evaluating my position here, and considering going to treating all patients with the two-stage, 3-injection regimen.  While it should give me more "complete kill", it also extends the patient's exercise restriction by four weeks, and adds to the expense.  This is not a clear-cut decision for these dogs.

10 thoughts on “Are Heartworms getting worse? A Seminar: Part 5 of 6

  1. Jessica says:

    My dog has been on preventive interceptor since I adopted him at age 2. When I took him to the vet when I adopted him he was negative. He now tested positive 2 years later even though he has been on interceptor since negative test. My vet is recommending a slow kill method using only heartguard and not using the arsenic shots. How do you feel about the slow method and not using the shots?

  2. Doc says:

    Hello again,Jessica,

    With the slow kill method you just don’t have any idea when the worms will die, other than it will be sometime one or two years from now. Maybe they’ll die a week apart, maybe a month apart, maybe they will all hold hands and drink the Kool-Aid together. {}

    It is best to keep the dog’s activity level as low as possible while the dead worms are being gradually eaten away by the microscopic white blood cells. This is a process that takes several weeks. Until the dead worms are gone, they are clogging the blood vessels. This causes back-pressure behind them, and a high-pressure jet downstream. Increasing the heart’s output (like when you run) further increases both pressures. This makes it more likely that the blood vessel would break and bleed into the lungs.

    You cannot keep a dog still for six weeks, much less two years. We just do the best we can – no Frisbee, no going to the country to run, just staying as laid-back as possible.

    The reason that restricted activity is not recommended for the slow-kill is that it would be impossible, or highly impractical. Same reason you don’t “need” seat-belts on school buses.

    Certainly, when fewer worms hit the downstream vessels at any given time you are less likely to experience ill effects from the event. Thirty are worse than ten are worse than one. SO, if only one at a time dies, then even if the blood vessel ruptures, maybe it will be a minor hemorrhage.

    We want to deal with smaller numbers of smaller worms. That’s why we pre-treat with doxycycline to shrink and weaken the worms. That’s why we treat in two stages (single Immiticide injection, wait one month, then two Immiticide injections, 24 hours apart): kill half now, and half later.

    I do not believe that I can say that there is a hard and fast “right vs. wrong” answer. I treat lots of dogs for heartworms, but I don’t treat YOUR dog, in YOUR part of the country.

    Your veterinarian is taking responsibility for doing what he/she thinks is best for your dog. I am in no position to criticize that. I can offer general information, and information about what I usually do here. Each case is an individual, and is evaluated individually. There is no standard formula that I “ALWAYS” use.

    I understand that you are confused in this area. Any time that you see fifteen different ways to do something, it seems that there must not be one “best way”. When even the real experts (of which I am not one) have differences of opinion, it’s a little hard to know what’s best.

    If your dog is young and strong and otherwise healthy, you probably have plenty of time for the slow-kill method.

    Again, you and your veterinarian who actually sees your dog need to make the decision together.

  3. daphne says:

    What is the recommended treatment if the preventative dose of ivermectin gives a dog seizures? He can take Advantage Multi, but I think I understand this does not kill adult worms.

  4. Doc says:

    Hello, Daphne,

    It is had for me to believe that the standard preventive dose of Ivermecin in medicines like Heartgard and Iverhart is causing seizures. The dose is just so tiny.

    Advantage multi contains moxidectin, which is a related drug.

    I really do not think that relying on ivermectin to kill adult worms is a very reliable plan.

    If your dog tolerates Advantage Multi, but not Heartgard, then I’d go with the Advantage Multi. I feel that it is a very reliable heartworm preventive medication.

  5. Aixa says:

    Hello, I need help. My beloved Jack Russell named Vigo, tested positive for heartworm a year and a half ago. We did the Immiticide treatment with the first 2 doses (day 1 & day 2), then a 3rd shot 4 weeks later. I had him tested 6 months later and he still had a positive result!! So we did the Immiticide treatment again, but with 2 back to back dosages only. Here we are August 2013 and he tested positive again, 7 months after the 2nd treatment!! Why?? Shouldn’t the 2 treatments of Immiticide have killed all the worms? What do you suggest I do now? He’s been on Revolution, should I switch to Heartgaurd for a year and test him again next year? He has no major symptoms and besides the worms is really healthy. Only thing I’ve noticed, is that he gets tired quickly after playing or running hard. I don’t know what to do. I’m in California and all the vets I’ve seen have never treated heartworm as it’s so rare out here. I want to get the opinion of someone who has successfully treated heartworm and has experienced a similar case to mine.

  6. Doc says:

    Hello, Aixa,

    Ordinarily, the treatment is a single dose, followed by two doses (24 hours apart) in 4 weeks.

    There are some dogs that never get totally cleared, but they should have a small enough worm burden that they feel okay.

    For dogs that are repeat offenders, we recommend switching to Advantage Multi, as we have seen virtually no apparent “lack of efficacy” cases with that product. We recommend it for high risk patients, though we still use Revolution, Sentinel, and Trifexis, and Heartgard, as well.

    If he has endurance problems, then I would want to get a chest X-ray to evaluate heart and lungs, and a general checkup with a blood screen to look for other problems if the chest looks okay.

    There can certainly be other reasons for poor endurance.

  7. Brian Dubs says:

    Hello, I have really enjoyed reading your posts and responses and have found them incredibly helpful.
    Dodger is a 60lb german shepherd mutt and I would do anything for him – he is truly my best friend. I adopted Dodger in New York in February 2015. He was a stray from Alabama, rescued at 3months and adopted by me at 7months. In April 2015, the vets found worms in his stool and started him on Sentinal because it fought one specific worm (that they said could last up to 3 years, maybe whipworm?). At this time, he tested negative for heartworm via an antigen test (I believe this was a mistake on the vets part because of his young age).
    In August 2015, we moved to Montana. he has been back to NY for 3 weeks total since our move, 2 of those weeks were in December (though uncharacteristically warm), and one week was just recently in June. During that week, we visited his old vet for his annual checkup, and he tested heartworm positive. To confirm, the vet performed a snap test, which came up negative. He was retested at the original, as well as a second laboratory, antigen and microfilariae, and came back positive for the antigen and negative for microfilariae.
    Given this news we immediately started 30days of doxy and switched to heartgard. His 30 days of doxy will be ending in the middle of this month as per my NY vet recommendation.
    Since I have returned to Montana, where heartworm is extremely rare, I have been searching for a vet that feels comfortable with the treatment and that is also knowledgeable. In this time, I have also done a lot of my own research and have ruled out the slow kill method due to ineffectiveness (re: antigen-antibody complex), but am having a very hard time on determining whether to perform the 2shot/24hr, or 3shot/2stage protocol.
    Dodger is totally asymptomatic. He is now 2 years old and loves people (gets very excited when we get home) and other dogs. He can sit around all day (although generally whines and runs around when he sees other dogs out the window), but could also hike 20 miles. In fact, a week before his positive heartworm test, we hiked 20 miles together in one day and he was as happy and mobile as ever. He is well trained and crate trained (although we haven’t used it, or even a leash, very much lately), and doesn’t “run off”, but would run for hours given the opportunity.
    He had x-rays that showed a normal heart and no lesions in the lungs. Because he is a “preventative failure case” and the drug company is actually covering the majority of his treatment, we were also able to perform an echocardiogram. The results of that were that his heart looked and functioned excellent, no visible worms.

    So, it is clear that Dodger has a low worm burden. Because of his, and my, active lifestyles, the idea of 8+ weeks of inactivity is daunting. Also, in the middle of that treatment, I would have family visiting for a week and am concerned about him getting excited while they are here. I understand the AHS suggests the 3shot protocol because it is more effective, but mostly so that there isn’t a mass worm death that can cause serious damage. In Dodger’s case, I don’t think a mass worm death is a concern, although I do also understand that even one worm can cause serious damage.
    I also had read from you that one injection kills male worms, while two injections kill female worms. Although it is totally possible that Dodger has both male and female, could it also be possible that he only has female worms (only females release the antigen, and there are no microfilariae in the blood, although that could just be due to his monthly preventative) and that only the two injections would clear them?
    Given his high activity, and his low worm burden, I am more concerned about keeping him calm for 2-3 months than I am concerned about a mass worm death harming him while only keeping him calm for 4-6 weeks. I also am starting to feel that a 10% chance of it not clearing all the worms is worth the risk – if it does clear all the worms, great! If it does not, we proceed with another round of treatment, but at least he isn’t restricted for 2-3 months back to back.

    I know you always suggest consulting the vet that is actually caring for him, but at this time, we are in between vets while we try and determine the best course of action. Many websites and vets seem to blindly suggest the 3shot/2stage protocol because “that’s what the AHS suggests”, but I believe that each dog needs to be handled as an individual case. I see that you take that approach, and so I would greatly appreciate your input in how to proceed with treatment for Dodger. Thanks!

  8. Doc says:

    Hello, Brian,

    The single injection will kill weaker worms, usually males (weaker than females), very young, very old.

    Two injections should kill any of the worms, male or female. That used to be the standard regimen.

    Studies have been done that show two injections may miss the occasional worm, but so may three injections. Some dogs will be 100% cleared with two injections, and some dogs will never be 100% cleared. The odds are greater of 100% clearing with 3 injections than with two.

    In your situation, we feel the worm burden is low due to his age (seasons in the mosquitoes), and his geographic history (areas with low mosquito populations). The other diagnostic tests do not suggest any problems, either.

    I wouldn’t have any problems with two injections at this point. Slightly less likely to clear all the worms, though.

    In this situation, you’d only be dealing with about six weeks of restricted activity.

    Restricted activity really means: no running loose, no vigorous play. It’s okay to trot leisurely around the yard, walk on a leash, play around the house.

    I base my comments on the information provided. As I have stated repeatedly, I really cannot prescribe for a patient that I have not seen and cannot follow up on.

  9. Kristy says:

    Hi, My husband and I just recently found out that our precious pup “Molly” has tested positive for heartworms. She’s a Jack Russell Terrior, 6 years old, and weighs 12lbs. I know that this is our fault- we have never had her on a heartworm preventative. She’s mostly an inside dog so we were never really concerned with it. She’s a very active dog- loves to play outside, chase squirrels, etc. Last week we took her to a local veterinarian because she wasn’t showing any interest in playing which is very unlike her and also she didn’t act like she wanted to eat. We did get a positive heartworm test. The vet placed her on doxycycline, prednisone, and cardizem. We also got blood work (which was fine) and a chest x-ray (her heart is a little enlarged). We have talked a little about an action plan, but I’m not sure if it’s the right thing to do or not. We took Molly back a week after being dx and she’s feeling better (eating, urinating fine, perked up), but now she’s developed ascites. He has added lasix to her medication regimen and would like to see her back in a week to re-check blood levels. I know that he’s a professional and he knows what he’s doing, but I guess I just would like some more guidance? I feel like we’re just wasting time when we could be getting rid of these worms! I do realize we need to get her stable first, but he recommends the “slow kill” because she’s such a little dog and because well it would be impossible to keep her restricted from activity for the fast kill. I just don’t want her to get worse before she gets better if that makes sense. Her poor little heart is racing (which could be from the pred??) and I don’t want this getting worse than it already is. Any recommendations or advice?


  10. Doc says:

    Hello, Kristy,
    I don’t believe that the prednisone is likely to be directly affecting the heart. It can be very helpful in shrinking the swelling in the pulmonary arteries, so they open up to give less resistance to flow, thus making it easier for the heart to work. Unfortunately, it can also contribute to fluid retention. The lasix should help with this, by allowing more fluid to escape in the urine.

    The ascites can also develop with right-sided heart failure. The right side of the heart is the one that suffers with heartworm disease. In severe cases, the lasix may not control the fluid build-up. Sometimes it is necessary to drain the fluid directly, using something similar to a large needle. This borders on a surgical procedure, as it needs to be done very carefully and in as sterile a manner as possible.

    I think your veterinarian is working to get your little friend stable, and that is the first thing that needs to be done before you start thinking about trying to kill the heartworms.

    Stay in close contact with your veterinarian. The doctor who is actually seeing your dog is the best person to advise you.

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