Heartworm preventive medicines may not be 100% effective.

Hwprevn_2 This is the sort of "can of worms" one hates to open. Yet I feel the need to open a discussion. I am concerned that heartworms (at least in our area) are starting to become resistant to all the preventive medications that we have.

I have been practicing in a severely endemic area for heartworm disease since 1978. Starting in 2006, it seems as though "the rules have changed" with regard to the prevention and treatment of canine heartworm disease. The problems that I have encountered are mirrored by other veterinarians in my area. They are experiencing the same frustrations. My Novartis representative has been working from Missouri south to Louisiana. In an unofficial conversation, he indicated that he had heard it was primarily a problem in the Mississippi valley, with the problem being worse as one goes farther south.

We routinely test all dogs for both microfilariae (baby heartworms in the blood) and heartworm antigen (adult heartworm protein in the blood) at their annual examination. In years past, I always detected a small number of heartworm-positive dogs which had allegedly been on year-round prophylaxis (as we recommend). Some few of these would prove to have an obvious deficiency in compliance. When I asked, "Is it possible that he missed a dose?", the folks would start looking at the floor and shuffling their feet. "Well, you see, Aunt Tillie was sick, and the boy was on a traveling team with the baseball season, and the motor in our car blowed up and…". Which is to say, yes, it is possible that he missed a dose.

On the other hand, each year we had four or five patients whose owners had bought plenty of preventive medication. These were folks whom I believed perfectly honest in their report of administration, and competent in their administration. With this small number of apparent product failures each year, it was easy to attribute the failure to things like the dog clandestinely vomiting under a bush. Nobody watches his dog 24 hours a day. So even though the owner was 100% conscientious in giving the medicine, and the medicine near 100% effective, these rare failures were easy to attribute to the weak link in the chain: the dog.

In 2006 and 2007, we have gone from 4 or 5 apparent product failures per year to 4 or 5 per month. Almost all are large-breed, outside dogs. We have the same percentage of failure among all the different products that we use. Interceptor/Sentinel (which we use in the majority of our patients), Heartgard 30, and Revolution. With this ten-fold increase in incidence of the problem, I am very concerned that the parasites are becoming resistant to the avermectins (EVERY heartworm preventive on the market except for Sentinel and Interceptor) and to milbemycin oxime (the active ingredient in Sentinel and Interceptor).

The drugs are far from useless, however. In fact, I believe that they are still preventing almost all of the heartworm infections from reaching maturity. In a previous post, I noted that a dog’s problem with heartworm treatment is pretty directly related to how many adult worms he has to deal with. These dogs who have been on year-round preventive (and turn up positive for heartworms anyway at their yearly physical) rarely show signs of thrombo-embolic complications after adulticide treatment. This would indicate that these dogs have very few adult worms present. For this reason, I believe that the preventives are largely effective, though no longer 100%.

Again, though the number of heartworm prevention failures has risen dramatically, it is still a very small percentage of our patients. Almost all have been large-breed, outside dogs, and our practice is in an area with a heavy mosquito population during the warmer months (and a few skeeters nearly all year). I certainly would not stop giving the preventive medications.

I certainly would give the medicine year-round, and I certainly would have my dog tested every year. When we can document the dog’s previous "clean" status, and document the preventive medicine purchases, the manufacturers have been very good about paying for the dog’s treatment under their guarantee programs.

18 thoughts on “Heartworm preventive medicines may not be 100% effective.

  1. Sheila says:

    In the last few months, I have had 4 of my dogs test positive for HW, in spite of absolutely monthly year-round doses of Interceptor. 2 are large breed dogs, one is a beagle that has been mine since she was a puppy in 1999, one is a 25 pound mixed breed dog I received as an adult about 5 years ago. All of these dogs were negative at their last test. Two of them live outside, 2 of them live inside. In addition to the HW problem, we also have difficulties ridding them of whipworms, which the Interceptor package insert say it will handle adult whips. If you find a safe, effective alternative preventive, please blog about it, we would love to know.

  2. Doc says:

    I wish that I had an answer for you. To the best of my knowledge,there are no new drugs in the pipeline. I was giving my own dog Heartgard on the first, and Revolution on the 15th, (i.e. dosing twice monthly)and she has heartworms this year.

    While I have had generally good luck with the Interceptor for whipworms, I’d probably use Panacur monthly for 3 months in the dogs that have this problem. Knowing that you generally have a fairly sizable dog population, it is possible that there is a heavy re-exposure rate from fecal contamination of the soil.

    Thanks for reading and writing.

  3. WendyAnn says:

    When I lived in Florida I had a dog come up positive for heartworm when he had never missed a dose of Frontline Plus in his entire life.

    I had six dogs at that time and he was the only one to come up positive. I know none of the other dogs ate his dose because I administer them personally.

    Now that I live in New England again, my fear of hearworms has lessened, but I’m still afraid one of my dogs will get them even though they get heartworm preventative.

    He was treated successfully and is a very healthy dog.

  4. Bettina Hart says:

    I have 3 dogs who have been on Heartgard Plus their entire lives; never missed a dose. My large breed, mainly outdoor dog was just diagnosed with a positive heartworm (antigen test). He starts treatment Tuesday.He’s very active, barking and jumping, and I’m very concerned about keeping him quiet to avoid PTE. I live in Northwest Alabama. People should be told that the preventive is NOT 100% effective and should get their dogs tested twice a year. Thanks for your website-

  5. Doc says:

    Hello, Bettina,

    I guess nothing is 100%, but I really felt that our
    heartworm preventive were nearly that until 2006.
    This has been an almost daily source of disappointment
    for me in the last two years. I really appreciate the
    companies standing behind their guarantee, but it’s
    still not fun.

    Thanks for reading and writing.

  6. Pietoro says:

    I thought Ivermectin was a treatment, not a preventative? It will kills worm larvae if they’re there, but it doesn’t stop them from appearing at all.

  7. Pietoro says:

    In other words, even if a dog gets some worm larvae while between doses, as long as he gets the next dose he’s due for on time, he should be just fine. Since the drug doesn’t -prevent- worms, of course they’re going to appear once in a while. But the drugs will shut them down pretty fast before they even start to hurt the dog. It seems like not that big of a problem, it’s just the drug working the way it always has…

  8. Doc says:

    Ivermectin is considered a “heartworm preventive drug” in that it is supposed to prevent the development of adult heartworms.

    Mosquitoes infect the dog with microscopic larvae daily and it takes these larvae at least six months to mature into adult worms in the heart.

    When one gives the ivermectin each month, he is (it is to be hoped) killing all the heartworm larvae that the mosquito has brought there in the previous few weeks.

    The apparent change in my geographic area is this: there is a significant increase (tenfold) in the number of dogs who develop adult heartworms, despite regular monthly doses of ivermectin and other heartworm preventive drugs (selamectin, milbemycin oxime, etc.). If the ivermectin were killing all the developing larvae, this would not be happening.

  9. Amy Walker says:

    I have a 5yr old german shepherd that has just been diagnosed AGAIN with heartworms. Two years ago he tested positive after being on Interceptor since he was a puppy. I was shocked and so was the vet! He was treated and switched to Heartgard Plus. We live in Louisiana, so mosquitos are prevalent; but the cases of heartworms in dogs treated with preventatives still appears to be fairly unheard of. My concern is that 2 preventative medicines have not worked, so what do we do now for prevention once he is treated? I am not so confident that any of them are going to work for a long period of time. I will still give my dog the preventative medicine recommended by my vet, but I’ll just be waiting for the eventual bad news of a re-occurance

  10. Doc says:

    Hello, Amy,

    I am surprised that your veterinarian has not had previous problems.

    Here is a link to a series of articles describing the problem in the Mississippi valley over the last 3&1/2 years.


    I recently attended a seminar given by Dr. Blagburn of Auburn University who believes that a resistant strain of heartworm may be developing.

    We have had a lot of cases like yours: owner conscientious, giving the meds, dog shows a positive heartworm test. The good thing is that these dogs have very few worms, so they almost all do fine with the treatment. The preventive IS working, just not 100%. The other good thing is that the companies have been very good about honoring their guarantee and paying for the treatment cost.

    Do NOT stop giving the preventive, DO get the dog treated. Repeated treatment does not appear to be harming the dogs (I have one patient who has been treated five times, and YES, he has taken 3 different preventive drugs, and NO, he hasn’t been positive every year, just a lot of years.).

    Feel free to write if you have further questions.

  11. Cathy Wells says:

    I have a two year old min pin that has had Ivermectin poisoning from HeartGuard. Out vet has been treating her for at least four months. She seems to get better and then backslides. At times we have to force feed her and then she will start eating. She was doing better until she started coming in heat and now she won’t eat again. She’s really losing weight and I’m really worried. Any ideas on something she might eat? We have tried everything we can think of. Any suggestions would be appreciated.

    Cathy in Texas

  12. Doc says:

    Hello, Cathy,

    Obviously, I don’t know the details and have not seen your dog. Therefore it is not appropriate for me to second-guess your veterinarian.

    On the other hand, that does sound bizarre. Ivermectin toxicity is seldom seen in breeds outside the herding group (Collies, Border Collies, Shetland Sheepdogs, Australian Shepherds, Australian Cattle Dogs, etc.). When it is seen there, it generally causes neurological signs.

    Dogs with a very large number of microfilariae (baby heartworms) can suffer a reaction if those tiny parasites all die at once when given ivermectin.

    The four months of illness is dramatically different from that.

    How much Heartgard did your dog eat? It is not uncommon to give the equivalent of 400 Heartgard doses at once when treating Demodectic mange with ivermectin.

    Even if my original diagnostic tests suggested the ivermectin to be the dog’s problem, after 4 months, I’d be backing up a little. Sometimes you have to really start over again, pretend you don’t know anything about what has happened and take a fresh look. Re-do the blood tests and so forth. At that point, I’d be calling a specialist to help me review the case.

    If your doctor has suggested repeating some of the diagnostic tests, I’d sure be inclined to let him do that.

    Good luck.

  13. Bettina Hart says:

    Hi Doc. I wrote you in 2008 when my 6 year old border collie mix was diagnosed with Heartworms, even after being given his Heartgard year round. He survived treatment and had two negative tests in 2009. Yesterday he tested positive, not just on the antigen test, but for microfilaria. The vet performed 3 diferent tests twice, with two different samples.I am sick that he has to undergo treatment again, especially since he is showing some liver problems. I dose him religiously, he chews his Heartgard, doesn’t vomit.I even increased his dose to 340mcg instead of 272mcg (he’s 90 pounds.) SOMEONE nedds to investigate this Heartgard failure! I have read with interest the articles re preventive failure in the Miss. Delta and the latest Heartworm Symposium in Memphis. What do I use for a preventive if he survives this treatment? Thank you for your website and blog.
    Bettina in Northwest Alabama

  14. Doc says:

    Hello, Bettina,

    I feel your pain. I have had dogs come up positive and do fine on the same preventive the next two years, then come up positive again. I have had dogs come up positive after being cleared for several years in a row.

    I have tried changing preventives, giving two different preventives per month (every two weeks), increasing doses, and still had some failures.

    There are people who believe that Advantage Multi might work better, IF the problem is a more rapid maturing of the heartworm larvae. This has not been proven.

    I have seen fewer failures in 2009 and 2010 than in 2006,07 and 08. I’m still seeing them though.

    Your dog will probably tolerate the Immiticide again, even with the liver trouble.

    People ARE investigating this now, rather than stonewalling like they were 3 years ago.

    Good luck.

  15. Beckett says:

    I have a question: Why didn’t every dog, particularly every outside dog, die from heartworms before the advent of heartworm prevention meds? And why don’t coyotes all die from heartworms? Surely spending their entire lives outside, means that they are ALL bitten repeatedly.

    I read an article recently stating that domestic dogs dying from heartworms is due mainly to us giving them massive amounts of routine vaccinations (I am not anti-vaccination, btw) and serious poisons in our effort to keep them flea, tick and, heartworm free. The article stated that all of these things actually compromise the dogs immune system, making them far more susceptible to actually becoming sick and dying from heartworms. This makes a lot of sense to me seeing that the dog population thrived for thousands of years prior to heartworm prevention meds, and the coyote population is still thriving without them.

    • Doc says:

      Hello, Beckett,
      In my area, where mosquitoes are incredibly bad, and the incidence of heartworm is as great as it is anywhere on the planet, we also have many stray dogs and many dog owners who do absolutely nothing in the way of veterinary care. Their dogs are certainly not being compromised by protection from deadly diseases.

      When I first returned to my home area 42 years ago, I saw dogs dying of heartworm disease every single day. These dogs had received no heartworm preventive because the disease had only become endemic in our area in the previous decade. There were a lot of dog-owners who were just ignorant about the parasite. When I was growing up, the parasite had not yet entered our part of the country and dogs lived a normal lifespan (if they didn’t get run over or die with canine distemper virus, as many unvaccinated puppies did).

      With our enormous mosquito population, once the parasite entered our part of the country, conditions were perfect for it to spread.

      The parasite is very successful because it doesn’t kill the host very quickly. Even dogs with no heartworm preventive, total outside dogs, living in an endemic area with horrendous mosquitoes will live about half a normal lifespan. In my experience, larger dogs start to have heart problems between 3 and 5 years, smaller dogs between 5 and 7 years. That is plenty of time for the species to thrive and reproduce.

      The most susceptible dogs died off, and we do have dogs that are more resistant, and live a nearly normal lifespan with heartworm exposure (though these are rare). I had a case where the folks had moved here from the Pacific northwest mountains and brought a pup with them. That population had not previously been exposed to heartworms. The dog died with a massive infestation at the age of 14 months, having no resistance at all.

      I would love to see the article and see the credentials of the individual who wrote it. The last time a client brought me some erroneous crap, we googled the author and found she was “noted for her articles on gardening”.

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