Heartworm Prevention Fears

February 12, 2011

Resistance to Heartworm Prevention:

Is it really happening?

We have heard it over and over again. “If your dog is on monthly heartworm prevention, he won’t get heartworms.” Or “Heartworms are 100% preventable”. So, what is all this hoopla about dogs in the Mississippi Delta region getting heartworms, even though they are on heartworm prevention?

I should back up by reminding our reader what heartworms are and where they come from. To put it simply, heartworms are carried by mosquitoes, something we have plenty of here and that the Mississippi Delta region may have in even greater abundance. When a mosquito bites a dog, it deposits one or more microscopic larvae. The infective larvae make their way to the heart where they can grow into a spaghetti length worms. As the number of worms mount from repeated mosquito bites, they eventually impede the flow of blood and interfere with the valves of the heart. Lung disease and heart failure often ensue. Cats have some level of resistance to the heartworms, but we now understand that they too are affected by the parasite, although differently than dogs. Signs of heartworms in cats can be mistaken for asthma and can actually lead to sudden death. Fortunately heartworms don’t like people, so the larvae dies in our bodies before making its way to our hearts.

So, we have been diligently putting our dogs and cats on heartworm prevention expecting our pets to be free of the disease. Fortunately, this is pretty much how it works. But, in the Mississippi Delta region veterinarians have been reporting positive tests in dogs who have been getting their heartworm prevention. Eighty percent of these reports, upon further study, were found to have missed a dose at some point or they were in-adequately tested before they got started on heartworm prevention. However, even if you take those dogs out of the picture, there are still some dogs who developed heartworm infections while taking preventive medication. But, don’t panic. This is not a growing problem. It has been noted since 2005 and the numbers of prevention failures reported are not growing. In fact, despite the increased awareness and more thorough testing by veterinarians in the region, the number of reported prevention failures in seemingly properly medicated dogs has actually decreased.

At this time we aren’t sure that there is a true resistance to heartworm preventives, like you see with antibiotics today. We may simply be looking at less than 100% efficacy of heartworm preventions. There certainly is anecdotal evidence and there are some preliminary studies to indicate that certain populations of heartworms are responding differently to our heartworm preventions, but the real data simply is not yet in.

A recent study by Dr. Byron Blagburn suggested that one of the heartworm preventions may have better efficacy in preventing heartworms, but, according to the American Heartworm Society, it is too early to make any distinct conclusions based on this single study. Previous studies, although designed slightly differently, do not support Dr. Blagburn’s findings , and because veterinarians in the region have reported failures with all heartworm preventions, at this point, there doesn’t seem to be a clear distinction in protection between one product and another.

There are aggressive studies underway at University of Georgia and Tennessee, Auburn University, Arkansas State University and several other centers in the USA, Canada and Italy. So stay tuned to your veterinarian for updates. Meanwhile, what can you do to help protect your pet?

First of all, keep your pet on his current heartworm prevention. Topical and oral medications must be dosed at 30 day intervals and the Pro Heart injection must be given at 180 day intervals to be effective. Lapses in these treatment intervals could ultimately lead to resistance, so don’t get lazy. Use reminder systems like medminders in PetPortals or remindmypet.com to email and text you when it is time to dose your pet. You can even set up reminders in your Google or Outlook calendars. Be creative, but do whatever it takes to remind you to give these drugs as directed.

Secondly, reduce your pet’s exposure to mosquitoes by keeping them in at night and using approved mosquito repellents.

Finally, have your dog tested at least once a year. So far, we are not seeing this lack of prevention efficacy in the lowcountry or in any other areas of the outside the Mississippi Delta region. However, annual testing will help detect problems early if or when they arise.





4 Responses to “Heartworm Prevention Fears”

  1. Carol Brown Says:

    We have a new rescue dog and took him to our vet who took a blood test and told us he had heartworms ( not bad), but requiring the treatment.
    Then we went to our local clinic for rabies shot and they took a heartworm test and said it was negative.
    Is there a more sensitive test that my vet used, or should I believe the clinic?
    We are not wealthy and treatment is quite expensive. Thanks for your reply. Carol

    • beesferryvet Says:

      Interesting. They are probably using the same test. I would recommend that you go back to your regular vet. They may wish to test again – maybe confirm result with an offsite lab (this is often free). Then go with their recommendation.

  2. Oxana Says:

    I have a rottweiler and he was diagnosed with heartworms. He is on medications such as Heartgard and Doxycicline. Can you please tell me are these medications good to define disease

    • beesferryvet Says:

      Heartgard (once a month) and Doxycycline (for 4 weeks) are the first step in the recommended Heartworm treatment. The next step, 2 months after starting the Heartgard, is a drug called Immiticide that will be given three times over a 2 month period. Strict rest and careful observation is imperative during the entire course of treatment until he is proven clear of heartworms. Prednisone may be used to reduce the risk of reaction to the death of the worms.

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