Heartworm Preventive Failure

Last night we were blessed to have Dr. James Crisman, of Pfizer Animal Health, come to our South Mississippi Veterinary Medical Association (SMVMA) to speak on the topic of heartworms.

Now, you might well think that there is nothing

Even little cuties like Annabelle may be susceptible to heartworm disease.
Even little cuties like Annabelle may be susceptible to heartworm disease.

new to know about a parasite that has been recognized for well over one hundred years. Not so, like most living things, Dirofilaria immitis undergoes constant change.

For example, we have known for about five years that dogs on heartworm preventive have been getting heartworms at an increasing rate. Study after study shows that the leading cause is failure to administer heartworm preventive on schedule. Here is a list of common mistakes:

  • failing to give the medicine on the same day every month.
  • thinking that being a few days late is not a problem.
  • missing entire month(s).
  • forgetting to refill the medicine when refills fall between doctor visits.

Apparently, much of heartworm preventive failure is actually failure to prevent!

These same studies show that pet owners (dogs and cats) who purchase their heartworm preventive 12 doses at a time are less likely to miss doses.

I must emphasize again: Heartworm preventive is a monthly medication. It is not an every 33 days medicine. It is not an every-other-month medicine. It is not a whenever-you-think-of-it medicine.

Having said that, we also know that while owner compliance is a major factor in this trend, it is not the only factor. Researchers are looking at a number of parameters about the heartworms themselves that are contributing to this trend.

Some of the things they are examining have to do with the adult worms. Some pertain to the microfilaria, the offspring of the adult worms. Some even have to do with changes in the intermediate host, the mosquito. Case in point: The introduction of the Asian Tiger mosquito, Aedes albopictus, changed the picture of heartworm transmission significantly. An Oriental invader, she is a much more aggressive feeder than the native mosquito. Natives may feed primarily at dawn and dusk. This striped menace feeds all day long. Every bite has the potential to transmit heartworms to your pet.

That’s right. It takes only one bite from one mosquito to transmit heartworms.

Researchers have no firm conclusions on the biological factors involved in heartworm preventive failure. MyPetsDoctor.com will keep you informed as more information comes to light.

Check back tomorrow for more on heartworm disease, Dr. Randolph.



  1. Phyl, sorry to hear about your dog. We received word today that our dog is heartworm-positive as well, and she has been on preventive too. Ours was on Interceptor.
    With the treatment shortage, our veterinarian has no meds to treat ours, so you are blessed that your veterinarian is able to! I am making calls tomorrow to see if any of our veterinarians have stock! Hope the treatment goes well. We had one go through it before and did well. (We did miss a dose that time.)

    • GG, thanks for reaching out to our fellow pet lover in their time of emotional discomfort. While the statistics are greatly in Phyl’s pet’s favor, none of us wants to see his pet go through any sort of medical problem. As you pointed out, heartworm preventive failure occurs with every form of heartworm prevention. You also made another good point: ONE miss of a dose of heartworm prevention increases a dog’s risk of contracting heartworms.

  2. I was contacted by my veterinarian’s office the other day and they have records verifying that I purchased the heartworm preventive medicine every 6 months as required and have informed the company about my purchases right on schedule. I am waiting to hear from them about payment. I was also informed by my veterinarian’s office they are stopping selling that particular brand of heartworm preventive. We are begining heartworm treatment Monday, August 22, and for the next 3 months, my life as well as my dog’s life is going to change, and not for the good. I will keep you posted.

    • Great news! I’m sorry you and your pet are having to go through this, but at least the financial burden is taken care of. The other aspect of good news is that modern heartworm treatment carries very low risk if you follow your pet’s doctor’s recommedations for home care. Restriction of activity is crucial. Best wishes, and keep us posted on how he does with the treatment, Dr. Randolph.

  3. I want you to know that my dog went for her yearly check-up this month, she is 3 years old, and her heartworm test came back positive. She has been on heartworm preventive medicine given to me by the veterinarian since she was 6 months old. I have never missed a pill and have given it to her on the same day each month. I am very attentive to my dog and always take her to the veterinarian if I sense anything possibly wrong. Her being positive has been so heartbreaking to me; I done every thing right; she shouldn’t have heartworms. I feel like I failed her and the preventive medicine failed her as well.

    • Read the story of one of my employees who became pregnant after taking her birth control medication right on schedule every month. There are no perfect medications. You need not beat yourself up because your dog got heartworms IF you gave your heartworm preventive every 30-31 days. I’m constantly amazed at the volume of clients who become defensive when we print out their heartworm preventive purchase history, showing that they couldn’t have maintained heartworm preventive administration monthly when they bought six doses ten months ago. Not that I’m saying you did that, mind you, it’s just that too many people think that being a week or two late is OK, when, in fact, we know that raises the risk of contracting heartworms substantially. If you have been purchasing your heartworm preventive regularly from your veterinarian and your purchase history in his computer shows no gaps, the manufacturer may be willing to pay for some or all of your dog’s heartworm treatment. Be sure to ask. That won’t make your heartache go away, but it will easy the pain in your pocketbook. Remember, too, that a heartworm-positive-diagnosis is not a death sentence, and hasn’t been for the last forty years. In the last twenty years heartworm treatment is safer than it has ever been. Please keep us posted on your dog’s progress. Best wishes, Dr. Randolph.

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