For many years we have been fighting a battle against heartworms. We have great products on our side to prevent this disease, but recent findings about heartworm resistance have many pet owners and veterinarians concerned.

Is it possible that heartworms are winning the war?

 Heartworm disease has been known to veterinarians for more than 120 years. The heartworms are transmitted from any of more than 70 known species of mosquito, and the disease attacks the pulmonary arteries and right side of the heart in dogs.  Heartworms are spread directly to the dog from the mosquito, with no dog-to-dog transmission. 

For more than 4 decades, heartworm disease has been effectively prevented in dogs by using available products.  But recent research indicates this might be changing. At a veterinary conference in 2010, information was released detailing a genetic mutation in heartworms that appears to confer slight resistance to current preventives.  Anecdotal reports in the last 4-5 years also point toward an increase in heartworm prevention product failures in the Mississippi delta region of the U.S.

While lack of efficacy (LOE) to heartworm preventives remains geographically limited, research is ongoing to determine the extent of this problem.  Historically, the LOE was attributed to poor owner compliance in the geographic area, the effects of Hurricane Katrina, increased heartworm numbers within the mosquito vector, and/or the increased sensitivity of heartworm testing.

Two prominent veterinary groups, the Companion Animal Parasite Council (CAPC) and American Heartworm Society (AHS) concluded at a recent council strategy session that most credible reports of LOE are geographically limited at this time.  In addition, the extent of this problem is not truly known.

The lack of conclusive evidence to LOE could also be due to several factors.

  • Poor Owner Compliance.
    • Are pet owners accurately following the veterinarian recommendations?
    • Is the pet given the preventive medication consistently without missing or delaying any treatment?
    • Is the pet ingesting the medication? (vomiting, pets spitting out or hiding the medications, swimming or bathing immediately following application can cause a missed treatment).
  • Imperfect Clinical Testing & Education
    • Dogs with inconsistent heartworm testing are at greater risk and must be tested more often.
    • Delayed maturity of heartworms can potentially indicate a “negative” antigen test.  This may lead to a false sense of security with dog owners and reluctance to retest.
    • Heartworm tests may also have become more sensitive (i.e. more accurate), than older generation tests.   

Whether there is indeed resistance to heartworm, this same study group (CAPC and AHS) concludes: “The potential for lack of efficacy of traditional control products is not a reason to abandon their use.  ” They place additional emphasis on the importance of annual heartworm testing.

Although research into heartworm resistance is on-going, the veterinary industry does recognize the dire consequences if resistance is confirmed.  The American Heartworm Society will continue to support and monitor research in this area.  And if resistance is confirmed, changes to preventive and therapeutic strategies may need to be implemented in the future.

Key recommendations for veterinarians include:

  • Proper use of current heartworm preventives remains effective in the vast majority of dogs.
  • Prevention strategies should not be abandoned.
  • In the case of confirmed heartworm disease, stage-specific medical management should be implemented.
  • CAPC and AHS guidelines should be followed in the “face of reports of lack of efficacy”.

Key recommendations for dog owners include:

  • Follow your veterinarian’s recommendation for heartworm disease; more often if preventives are missed or high risk. 
  • Don’t vary from label directions on the dose and frequency.
  • Make sure the dog ingests the medication.
  • Reduce exposure to mosquitoes
  • Get examined by veterinarian immediately if symptoms appear in your dog…i.e. persistent cough, exercise intolerance, body wasting.
  • Follow veterinary recommended treatment if dog is diagnosed with heartworm disease.

Heartworm disease is a complex issue.  Until more is known about the extent of the issue, pet owners should trust that current heartworm products are still the best choice for prevention in the vast majority of dogs.

No matter where you live, following your veterinarian’s recommendations and giving heartworm prevention every month, year-round as well as annual testing gives you the peace of mind that you are protecting your pets.

To learn more about heartworm disease, visit our Pet Health Library at