Recent cases of heartworm infection in dogs reportedly receiving appropriate heartworm prevention medicine have raised concern about the possibility that heartworms may be developing resistance to heartworm medications.

Are Heartworms Becoming Resistant to Heartworm Prevention Medicines?

Currently, the jury is still out on this question. The not so simple answer is that we simply don’t know right now. But even the American Heartworm Society confirms that we need to consider this possibility.

This article, Heartworm Prevention: Potential Resistance to Heartworm Medicines goes into detail about some of the factors that must be considered in determining whether true resistance is occurring. Some of these factors include lack of compliance in administering heartworm prevention medicines and advances in the ability to be able to detect heartworm infection leading to increased numbers of heartworm disease is correctly diagnosed.

How Should Worried Be Dog Owners About the Possibility of Heartworm Resistance?

Research in this area is ongoing and is taking place in many different institutions throughout the United States, Canada, and Italy. At this time, according to the American Heartworm Society (AHS), “given all known evidence, we cannot conclude resistance exists.”

The AHS also acknowledges the dire consequences that will ensue if heartworm resistance to medicines used for prevention is occurring.

How Should Worried Be Dog Owners About the Possibility of Heartworm Resistance?

How Should Worried Be Dog Owners About the Possibility of Heartworm Resistance?

What Can a Dog Owner Do to Help While Also Protecting Your Dog Against Heartworms?

The American Heartworm Association suggests the following:

  • Report any cases of heartworm infection that may be considered suspicious. (The AHS suggests reporting to the product manufacturer but I would go one step further and advise reporting the potential failure to the FDA as well.)
  • Continue using appropriate heartworm preventive medications carefully following all label directions. Administer these medications monthly, not every 6 weeks. (The exception to this is Proheart 6®, which should be administered every 6 months.)
  • Reduce exposure to mosquitoes by keeping your dog inside at night and using approved mosquito repellents. Also, consider contacting a pest control company to help control mosquitoes.
  • Treat all dogs with positive heartworm tests with melarsomine (Immiticide®), the only approved heartworm adulticide treatment. Discontinue using macrocyclic lactones (Heartgard Plus® and other commonly used monthly heartworm medicines) that are currently used as a “slow kill” method of heartworm treatment. (I have mixed feelings about this. I can understand the AHS position in that the “slow kill” method likely results in the dog being able to infect mosquitoes and spread the disease for a longer period of time than adulticide treatment. At the same time, not all dog owners are able to afford adulticide therapy and the “slow kill” method provides a more affordable alternative and, in my opinion, would be preferable to doing nothing to treat the dog, though I agree that adulticide treatment (i.e. Immiticide®) is the preferable method.)
  • Talk to your veterinarian about whether doxycycline during adulticide (Immiticide®) treatment is appropriate for your dog.
  • Participate in resistance studies if asked and encourage your veterinarian to do so as well.
  • Support organizations and studies investigating heartworm resistance.

Should Dog Owners Continue to Use Medicines for Heartworm Prevention? What About Increasing the Dosage?

You should definitely continue administering heartworm prevention medicine to your dog. In fact, administering these medications as specified on the label directions is extremely important. In most cases, that means administering the medication once monthly.

Sources that report that heartworm preventive medicines can be effective when administered every six weeks do not take into account that this risk breaks in protection for your dog. They also do not consider that using these medications off-label (i.e. not following the directions on the label) means that the drug manufacturer will not honor any guarantees should the product fail to protect your dog.

Another question that is sometimes asked is whether increasing the dosage of commonly used heartworm preventive medications might afford more complete protection or lower the risk of resistance. Currently, there is no evidence to suggest that increasing the dosage will make the medication more effective and this is not generally recommended.

Should Dog Owners Continue to Use Medicines for Heartworm Prevention?

Should Dog Owners Continue to Use Medicines for Heartworm Prevention?