Cases of heartworm disease have continued to increase in both number and geographical location throughout the United States in the last few years, according to the Companion Animal Parasite Council (CAPC).

Cassan N. Pulaski, DVM, MPH, PhD, clinical assistant professor and director of the diagnostic parasitology lab at the University of Georgia College of Veterinary Medicine and a small animal relief veterinarian, has followed trends in resistance of parasites to commonly used preventives and anthelmintics. Thankfully, she has seen an overall decline in the number of cases of heartworm disease resistant to commonly used preventives.

Causes of heartworm resistance

In 2020, the American Heartworm Society reported that all the macrocyclic lactones on the market had been shown in at least one study to be “less than perfect,” meaning that some dogs taking these drugs regularly still developed heartworm disease.

Pulaski reports that many contributing factors added to reported resistance. These involve a combination of natural genetic variation that resulted in drug resistant worms and human activity that has helped to select for these mutations.

The Asian tiger mosquito, Aides albopictus, originally entered the country on tire barges. This mosquito is very aggressive and feeds all day long, unlike other mosquito species that mainly feed at dusk and dawn. Particularly in rural areas and regions where dogs spend more time outside, there was a greater chance of microfilaria transmission from one dog to another due to the increased potential feeding time.

Other factors include the movement of animals throughout the country. For example, after Hurricane Katrina, many dogs were displaced from their homes in Louisiana and other areas. Many of these dogs ended up being transported to shelters in surrounding states or in other parts of the country. This population included some heartworm positive dogs, some of whom carried worms that were resistant to one or more macrocyclic lactones.

The movement of dogs has not been limited to catastrophic weather events. As people travel and move around more, they are bringing their pets with them. Colorado, a state that previously had very low heartworm prevalence, has been experiencing an increase in heartworm cases since 2013, and the only significant causative factor was determined to be the import of over 100,000 dogs into the state.

What has helped reduce resistance

Pulaski reports that heartworm resistance is not considered to be as big of a problem now as it was in previous years. She attributes this to a few important factors:

• Melarsomine works. There have been no documented cases of heartworm resistance to melarsomine.
• The widespread use of doxycyline has “kept this problem from erupting,” according to Pulaski.
• Killing the Wolbachia organism that lives in symbiosis with the heartworms weakens the worms and results in less pathology for the dog.
• Doxycyline also helps to block the transmission cycle of heartworms, Pulaski adds, because without Wolbachia, L3 are no longer infective.

Preventing future resistance

To help reduce the risk of a resurgence of resistant heartworm infections, Pulaski recommends abandoning any thinking that some areas are “heartworm areas” and others are “non-heartworm areas.”

While this may have been true in the past, it is no longer the case. Another important recommendation is to avoid using large animal parasiticides off-label for heartworm prevention in dogs. Of course, all efforts to improve compliance with heartworm prevention are extremely important.

Heartworm resistance may not be as prevalent as it once was, but there is the potential for that to change in the future due to genetic variation and human activity. And with the uptick of heartworm positive cases in locations where we didn’t used to see it, it is more important than ever to educate and encourage clients to continue heartworm prevention all year round for their dogs, no matter where they are.

If we can put heartworm disease on the radar of dog owners who never considered it to be a problem where they live, hopefully we can start to push heartworm disease incidence numbers in the other direction and make heartworm resistance just a memory of times past.

Further reading

American Heartworm Society Resistance Statement
https://d3ft8sckhnqim2.cloudfront.net/images/Resistance_Statement_20200716.pdf?1594925621

CAPC Heartworm Disease Guidelines
https://capcvet.org/guidelines/heartworm/

Dog Importation and Changes in Heartworm Prevalence in Colorado 2013-2017
https://parasitesandvectors.biomedcentral.com/articles/10.1186/s13071-019-3473-0

Emily Singler, VMD, is AAHA’s Veterinary Content Specialist.

Cover photo credit: © GordZam E+ via Getty Images Plus

Disclaimer: The views expressed, and topics discussed, in any NEWStat column or article are intended to inform, educate, or entertain, and do not represent an official position by the American Animal Hospital Association (AAHA) or its Board of Directors.