AHS updates heartworm guidelines for dogs

The American Heartworm Society (AHS) published on April 9 an updated version of its Canine Heartworm Guidelines on heartworm prevention, diagnosis, and treatment.

The AHS continues to recommend year-round administration of macrocyclic lactone heartworm preventives as the cornerstone of heartworm management. In addition, the revised guidelines now recommend both Food and Drug Administration (FDA)–approved isoxazolines and Environmental Protection Agency (EPA)–approved mosquito repellent products to kill mosquitoes and help prevent the spread of heartworms. Previously, only EPA-approved products were recommended.

The revisions were based on the latest research and understanding of heartworm management, and address frequently asked questions by veterinary practitioners, according to the AHS.

Guideline highlights

The AHS guidelines recommend annual antigen and microfilaria testing when screening canine patients, with microfilaria testing considered especially important when the dog’s prevention history is unknown or when the veterinarian suspects the dog is infected, according to the AHS announcement.

“Noting that one of the more common questions posed to the AHS relates to disagreement between antigen and microfilaria test results, the guidelines emphasize the importance of repeating heartworm tests when the results are unexpected—for instance, when a dog is microfilaria-positive with no antigen detected,” the announcement states. “If blocked antigen caused by an antigen-antibody complex has led to such a result, repeating the antigen test with a new blood sample that has been heat treated can unmask the blocked antigen and yield a more accurate result.”

Dr. Jennifer Rizzo, president of the AHS, added that, “Research suggests that at least one-fifth of heartworm-positive dogs will test negative for microfilaria, even more so if the patient has received doses of macrocyclic lactones. However, adult worms are still present, and treatment is still needed.”

The guidelines now include a spectrum-of-care section that addresses when optimal treatment protocol for heartworm-positive patients isn’t an option. This includes procedures with fewer melarsomine injections, alternatives to melarasomine treatment, and guidance when heartworm treatment is interrupted.

“If (veterinarians) must follow an alternative treatment protocol, the guidelines provide scientifically backed information for veterinarians on prioritization of certain treatments, efficacy, and complication rates,” Dr. Rizzo said.

Heartworm incidence

The AHS conducts a heartworm incidence survey every three years, working with veterinary practices and shelters that submit data from heartworm antigen tests. The latest survey was conducted in early 2023 and reflects data from testing conducted during 2022.

The survey results indicated heartworm rates continued to trend upward, with the range of infection expanding in Arkansas, the Carolinas, Georgia, Mississippi, and Texas, which are known for having the highest heartworm incidence.

In addition, several states with historically low heartworm rates—such as Connecticut, Kansas, Massachusetts, North Dakota, Oregon, and Washington—experienced unexpected increases.

These trends were confirmed by a January Frontiers in Veterinary Science study from Banfield Pet Hospitals, which also saw increases in the Northwest and New England.

“These data points speak to the importance of improved consumer education, access to care, and confident, unified recommendations from veterinary teams—all concepts kept at top of mind in the most recent update to our guidelines,” Dr. Rizzo said. “With so few dogs protected consistently, it’s easy to see why cases continue to rise.”

Dr. Rizzo explained that education and empathy are key when broaching the topic of heartworm with clients.

“It’s important to meet clients where they’re at. Take the time to understand their beliefs, their budget, their wants, their fears,” she said. “The better we understand these dynamics, the better we can help.”

The updated Canine Heartworm Guidelines were sponsored by Boehringer Ingelheim, Elanco, Merck Animal Health, Zoetis, Ceva, and Idexx Laboratories. According to AHS, these updated guidelines serve as the most comprehensive and rigorously researched piece of guidance on heartworm epidemiology; heartworm biology and life cycle; heartworm prevention; and heartworm treatment.

Correction: A previous version of this story included a different percentage for how many heartworm-positive dogs test negative for microfilaria.