AHS releases heartworm guidelines

BALTIMORE — The American Heartworm Society (AHS) now calls for year-round prevention and annual testing for heartworm disease.

The completed guidelines were unveiled at the American College of Veterinary Internal Medicine (ACVIM) Forum in early June. The guidelines constitute recommendations for dogs and cats for the diagnosis, prevention and management of heartworm infection.

Charles Thomas Nelson, DVM and president of AHS, tells DVM Newsmagazine that the guidelines call for annual testing (instead of every two to three years), additional testing when switching prevention methods and support for year-round prevention even in northern climates.

The guidelines state: "The ubiquitous presence of one or more species of vector-competent mosquitoes makes transmission possible wherever a reservoir of infection and favorable climatic conditions co-exist. A climate that provides adequate temperature and humidity to support a viable mosquito population, and also sustain sufficient heat to allow maturation of ingested microfilariae to the infective, third-stage larvae (L3) within the intermediate host is a pivotal prerequisite for heartworm transmission to occur."

Nelson says the new guidelines are based on the latest scientific research.

While there are significant differences between canine and feline heartworm disease, Nelson adds, the AHS committee felt it was important to address both species and the real threat the disease poses.

As part of the new canine guidelines, AHS says annual testing is necessary. The guidelines make the recommendations due to reports of positive heartworm cases while on preventives. "It may be too difficult to document when an animal hasn't been checked in three years, and therefore annual testing will ensure that an infection is caught in plenty of time to effectively manage it," AHS reports.

Nelson wants veterinarians to remember, "the longer in, the more damage is actually occurring." The testing recommendation also was based on a request from the Food and Drug Administration (FDA) for "a more conservative testing protocol."

The reason, Nelson says, is that FDA is receiving reports of preventives lacking efficacy. He believes the issue is focused squarely on compliance. In fact, surveys suggest only 75 percent of the heartworm preventives prescribed are actually being used. "The science indicates this is a compliance issue."

The guidelines call for additional testing when prevention medications are changed. As well, year-round testing is supported because it improves compliance. AHS reports that monthly heartworm preventives have activity against intestinal parasites that inadvertently infect 3 million to 6 million people every year.

AHS says the clinical importance of heartworm disease is amplified in cats because the small numbers of heartworms are potentially life-threatening. "And you don't have to have adult worms in the cat to cause disease," Nelson says.

Therefore, AHS is recommending prevention for cats in heartworm-endemic areas. While the diagnosis of heartworm disease in cats is much more elusive, serology, thoracic radiography and echocardiography are the most useful methods of clinical confirmation.

The guidelines are available for review at www.heartwormsociety.org.