Canine Heartworm Guidelines


Written by:

Bio not available.

American Heartworm Society updates guidelines on management of canine heartworm disease.

The American Heartworm Society (AHS) announced this spring that it has published an updated version of their Canine Heartworm Guidelines on heartworm prevention, diagnosis, and treatment. The AHS guidelines were previously revised in 2020.

The update, available on the AHS website, was completed after a thorough scientific review by a team of authors from the fields of parasitology, cardiology, and clinical practice, the society said in a release. The revisions were based on the latest research and understanding of heartworm management, while also addressing questions frequently posed to the AHS by veterinary practitioners.

The current update was spearheaded by a committee of AHS board members, including:

  • Tom Nelson, DVM
  • John McCall, PhD
  • Andrew Moorhead, DVM, MS, PhD, DACVM (Parasitology)
  • Lindsay Starkey, DVM, PhD, DACVM (Parasitology)
  • Marisa Ames, DVM, DACVIM (Cardiology)

“The goal of this update was to ensure that our recommendations continue to be both scientifically grounded and practical for veterinarians to implement,” said Jennifer Rizzo, DVM, president of the AHS. “The update includes changes to our guidance as well as reinforcement of key points that remained unchanged.” In addition, she noted, the guidelines update includes spectrum-of-care guidance for veterinarians treating dogs under less-than-optimal circumstances.


Following are highlights of the revised canine heartworm guidelines:

  • Heartworm prevention. The AHS continues to recommend year-round administration of macrocyclic lactone heartworm preventives as the cornerstone of heartworm management. In addition, the document now recommends both FDA-approved isoxazolines and EPA-approved mosquito repellent products to kill mosquitoes and help prevent the spread of heartworms (previously only EPA-approved products were recommended).
  • Heartworm diagnosis. The AHS recommends 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. In addition, dogs over the age of 7 months should be tested for heartworms prior to starting a heartworm preventive to ensure that untested and heartworm-positive dogs are not inadvertently administered “slow kill” treatment, AHS said. In cases where a dog is known to have missed a dose of preventive, dogs should be tested 7 months after the lapse.
  • 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. 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.
  • Patient evaluation and heartworm treatment. Depending on whether the dog is symptomatic, veterinarians can consider additional patient evaluations, including thoracic radiographs, ultrasonography, and echocardiography. The updated guidelines include a section with detailed advice for using point-of-care ultrasonography (POCUS).

Meanwhile, the AHS treatment protocol remains largely unchanged in the latest guidelines and includes administration of an ML medication on Day 0, followed by 4 weeks of doxycycline administration to eliminate Wolbachia; another month of macrocyclic lactone on Day 30, 60 and 90; and a series of three injections of melarsomine administered on Days 60, 90 and 91. Activity restriction is recommended throughout the treatment period and for at least 6-8 weeks after treatment has been completed.

  • Spectrum-of-care principles in treating heartworm disease. In addition to AHS’ recommendations on prevention, diagnosis and treatment of heartworm, the guidelines now include a spectrum-of-care section intended for use in clinical situations when a practitioner is unable to follow the optimal treatment protocol for heartworm-positive patients and needs access to information on efficacy and complication rates. This new section includes a discussion of alternative treatment protocols, including protocols with fewer melarsomine injections, alternatives to melarsomine treatment, and guidance when heartworm treatment is interrupted.


“Heartworm disease in dogs continues to be far too common, despite the availability of highly effective medications to prevent and treat it,” said Dr. Rizzo. “Our hope is that the updated AHS guidelines can provide the practical and proven support veterinarians need to significantly reduce the rate of heartworm disease.”


Photo credit: