May 2024

In this VETgirl online veterinary continuing education blog, we discuss the updated American Heartworm Society Guidelines.

American Heartworm Society Announces Updated Guidelines on Management of Canine Heartworm Disease

by Jenni Rizzo, DVM

Despite the availability of highly effective medications to prevent and treat heartworm disease in dogs, heartworm disease in the U.S. continues to be far too common. Heartworm incidence surveys conducted by the American Heartworm Society (AHS) have documented the spread of heartworm to all 50 states. The most recent survey saw infection rates rise in many parts of the Mississippi Delta and Gulf Coast states, while also becoming more common in regions such as New England, the Upper Midwest and the Southwest.

A primary responsibility of The American Heartworm Society (AHS) is to provide guidance to the veterinary profession on heartworm prevention, diagnosis, and treatment. The AHS recently completed a thorough scientific review and update of their Canine Heartworm Guidelines. Revisions were based on the latest research and understanding of heartworm management, while also designed to address questions that are frequently posed by veterinarians in practice to the AHS. The goal was to ensure that AHS guidance continues to be both scientifically grounded and practical for veterinarians to implement.

Following is a summary of the key points of the updated guidelines, including both new information and guidance that remains unchanged after considering the latest heartworm research. In keeping with the structure of the AHS Guidelines document, the key points pertain to heartworm prevention; heartworm diagnosis and assessment; and heartworm treatment. In addition, the revised guidelines also include new spectrum-of-care guidance for veterinarians treating dogs under less-than-ideal circumstances.

Heartworm prevention:
Prevention is the cornerstone of heartworm management in dogs. While adult heartworms can be successfully treated in dogs, damage from heartworm infection takes place months before a dog can test positive on a heartworm antigen test, and the damage can be long-term—even lifelong.

Macrocyclic lactones (MLs)—the class of medications used in heartworm prevention—are among the safest medications used in veterinary medicine. The AHS recommends that heartworm prevention begin by the time a puppy is 8 weeks old—and earlier if possible. Some MLs can be used in puppies as young as 4 weeks of age. Prevention should be administered year-round to optimize efficacy and reinforce owner compliance.

Vector control is an important component of heartworm prevention in dogs. A change made to the latest AHS guidelines is that both FDA-approved isoxazolines and EPA-approved mosquito repellent products can be used to kill mosquitoes and help prevent the spread of heartworms (previously only EPA-approved products were recommended in the guidelines). It is important to note that while vector control is an important component of multimodal heartworm prevention, it should not be used as monotherapy in dogs.

Heartworm diagnosis:
Dogs over the age of 7 months should be tested for heartworms prior to starting a heartworm preventive. The rationale is that giving an ML to an untested dog could become an inadvertent form of “slow kill” if the dog were to be heartworm positive. Meanwhile, even when dogs are maintained on year-round prevention, annual testing is recommended to ensure a dog has not become infected due to lapsed compliance or a failure to consume or fully absorb the preventive medication.

In cases where a dog is known to have missed a dose of preventive, dogs should be tested 7 months after the lapse, as it can take this long for a sample to test positive.

Annual screening is an integral component of heartworm management. It both assures the practitioner that prophylaxis has been achieved and maintained and ensures that any heartworm infections are detected and treated early. The AHS recommends both antigen and microfilaria testing when screening canine patients. Microfilaria testing is especially important in cases where the dog’s prevention history is unknown, or veterinarians have reasons to suspect the dog is infected.

One of the more common questions practitioners ask is “What should I do if when antigen and microfilaria test results don’t agree?” In approximately one in five cases, testing yields a positive antigen test along with a negative microfilaria test, with this being most common in cases where the dog has been on a heartworm prevention program. Meanwhile, if the dog is microfilaria-positive with no antigen detected, the likelihood that heartworm antigen has been blocked due to the presence of an antigen-antibody complex is high. In the latter case, heat pretreatment of a blood sample can unmask the bound antigen and produce a more accurate test result.

It should be emphasized that whenever heartworm test results are unexpected—whether the finding is positive or negative—repeating the test and/or conducting additional testing with a new blood sample is recommended. Finally, while veterinarians often question how to interpret test results when the color change on an antigen test is faint (a common assumption is that this indicates the animal has a light heartworm burden), such interpretations are highly speculative and should be avoided.

Patient evaluation and heartworm treatment:
Depending on whether the dog is symptomatic, the veterinarian can consider additional evaluations, including thoracic radiographs, ultrasonography, and echocardiography. The updated guidelines include a section with detailed advice for using point-of-care ultrasonography (POCUS).

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. While veterinarians sometimes raise the question of whether a 30-day waiting period between completion of doxycycline and the first melarsomine injection is needed, the AHS has left the timing recommendations in place. A recent study indicates no further pathology occurs over the 30-day period.

Spectrum-of-care principles in treating heartworm disease:
While the AHS’ recommended treatment protocol is as outlined above, the newly revised guidelines include a discussion of alternative treatment protocols, including protocols with fewer melarsomine injections, alternatives to melarsomine treatment, and guidance when heartworm treatment is interrupted. This new spectrum-of-care section in the guidelines is 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. Within this section, the guidelines committee also stipulate that any non-arsenical treatment protocol must include doxycycline (both at the beginning of the protocol and repeated in six months), a macrocyclic lactone (ivermectin and moxidectin have been evaluated in conjunction with doxycycline; milbemycin oxime and selamectin have not) and activity restriction throughout the treatment period.

The AHS Heartworm Management Guidelines for dogs and cats are living documents that are updated as appropriate when new information and evidence comes to light. The AHS Board of Directors would like to thank the following board members who serve on the Heartworm Guidelines Committee:

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

This VETgirl online veterinary continuing education blog is sponsored by the American Heartworm Society, whose mission is to lead the veterinary profession—and the public—in the understanding of heartworm disease. If you’re one of thousands of busy practitioners who have recently diagnosed a dog with heartworms, visit With this treatment calculator, you can instantly generate a customized treatment timeline for your patient, then email the treatment plan to your client and save it in your medical record. Visit today!

This VETgirl online veterinary continuing education blog is sponsored by American Heartworm Society. Please note the opinions in this blog are the expressed opinion of the author(s), and not directly endorsed by VETgirl.

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