FAQs and Answers About Canine Heartworm Treatment

The American Heartworm Society (AHS) guidelines provide a research-based plan of action for veterinary practitioners who treat heartworm patients. The AHS Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs, which were updated in 2024, include comprehensive recommendations for patient assessment, medications, medication timing, and monitoring and are viewed by both veterinary specialists and general practitioners as an optimal roadmap to follow when treating heartworm-positive dogs.¹

Heartworm treatment ideally involves a committed owner with the time and resources to work closely with the veterinary team over a period of months. The reality, however, is that it is not always possible for practitioners to adhere to every step of the AHS treatment plan—particularly when they must grapple with missed appointments, financially challenged clients, time constraints, and patients with undocumented health histories. Some of the most common treatment-related questions posed to the AHS by veterinarians concern what to do when an unexpected turn of events disrupts treatment or when less-than-ideal circumstances dictate an altered treatment plan.

Between 2020 and 2021, the AHS was inundated with emails about these types of scenarios. COVID-19 set back many treatments due to temporary clinic closings or clients infected with COVID. Six months proved to be the limit for safely suspending the melarsomine injections, whether after the doxycycline administration or after a first injection had been administered. While not ideal, in both scenarios just about all patients became antigen negative within 9 months of resuming adulticide treatments. Following is a discussion of 3 of the most frequently asked questions.

Q: I recently initiated heartworm treatment for a canine patient using the AHS protocol. The dog was administered a heartworm disease preventive and 4 weeks of doxycycline, but a family emergency prevented the owner from bringing the dog back to the hospital for adulticide treatment. Six months have passed, the dog did receive regular monthly preventive medication, and the client is now willing to have the dog finish treatment. Do I need to repeat doxycycline before administering melarsomine?

The AHS heartworm treatment protocol for dogs starts with administration of an approved heartworm disease prevention product, followed by 4 weeks of doxycycline given at a dosage of 10 mg/kg q12h. The dog then receives a second dose of preventive, followed by a 30-day waiting period before the first injection of melarsomine is given. A second melarsomine injection is given 30 days later, followed by a third injection the following day. Meanwhile, the dog needs to remain on preventive medication to avoid any new infections.

This patient faces 2 challenges: (1) heartworm infection has been allowed to progress during the “pause” between doxycycline and melarsomine, and (2) extending the treatment period means extending the period when the dog needs to be activity restricted to avoid treatment complications. While the situation is not ideal, the upside is that the owner is now ready and willing to have their dog resume heartworm treatment.

Reducing Wolbachia is a critical component of a heartworm treatment protocol; therefore, the full 30-day course of doxycycline at 10 mg/kg q12h should be repeated if 6 months or more have passed between the initial doxycycline dose and the first melarsomine injection. After completion of the new doxycycline regimen, the dog can be given its first injection of melarsomine, followed by the second and third injections 30 and 31 days later. If less than 6 months have passed since doxycycline administration, and the patient has been on a heartworm disease preventive, it is acceptable to proceed to melarsomine treatment.

Another common circumstance is for heartworm treatment to be interrupted after the first melarsomine injection. In this case, if less than 6 months have passed since the first injection, it is acceptable to proceed immediately with the second and third injections, 24 hours apart, assuming the dog has remained on heartworm disease prevention. If the pause between the first and second injection is greater than 6 months, the full 3-injection protocol should be restarted.

Q: I have a client who adopted a dog from a shelter. The dog was treated for heartworms in the shelter; however, due to time and money constraints, the shelter was unable to follow the full AHS treatment protocol and instead gave the patient 4 weeks of doxycycline followed immediately by 2 consecutive melarsomine injections. Is this treatment considered adequate? What follow-up treatment and testing are needed now?

Many pet shelters face time and financial constraints and are compelled to streamline heartworm treatment, adopting protocols such as the one described. The AHS heartworm treatment protocol was designed with 3 melarsomine injections to deliver the highest level of efficacy. In studies, the 2-injection protocol yielded a 90% reduction in heartworm burden versus the 98% to 99% reduction that can be expected from the 3-injection protocol.²

• At this point, the plan of action should include the following steps:
• Unless it was performed at the shelter before the dog’s release, perform a microfilaria test. If the dog had microfilariae at the time of treatment, melarsomine would likely not eliminate them. If the microfilaria test result is positive, topical moxidectin (an approved microfilaricide) should be administered.
• Do not perform antigen testing at this point, as it will likely yield an unreliable (i.e., false-positive) result. The AHS recommends antigen testing no sooner than 9 months after treatment.
• Maintain the patient on a heartworm preventive to prevent reinfection.
• Nine months after the last melarsomine injection, retest the dog with an antigen test. If the result is negative, it can be assumed that treatment was successful. If the test result is positive, the AHS recommends repeating 4 weeks of doxycycline followed by 2 melarsomine injections on consecutive days.

Q; Heartworm treatment can be expensive, and unfortunately, many heartworm-positive patients belong to clients with limited financial means. Are there ways to minimize treatment costs for the client—and my practice—without significantly sacrificing care?

In an ideal world, every patient with heartworm disease would undergo comprehensive blood analysis and thoracic radiography; if needed, echocardiography or cardiac ultrasonography would also be performed. However, when cost is a concern and the patient is asymptomatic, the need for diagnostic testing beyond antigen and microfilaria tests can be considered lower in priority. Instead, the focus should be to conduct a thorough physical examination to determine if the dog is showing clinical signs of disease. If the dog is asymptomatic, the examination and antigen and microfilaria tests are usually sufficient and treatment can be commenced. Additional blood analysis and thoracic radiography can be reserved for dogs exhibiting clinical signs of disease.

Another strategy a practice can employ to help reduce time and expenses when treating patients with heartworm disease is to group these patients together so that melarsomine treatment is administered in the veterinary clinic on 1 or 2 days a week, or just 1 or 2 days a month in smaller practices. Once a vial of melarsomine is opened, it must be used within 24 hours or discarded. By grouping heartworm-positive patients together on “treatment days,” it is possible to reduce adulticide medication waste while optimizing practitioner and staff time.

Finally, the value and importance of client education should not be shortchanged. Ensure the client understands how heartworms are affecting their dog, how the disease will progress if left untreated, and why adulticide therapy is recommended. Inform the client that it is critical to restrict exercise for dogs undergoing treatment, as the lung pathology associated with dead and dying worms is potentiated with exercise. Emphasize the veterinary team’s commitment to working with the owner to determine a plan of action that will work for both them and their pet. Finally, remember that the worst-case scenario is for the client and patient to leave and pursue no treatment at all. This not only puts the life of the patient at risk but also risks the patient becoming a source of heartworm infection that could jeopardize the lives of other pets.

Summary

The AHS heartworm treatment guidelines lay out the best plan for treating heartworm infection in dogs. However, streamlining treatment is possible if necessary, and an interrupted treatment plan can be resumed on a revised timeline depending on the length of the delay between steps.