Heartworm disease may be detected on routine health screenings — for example, when a heartworm test is performed at a dog’s annual veterinary wellness visit — or when a dog is tested based on clinical suspicion. Regardless of how heartworm disease is diagnosed, prompt treatment is essential.
Goals of heartworm treatment
Pet owners and veterinarians should have a clear understanding of the goals of heartworm treatment. While many dog owners administer heartworm prevention on a regular basis, too many dogs do not receive these treatments and eventually develop the disease.
When adult heartworms become lodged within the pulmonary vessels, they cause significant inflammation within the vessel walls and surrounding tissues. This leads to the clinical signs associated with heartworm disease: coughing, exercise intolerance, syncope, abnormal heart and lung sounds, ascites, and a host of other symptoms.
The primary goals of heartworm treatment are to minimize the clinical signs associated with heartworm disease and to eliminate all heartworms (adults, juveniles, larvae, and microfilariae) from the body.
While live worms cause significant inflammation in the pulmonary vessels, dying and dead worms can result in even more damage. As such, treating heartworms does not result in an immediate risk reduction or the immediate resolution of inflammation. In fact, dogs are often at a higher risk of clinical signs of heartworm disease as heartworms die, regardless of whether the parasite’s death is caused by adulticide treatment or the natural end of the life cycle.
Treating heartworms early, using an adulticide, prevents worsening damage and leads to faster resolution of clinical signs.
Avoiding complications: pre-treatment heartworm diagnostics
Prior to beginning treatment for heartworm disease, diagnostics should be performed to assess the patient’s health status. These diagnostics can better prepare the veterinarian and pet owner for complications that may arise during treatment and suggest approaches to minimize these risks.
Pre-treatment thoracic radiographs are often recommended for patients with heartworm disease. A technical proficiency in diagnostic imaging and proper positioning on the vet table are paramount in assessing the degree of pulmonary damage and predicting the likelihood of treatment-related complications.
Bloodwork, including a complete blood cell count and serum biochemistry, is also often recommended prior to treatment. This can help in assessing for uncommon complications of heartworm disease, such as liver and kidney damage, while also assessing the dog’s overall health status.
Adulticide treatment side effects
Adult heartworms are treated with melarsomine dihydrochloride. This medication is administered by intramuscular injection, deep into the epaxial muscles.
Melarsomine injections are often associated with localized, injection-site swelling and soreness. These local effects may vary from mild to severe, depending on the patient. Many patients benefit from the administration of pain medication in conjunction with their melarsomine injections.
A more significant side effect associated with melarsomine treatment is pulmonary thromboembolism: dead or dying worms dislodge and travel into distal pulmonary arteries, where they are broken down by the immune system. Some degree of pulmonary thromboembolism is virtually guaranteed with adulticide therapy, but not all dogs will show clinical signs.
Clinical signs can be minimized by ensuring that owners strictly restrict their pet’s activity during and after treatment, which reduces the degree of pulmonary thromboembolism that occurs.
Heartworm treatment protocol: melarsomine dosage and adjunct medications
Melarsomine label recommendations discuss two treatment options: a two-dose protocol and a three-dose protocol.
The manufacturer recommends the two-dose protocol for dogs with Class 1 (asymptomatic or mild) or Class 2 (moderate) heartworm disease. The three-dose protocol is recommended for use in dogs with Class 3 (severe) heartworm disease, including dogs with cough, dyspnea, muscle wasting, and/or fatigue.
In contrast to the manufacturer recommendations, however, the American Heartworm Society recommends that all infected dogs be treated with the three-dose protocol. The two-dose protocol kills only 90% of adult worms (according to label claims), while the three-dose protocol kills 98% of adult worms. Additionally, the three-dose protocol is associated with a lower risk of effects related to pulmonary thromboembolism than the two-dose protocol.
In addition to melarsomine, a number of adjunct medications are recommended in dogs undergoing heartworm treatment. These medications, which help improve treatment efficacy and minimize side effects, include:
Anti-inflammatory doses of steroids minimize clinical signs associated with pulmonary thromboembolism.
This antibiotic is effective against Wolbachia, a symbiotic bacterium found within heartworms. Doxycycline is thought to decrease pulmonary inflammation while making adult heartworms more susceptible to adulticide therapy.
- Macrocyclic lactones
These are used to kill microfilariae and 3rd and 4th stage larvae. Because melarsomine is ineffective against heartworm larvae and young adult worms, macrocyclic lactones should be administered for two months prior to adulticide therapy, thus ensuring that all heartworms in the dog are mature enough to be susceptible to melarsomine.
Macrocyclic lactones should be used with caution in dogs with high microfilarial counts, as rapid microfilarial die-off may lead to significant clinical signs.
Dog Heartworm treatment timeline
The treatment timeline recommended by the American Heartworm Society is as follows:
- Day 0 (diagnosis): Discuss heartworm disease and treatment with the client.
Begin exercise restriction to reduce risk of pulmonary thromboembolism.
Administer prednisone if the dog is symptomatic for heartworm disease.
Begin doxycycline (10 mg/kg PO BID for 4 weeks).
- Day 1: Administer heartworm prevention in the veterinary clinic, then monitor for 8 hours.
Consider pre-treatment with antihistamines and/or steroids, to reduce reaction risk.
- Day 30: Owner administers heartworm prevention at home.
- Day 60: Owner administers heartworm prevention at home.
Administer first melarsomine injection (2.5 mg/kg IM).
Consider pain medication for injection.
Begin prednisone (four-week taper, starting at 0.5 mg/kg BID for the first week).
- Day 90: Owner administers heartworm prevention at home.
Administer second melarsomine injection (2.5 mg/kg IM).
Consider pain medication for injection.
Repeat four-week prednisone taper.
- Day 91: Administer third melarsomine injection (2.5 mg/kg IM).
Continue year-round monthly heartworm prevention and perform follow-up as recommended.
The “slow-kill” heartworm treatment method: cheaper, but riskier
The “slow-kill” method has long been used in heartworm cases that cannot receive adulticidal treatment, often due to financial constraints. In this treatment, monthly heartworm prevention (macrocyclic lactones) is initiated without adulticidal treatment.
While many view “slow-kill” as an alternative to adulticidal therapy, it is important that clients understand that it is more a salvage procedure than an actual medical treatment. With this treatment, it can take upwards of a year for the dog to clear their heartworm infection. Activity must be restricted throughout that entire time period, since there is no way to predict when adult worms will die.
Studies have shown that dogs treated with the “slow-kill” method show radiographic progression of disease that is indistinguishable from dogs receiving no treatment at all. Additionally, dogs receiving “slow-kill” treatment are at a higher risk of pulmonary thromboembolism than dogs receiving adulticidal treatment.
Aftercare and post-treatment monitoring
Clients should be informed of the risk of pulmonary thromboembolism during and after heartworm treatment. Dogs should have their activity restricted before treatment, during treatment, and for two months after treatment. Activity during or after treatment is one of the most significant predictors of complication associated with adulticidal treatment.
Clients should also be advised to monitor their dog closely for signs of pulmonary thromboembolism. Signs may include coughing, hemoptysis, shortness of breath, weakness/lethargy, or pale mucous membranes. If these signs are observed, the dog should receive immediate veterinary attention.
Recommended follow-up includes a microfilaria test, one-month post-treatment, and a heartworm antigen test and microfilaria test nine months post-treatment.
Prognosis: heartworm treatment success rates
With the three-dose adulticide protocol described above, in conjunction with doxycycline and macrocyclic lactones as recommended by the American Heartworm Society, 98% of dogs will be cleared of heartworm infection.
In the event that a heartworm antigen test is positive at nine months post-treatment and the client has been administering heartworm prevention as directed, treatment failure is suspected. The patient should be treated with doxycycline and then given two doses of adulticide, 24 hours apart, and then be retested with a heartworm antigen test nine months after.
The bottom line
While heartworm treatment is associated with a significant risk of side effects, these risks can be minimized through early treatment and client education. Activity restriction remains the most important aftercare instruction for dog owners to follow.
Sources and additional reading
- American Heartworm Society. 2018. Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs. Available at: https://d3ft8sckhnqim2.cloudfront.net/images/pdf/2018-AHS-Canine-Guidelines-181114.pdf?1542248135
- Dixon-Jiminez A, et al. 2018. Approaches to Canine Heartworm Disease Treatment Among Alumni of a Single College of Veterinary Medicine. JAAHA. Available at: https://www.aaha.org/public_documents/professional/resources/jaaha54.5_approaches_to_canine_heartworm_disease_treatment.pdf
- Immiticide Sterile Powder. Available at: https://dailymed.nlm.nih.gov/dailymed/getFile.cfm?setid=e81ad4b2-35d8-4f13-a07f-3e8d95ef2307&type=pdf&name=e81ad4b2-35d8-4f13-a07f-3e8d95ef2307
- Moorehead A. 2018. The AHS Protocol vs. “Slow Kill.” Available at: https://www.heartwormsociety.org/veterinary-resources/veterinary-education/ahs-board-speaks-out/507-the-ahs-protocol-vs-slow-kill