Dog Heartworm Treatment: Side Effects, Aftercare, and Success Rates

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:

• Steroids

Anti-inflammatory doses of steroids minimize clinical signs associated with pulmonary thromboembolism.

• Doxycycline

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.