How To Treat Dog Heartworm Disease By Medications

Most dogs infected with heartworm can be successfully treated. The goal of treatment is to kill all adult worms with an adulticide and all microfilariae with a microfilaricide. It is important to try to accomplish this goal with a minimum of harmful effects from drugs and a tolerable degree of complications created by the dying heartworms. Heartworm infected dogs showing no signs or mild signs have a high success rate with treatment. Patients with evidence of more severe heartworm disease can be successfully treated, but the possibility of complications and mortality are greater. The presence of severe heartworm disease within a patient in addition to the presence of other life-threatening diseases may prevent treatment for heartworm infection.

1. Adult Heartworm Therapy (Adulticide Therapy).

There is currently one drug approved by the FDA for use in dogs for the elimination of adult heartworms. This drug is an organic arsenical compound. Dogs receiving this drug therapy will typically have had a thorough pretreatment evaluation of its condition and will then be hospitalized during the administration of the drug.

Melarsomine dihydrochloride (Immiticide®, Merial) has demonstrated a higher level of effectiveness and safety than anything previously available. It is administered by deep intramuscular injection into the lumbar muscles. With this drug, veterinarians have the advantage over its predecessor of minimizing post-adulticide complications in dogs with high heartworm numbers. In addition the drug itself is less toxic to dogs than the drug that was its predecessor. For complete information on the classification and treatment for heartworm infected dogs using this product, consult your veterinarian.

2. Post-Adulticide Complications

The primary post-adulticide complication is the development of severe pulmonary thromboembolism. Pulmonary thromboembolism results from the obstruction of blood flow through pulmonary arteries due to the presence of dead heartworms and lesions in the arteries and capillaries of the lungs. If heartworm adulticide treatment is effective, some degree of pulmonary thromboembolism will occur.

When dead worms are numerous and arterial injury is severe, widespread obstruction of arteries can occur. Clinical signs most commonly observed include fever, cough, hemoptysis (blood in the sputum). These patients require both the administration of anti-inflammatory doses of corticosteroids and a strict reduction in exercise.

3. Elimination of Microfilariae

The most effective drugs for this purpose are the macrocyclic lactone (ML) anthelmintics, i.e.,milbemycin oxime, selamectin, moxidectin and high doses of ivermectin. These drugs are the active ingredients in the commonly used heartworm preventives. Although their usage as microfilaricides has not been approved by the FDA, they are widely used as such because there are no approved microfilaricidal drugs currently available. It is recommended that microfilariae positive dogs being treated with these drugs be hospitalized for at least eight hours following treatment for observation of possible adverse reactions resulting from rapid death of the microfilariae.

Circulating microfilariae usually can be eliminated within a few weeks by the administration of the ML-type drugs mentioned above. Today however, the most widely used microfilaricidal treatment is to simply administer ML preventives as usual, and the microfilariae will be cleared slowly over a period of about six to nine months.

4. Confirmation of Adulticide Efficacy

The goal of adulticide treatment is the elimination of all adult heartworms. However, clinical improvement in dogs treated for heartworm infection is possible without completely eliminating the adult heartworms. Heartworm antigen testing is the most reliable method of confirming the efficacy of adulticide therapy. If all the adult worms have been destroyed or very few survive, heartworm antigen should be undetectable by four months post-adulticide. Dogs that remain antigen positive at that time could be considered a potential candidate for repeat treatment with an adulticide only after a full review of each case. In some cases, an alternative is to not retreat with the arsenical but to continue with a preventive such as ivermectin which will gradually eliminate the remaining worms.