Treating for heartworms is not just a simple pill. The treatment must kill the microfilariae and the adults. Remember that a dog can have a heartworm infestation and not test positive. A heartworm infestation will only become apparent if there are adult females present because the standard tests looks for microfilariae (baby or the first larval stage of heartworm parasite development).
Here’s what the Heartworm Society writes about the standard treatment:
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 any other adult heartworm treatment previously available. It is administered by deep intramuscular injection into the lumbar muscles. For complete information on the classification and treatment for heartworm infected dogs using this product, consult your veterinarian.
Prior to treating the adult heartworms, your dog will be put on a 4 week course of doxycycline (or a related antibiotic). This helps reduce the viability of the parasite and decreases the reaction to the dying and decaying heartworms following adulticide treatment.
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) and potentially sudden death. It is extremely important to not allow exercise in any dog being treated for heartworms. Often dogs with severe infections will also require the administration of anti-inflammatory doses of corticosteroids.
Elimination of Microfilariae
The most effective drugs for this purpose are the macrocyclic lactone (ML) anthelmintics, i.e.,milbemycin oxime, selamectin, moxidectin and ivermectin. These drugs are the active ingredients in commonly used heartworm preventives. Although their usage as microfilaricides has not been approved by the FDA, they are widely used by veterinarians as there are no approved microfilaricidal drugs currently available. It is recommended that microfilariae positive dogs being treated with these macrocyclic lactones be hospitalized for at least eight hours following treatment for observation of possible adverse reactions, including those 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.
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 after six 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.