It May be beneficial to administer a macrocyclic lactone for up to six months prior to administration of melarsomine, when the clinical presentation does not demand immediate intervention. The reasoning for this approach is to reduce circulating microfilariae and kill migrating D. immitis larvae, and in the case of ivermectin, stunt immature D. immitis and reduce female worm mass by inhibiting the reproductive system. Milbemycin also sterilizes female worms, but it does not affect worms older than four months. Administration for greater than three months should result in reduced antigenic mass, which in turn may reduce the risk of pulmonary thromboembolism. Depending on the season and geographic locale, administration for three months also will allow immature worms to reach an age at which they are known to be susceptible to killing by melarsomine1.
...Continuous monthly administration of prophylactic doses of ivermectin, alone or in combination with pyrantel pamoate, is highly effective against late precardiac larvae and young (<7 month="month" post-infection))" adult heartworms. Comparable adulticide capability of the other macrocyclic lactones has not been reported. The adulticide effect of ivermectin generally requires more than a year of continuous monthly administrations and may take more than two years before heartworms are eliminate completely. The older the worms when first exposed to ivermectin, the slower they are to die In the meantime, the infection persists and continues to cause disease. Therefore, long-term continuous administration of ivermectin generally is not a substitute for conventional arsenical adulticide treatment. If arsenical therapy is declined, a lengthy course of prophylactic doses of ivermectin will gradually reduce the number of adult heartworms, but in chronic mature infections this may not be clinically beneficial. Exercise should be restricted in dogs treated with prophylactic doses of ivermectin as the adulticide.
The results of a recent study in which monthly ivermectin was administered to client-owned heartworm infected dogs for two years indicated that this method of killing adult heartworms should not be used in dogs with signs of heartworm disease or very active dogs, and if used in asymptomatic dogs, the dogs should be examined by a veterinarian at least once every four to six months until all of the worms are dead As worsening of radiographic signs may be observed, periodic radiographic evaluations may be useful in monitoring the treatment.[/b]
