Prophylactic activity of ivermectin against Dirofilaria immitis infection in dogs. Larvicidal activity of ivermectin against D. immitis larvae 30 days after infection.

Larvicidal activity of ivermectin against Dirofilaria immitis larvae of 30 days old was evaluated. A single oral dose of ivermectin to demonstrate complete larvicidal activity was 3μg/kg of body weight or more. Administrations of ivermectin at 0.5, 1 and 2μg/kg dose-relatedly showed incomplete antilarval activities. Administration of ivermectin at 0.5, 1 and 2μg/kg 30 days after infection caused lower levels of the sex ratios (female/male) than that of the control showing definite decrease in numbers of female worms. The average body lengths of both female and male worms recovered from dogs receiving ivermectin at 0.5, 1 and 2μg/kg, except for that of female worms at 1μg/kg, were significantly shorter than that of the worms recovered from the control dogs.