Efficacy of ivermectin against Dirofilaria immitis larvae in dogs 30 and 45 days after induced infection.

Forty-two Beagles, 14 to 15 weeks of age, were injected subcutaneously with 50 infective larvae of Dirofilaria immitis and were allotted by weight, within sex, to 6 treatment groups. Group 1 served as nonmedicated vehicle-treated controls; groups 2 through 5 were given an oral tablet form of ivermectin at dosages of 0.3 micrograms/kg, 1.0 micrograms/kg, 2.0 micrograms/kg, and 3.3 micrograms/kg at 30 days after inoculation; group 6 was given the 2.0 micrograms/kg dosage at 45 days after inoculation. Dogs were euthanatized and necropsied 154 days after treatment (day 139 for dogs in group 6) and examined for heartworms. On the numerical bases of helminths recovered in the groups, the efficacies for preventing heartworm maturation were 0% (group 2), 53.2% (group 3), 97.2% (group 4), 98.1% (group 5), and 63.8% (group 6). Drug-related adverse reactions were not detected.