Comparative efficacy of oral preventatives against macrocyclic lactone-resistant heartworm in dogs

The two studies here evaluated the efficacy of consecutive monthly dosing of moxidectin against macrocyclic lactone resistant Dirofilaria immitis. The moxidectin dosing was then compared to drug combinations of ivermenctin/pyrantel and milbemycin oxime/praziquantel for efficacy against the resistant strains as well.

Dogs were inoculated 30 days prior to first treatment with 50 third-stage (L3) larvae of a ML-resistant strain of D. immitis, ZoeLA or JYD-34. In each study, dogs (six per group) were randomized to treatment with six monthly doses of placebo, four or six monthly doses of 24 microgram per kilogram moxidectin, or six monthly doses of ivermectin/pyrantel or milbemycin oxime/praziquantel at their label dose rates.

Efficacies against ZoeLA for moxidectin, ivermectin/pyrantel and milbemycin oxime/praziquantel were ≥ 96.1%, 18.7% and 21.2%, respectively. Efficacies against JYD-34 were ≥ 95.9%, 63.9% and 54.6% for moxidectin, ivermectin/pyrantel and milbemycin oxime/praziquantel, respectively.

Moxidectin administered orally at 24 microgram per kilogram to dogs for four or six consecutive months was ≥ 95.9% effective in preventing the development of two macrocyclic resistant-resistant heartworm strains and resulted in significantly fewer adult D. immitis than in dogs treated with the other medications.

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Comparative efficacy of oral moxidectin doses against marocyclic lactone-resistant heartworm in dogs