Preventive efficacy of four or six monthly oral doses of 24 µg/kg moxidectin compared to six monthly doses of Heartgard® Plus or Interceptor® Plus against macrocyclic lactone-resistant heartworm (Dirofilaria immitis) strains in dogs.

BACKGROUND

Recent reports indicated that increasing the monthly oral dosage and the number of consecutive monthly doses of moxidectin improved the efficacy against macrocyclic lactone (ML)-resistant Dirofilaria immitis. The two laboratory studies reported here evaluated the efficacy of four or six monthly oral doses of 24 µg/kg moxidectin compared to six monthly doses of either Heartgard® Plus (ivermectin/pyrantel) or Interceptor® Plus (milbemycin oxime/praziquantel) against ML-resistant D. immitis strains.

METHODS

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 µg/kg moxidectin, or six monthly doses of Heartgard® Plus or Interceptor® Plus at their label dose rates. Efficacy was evaluated by adult heartworm counts approximately nine months after L3 inoculation.

RESULTS

All negative-control dogs were infected with adult heartworms (geometric mean, 35.6; range, 24-41) for ZoeLA and (geometric mean, 32.9; range, 30-37) for JYD-34. Efficacies against ZoeLA for moxidectin, Heartgard® Plus and Interceptor® Plus were ≥ 96.1%, 18.7% and 21.2%, respectively. Adult counts for both moxidectin-treated groups were significantly lower than negative control (P < 0.0001), significantly lower than Heartgard® Plus and Interceptor® Plus (P < 0.0001), but not significantly different from each other (P = 0.5876). Counts for Heartgard® Plus and Interceptor® Plus were not significantly different than negative control (P ≥ 0.2471). Efficacies against JYD-34 were ≥ 95.9%, 63.9% and 54.6% for moxidectin, Heartgard® Plus and Interceptor® Plus, respectively. Counts for all groups were significantly lower than negative control (P ≤ 0.0001). Counts for six monthly doses of moxidectin were significantly lower than those for four monthly doses (P = 0.0470), and the counts for both moxidectin-treated groups were significantly lower than Heartgard® Plus and Interceptor® Plus (P ≤ 0.0002).

CONCLUSIONS

Moxidectin administered orally at 24 µg/kg to dogs for four or six consecutive months was ≥ 95.9% effective in preventing the development of two ML-resistant heartworm strains and resulted in significantly fewer adult D. immitis than in dogs treated with Heartgard® Plus or Interceptor® Plus when administered for six consecutive months at their approved label dosages in two laboratory efficacy studies.