Evaluation of ivermectin and milbemycin oxime efficacy against Dirofilaria immitis infections of three and four months' duration in dogs.

Objectives: To determine the efficacy of ivermectin (IVM) and milbemycin oxime (MBO) against induced heartworm infection, where monthly treatment is started 3 or 4 months after infection, and to monitor microfilaremia and antigenemia.

Animals: 21 heartworm-naive Beagles.

Procedures: Each of 21 dogs was given 50 infective larvae of Dirofilaria immitis by SC inoculation. One group of 5 dogs served as nonmedicated controls, 2 groups of 5 dogs received IVM at 6 micrograms/kg of body weight or MBO at 500 micrograms/kg for 12 months beginning at postinfection (PI) month 4, and 2 groups of 3 dogs received IVM or MBO for 13 months beginning at PI month 3. Blood collected at intervals not > 1 month beginning at PI month 4 was examined for microfilariae and antigen. Dogs were euthanatized at PI month 16.

Results: Adult worm counts, relative to controls, were reduced in the 4-month treatment groups by 95.1 (P < 0.01) and 41.4% for IVM and MBO, respectively. The difference between the IVM and MBO groups was significant (P < 0.01). Live worms were found in all MBO-treated (range, 8 to 27) and control dogs (range, 12 to 39) and in 3 of 5 IVM-treated dogs (range, 2 to 4). In the 3-month treatment groups, worm counts were reduced by 97.7 (P < 0.01) and 96.8% (P < 0.01) for IVM and MBO, respectively. Microfilariae were seen in all control dogs and in only 2 of the 16 treated dogs. The antigen response of MBO-treated dogs in the 4-month treatment group was only slightly weaker than that for control dogs. In all other treated dogs, this response was delayed and weaker.

Conclusion: Ivermectin is highly (> or = 95%) and significantly more effective than MBO against induced heart-worm infection when 1 year of monthly prophylactic dosing is started 4 months after infection.

Clinical Relevance: In some cases of owner compliance failure, monthly administration of IVM gives a high level of protection against young adult heartworms.