EVALUATION OF IVERMECTIN AND MILBEMYCIN OXIME EFFICACY AGAINST DIROFILARIA-IMMITIS INFECTIONS OF 3 AND 4 MONTHS DURATION IN DOGS

Objectives-To determine the efficacy of ivermectin (IVM) and milbemyci n oxime (MBO) against induced heartworm infection, where monthly treat ment is started 3 or 4 months after infection, and to monitor microfil aremia 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 mu g/kg of body weight or MBO a t 500 mu g/kg for 12 months beginning at postinfection (PI) month 4, a nd 2 groups of 3 dogs received IVM or MBO for 13 months beginning at P i month 3. Blood collected at intervals not > 1 month beginning at Pi month 4 was examined for microfilariae and antigen. Dogs were euthanat ized 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 MB O-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 group s, worm counts were reduced by 97.7 (P < 0.01) and 96.8% (P < 0.01) fo r IVM and MBO, respectively. Microfilariae were seen in all control do gs and in only 2 of the 16 treated dogs. The antigen response of MBO-t reated dogs in the 4-month treatment group was only slightly weaker th an that for control dens. In all other treated dogs, this response was delayed and weaker. Conclusion-Ivermectin is highly (greater than or equal to 95%) and significantly more effective than MBO against induce d heartworm infection when 1 year of monthly prophylactic dosing is st arred 4 months after infection. Clinical Relevance-In some cases of ow ner compliance failure, monthly administration of IVM gives a high lev el of protection against young adult heartworms.