Effects of an ivermectin otic suspension on egg hatching of the cat ear mite, Otodectes cynotis, in vitro.

An otic suspension containing 0.01% ivermectin (Acarexx; Blue Ridge Pharmaceuticals, Greensboro, NC) controls adult ear mites (Otodectes cynotis), but there are no data published on its efficacy against the eggs of this parasite. Three in vitro trials were conducted to examine the efficacy of this product for prevention of hatching of O. cynotis eggs. Eggs were placed in Petri dishes on 1 ml agarose only, agarose with 50 microl of the ivermectin otic suspension (low dose), or agarose with 0.5 ml of the otic suspension (full dose). In Trials 1 and 2, eggs were held at 28 degrees C; in Trial 3, eggs were kept at 37 degrees C. After 6 days, 22% to 38% of the eggs hatched in the control cultures, compared with 0% to 14% in the low-dose medium, and 0% to 8% of the eggs exposed to the full-dose medium. In the controls, larvae hatched throughout the 6 days and crawled away from the empty eggshells. Only one other larva (from a low-dose plate in Trial 2) crawled away from its eggshell. At the full dose of ivermectin, larvae hatched only within the first day of culture, and those few that did hatch died immediately. The 0.01% ivermectin otic suspension prevented the hatching of larvae from eggs or immediately killed the small number of mites that hatched.