Efficacy of a topical spot-on containing 65% permethrin against the dog louse, Trichodectes canis (Mallophaga:Trichodectidae).

The efficacy of a 65% permethrin spot-on formulation (Defend EXspot, Schering-Plough Animal Health Corp., Union, NJ) against the dog louse, Trichodectes canis de Greer 1778, was studied. Fourteen dogs naturally infested with T. canis were evenly and randomly allocated to treatment with 65% permethrin administered at the label dose rate of 1 or 2 ml per dog or to an untreated control group. Louse counts were performed for each dog by gently back-combing the hair at six designated anatomic sites (head, tail, belly, each side, and an 8-cm strip the length of the body on the back), and lice were counted without removal on Days 0 (pretreatment), 7, 14, 21, and 28. Lice were eliminated from all dogs treated with the 65% permethrin spot-on within 7 days after treatment, and no subsequent reinfestations due to hatching of eggs were observed during the 28-day evaluation period. Untreated control dogs were subsequently treated with the 65% permethrin spot-on after the initial phase was completed and lice populations were evaluated as previously described. All lice were cleared from these dogs by Day 7, and there were no signs of reinfestation. No adverse reactions to treatment were noted during the study.