The persistent efficacy of doramectin pour-on against biting and sucking louse infestations of cattle

A repeated-exposure challenge model was used to evaluate the pour-on formul ation of doramectin in preventing the establishment of louse infestations i n cattle. Twenty calves cleared of preexisting biting and sucking louse inf estations were randomly and equally allocated to either a doramectin-treate d or untreated control group, with five replicates per group. Doramectin po ur-on was administered topically at a dose rate of 500 mu g/kg body weight. Every 14 days, from a pool of seeder calves with infestations of at least 50 biting and 50 sucking lice each, 10 calves were selected and 1 was place d in each replicate pen. Every week during the 112-day study, 9 predilectio n sites on the doramectin-treated and untreated calves were examined to est imate the louse population density. A calf met the infestation criterion fo r a louse species when two or more live lice were counted on two or more bo dy regions for two consecutive count days. Because only 4 of 10 untreated c alves acquired Solenopotes capillatus infestations, the persistent efficacy of doramectin against S. capillatus was not evaluated. Bovicola bovis and Linognathus vituli infestations in the untreated calves developed shortly a fter exposure to infested seeder calves. The acquisition of B. bovis and L. vituli infestations in the doramectin-treated group was delayed for 77 day s and 105 days, respectively (C)2000 Elsevier Science B.V. All rights reser ved.