Evaluation of the effect of simulated rainfall on the efficacy of Ivomec Pour-on against Cooperia spp. infection in cattle

Four controlled studies, one each in Australia, Germany, the United Kingdom , and the United States, involving 30 calves each were conducted to evaluat e the effect of simulated rainfall on the efficacy of Ivomec Pour-On agains t infections of Cooperia spp. At 3 weeks before treatment the calves were i nfected orally with third-stage larvae of Cooperia spp. In each study a rec ent, locally derived field isolate was used. The calves were allocated by r estricted randomization based on body weight within sex to one of the follo wing treatments: unmedicated control with no rain, Ivomec Pour-On with no r ain, Ivomec Pour-On with rain starting at 40 min before treatment, Ivomec P our-On with rain starting at 10 min after treatment, and Ivomec Pour-On wit h rain starting at 60 min after treatment. Ivomec Pour-On was applied topic ally at a dose rate of 1 ml/10 kg body weight (500 mu g ivermectin/kg body weight). The simulated rainfall was equivalent to a heavy shower of approxi mately 12.5 mm of water during a 30-min period. The calves were necropsied for worm counting at 14 or 15 days after treatment. An evaluation of the po oled data showed that as compared with the untreated controls, the Ivomec P our-On-treated calves with no rain had significantly (P < 0.01) fewer C. on cophora (> 99%), C. punctata (> 99%), C. surnabada (> 98%), and combined Co oper ia spp. (> 99%). The reduction in Cooperia numbers noted for calves ex posed to simulated rainfall was >96% for all Cooperia species, regardless o f when the rainfall started relative to the application of Ivomec Pour-On. There was no significant (P > 0.1) difference between the Ivomec Pour-On-tr eated calves with no rain and the pooled groups with simulated rainfall or between the group with rain before treatment and the pooled groups with rai n after treatment. Ivomec Pour-On was highly effective against established infections of Cooperia spp. when applied to wet animals or to animals becom ing wet shortly after treatment.