Persistent efficacy of doramectin pour-on against artificially induced infections of nematodes in cattle

Two studies were conducted to determine the persistent efficacy of doramect in pour-on against an artificial, trickle challenge of mixed nematodes in c alves. In each study, 42, 4-8 months old calves were randomly assigned into four groups of 10 animals each (T1-T4), plus two larval-viability monitor animals. All animals were treated with fenbendazole (10 mg kg(-1)) 14 days prior to the start of the study to clear any existing infection. Doramectin pour-on at 500 mu g kg(-1) was used on each animal in Groups T2, T3, and T 4 with intervals of 1 week (Day 0, 7, and 14, respectively). Calves in Grou p T1 were treated with saline solution on Day 0 and at the same volumetric rate (1 mi 10 kg(-1)) as the doramectin treated animals. All treatments wer e applied in a single passage along the midline of the back, from the withe rs to the tailhead. Subsequently, trickle inoculations with infective larva e were administered to all calves for 22 consecutive days (Days 14-35), Dor amectin pour-on provided greater than or equal to 91.9% efficacy against ch allenge with Dictyocaulus viviparus, Haemonchus spp., and Ostertagia, oster tngi for up to 35 days post-treatment anh against challenge with Cooperica ancophora, Cooperia punctata, and Oesophagostomum radiatum for up to 28 day s post-treatment. (C) 1999 Elsevier Science B.V. All rights reserved.