Comparison of the persistent efficacy of the injectable and pour-on formulations of doramectin against artificially-induced infection with Dictyocaulus viviparus in cattle

The persistent efficacy of the injectable and topical formulations of doram ectin was compared against experimental challenges with infective larvae of Dictyocaulus viviparus in two separate studies. Four groups of 10 randomly -assigned calves, negative for lungworm larvae by the Baermann technique, w ere used in each study. Calves were treated subcutaneously in the midline o f the neck or poured down the midline of the back with saline (1 ml/50 kg, injection; 1 ml/10 kg, pour-on) on Day 0 or doramectin (200 mu g/kg = 1 ml/ 50 kg, injection; 500 mu g/kg = 1 ml/10 kg, pour-on) on Day 0, 7, or 14. Tw o additional calves from the same pool of animals were randomly assigned as larval-viability monitors and received no treatment. Calves were inoculate d daily with a gavage of similar to 100 larvae of D. viviparus from days 35 to 49 for the injectable study and days 28 to 42 for the pour-on study. Th e two larval viability monitor calves received similar to 3000 infective la rvae in the same manner on Day 49 or 42 for the injectable and pour-on stud ies, respectively. Equal numbers of calves from each treatment group as wel l as the larval viability monitor calves were necropsied on days 14 and 15 after the last lungworm inoculation to enumerate the worm burden. The worms recovered were quantified and identified. For each study, geometric mean w orm recoveries for each treatment group were back transformed from the natu ral log-transformed data (worm count +1) and were used to estimate percenta ge reduction. Doramectin injectable solution was 100.0% efficacious against lungworms for up to 49 days and the pour-on formulation was 100.0%, 93.1% and 81.5% effective in reducing lungworm infection resulting from challenge infection for up to 28, 35, and 42 days post-treatment, respectively. (C) 1999 Elsevier Science B.V. AU rights reserved.