PERSISTENT EFFICACY OF DORAMECTIN AGAINST EXPERIMENTAL NEMATODE INFECTIONS IN CALVES

Three studies were conducted involving cattle exposed to experimental nematode infections. These studies were designed to investigate the pr ophylactic activity of a single subcutaneous treatment of doramectin a t 200 mug kg-1 body weight against infections of Ostertagia ostertagi, Cooperia oncophora and Dictyocaulus viviparus. For each study, parasi te-naive calves were randomly allocated to either a treated or a match ed control group. One group received doramectin and the other received either no treatment or an injection of saline at 1 ml per 50 kg body weight by the subcutaneous route. Thereafter, all calves received a da ily oral challenge of infective larvae of the particular parasite spec ies on test in each study. Challenge of each pair of treatment/control groups continued for periods of 14, 21 or 28 days. An interval of 14- 21 days was then allowed to permit the parasites which had established to mature, after which all animals were slaughtered and their worm bu rdens determined using standard techniques. Geometric mean worm burden s were calculated from the log worm counts and used to estimate percen tage efficacy. Accumulated burdens of C oncophora in doramectin-treate d cattle resulting from a daily challenge infection for 14 or 21 days were reduced by 99.2% and 90.7% respectively, in comparison with those of non-treated control animals. For D. viviparus, burdens were reduce d by 100% and 99.9% after a 21 or 28 day challenge, respectively. The corresponding figures for 0. ostertagi were 99.9% after a 21 day chall enge and 93.7% after a 28 day challenge.