Long-acting moxidectin for the control of trichostrongylid infections of sheep in south-eastern Australia.

Abstract

OBJECTIVE: To compare the effectiveness of a single long-acting injection of moxidectin, given in either summer or winter, with a two-summer ('standard') treatment strategy for controlling gastrointestinal nematodes in spring-lambing Merino ewes. DESIGN: A replicated study over 2 years in the high winter rainfall environment of western Victoria. METHODS: Measures of worm infection were worm egg counts of ewes and total worm counts of tracer lambs. Measures of production were body weight, proportion of pregnant ewes and number of weaned lambs. RESULTS: The egg counts of ewes given the standard strategy followed a characteristic pattern, rising to 300 to 600 eggs per gram before the second summer treatment in February. During this time, ewes given long-acting moxidectin in December had zero or negligible counts. There were no consistent differences between tracer worm counts or production measures for the two groups treated in summer, but when data from both years were pooled, the total egg output from December to May was significantly lower for ewes treated with long-acting moxidectin in December. Ewes not treated during summer had lower body weights, and higher worm egg counts and tracer worm counts, showing that this was not a suitable strategy at that time of the year. However, following treatment with long-acting moxidectin in winter, ewes had rapid compensatory weight gain and lower egg output than ewes treated in summer. CONCLUSION: A single injection of long-acting moxidectin in December is an effective treatment strategy for Merino ewes lambing in spring.