Detection of anthelmintic resistant nematodes in sheep in southern England by a faecal egg count reduction test.

On five farms identified by in vitro tests to have benzimidazole resistance, and on one farm with apparent anthelmintic failure, faecal egg count reduction tests were performed using benzimidazoles, levamisole and ivermectin. With benzimidazoles efficacy was less than 70 per cent on all farms, confirming the validity of the in vitro tests to detect benzimidazole resistance. On two farms levamisole had an efficacy of less than 90 per cent but laboratory tests failed to demonstrate levamisole resistance. Ivermectin reduced egg counts by 100 per cent on all farms.