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Anthelmintic resistance in sheep flocks in Canada

L. C. Falzon, P. I. Menzies, K. P. Shakya, A. Jones-Britton, J. Vanleeuwen, J. Avula and others

ANTHELMINTIC resistance (AR) of gastrointestinal nematodes (GIN) is an escalating problem in most countries that rear sheep. This study was conducted to determine the frequency of AR in GIN in sheep flocks in Ontario, Canada.

Forty-seven sheep flocks were enrolled in the study, conducted for two consecutive grazing seasons (2010 and 2011). Their level of parasitism was monitored monthly throughout the grazing season by analysing faecal samples taken from 15 grazing lambs per flock. When the mean GIN faecal egg count (FEC) had reached a threshold of 200 epg, oral ivermectin, an anthelmintic licensed in Canada for use in sheep, was supplied to producers to check its efficacy. The reduction in mean FEC 14 days after ivermectin treatment was calculated and ‘drench failure’ was defined as a reduction in mean FEC of less than 95 per cent. Approximately 42 (89 per cent) farms farms reached the FEC threshold of 200 epg. Thirty-nine farms then performed an ivermectin drench check and of these, 34 (87 per cent) had drench failure.

In 29 flocks with apparent drench failure, researchers performed an FEC reduction test (FECRT), dividing sheep …

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