Worm worries : gastrointestinal nematodes, anthelmintic resistance, and worm management options

Abstract

Gastrointestinal nematodes cause significant morbidity and mortality in sheep. Anthelmintics are essential for worm management, but the sole reliance on these drugs to manage infections has led to widespread anthelmintic resistance (AR). Ivermectin is the only anthelmintic licenced for use in sheep in Canada, although off-label use of benzimidazoles and levamisole is common. In Nova Scotia during 2012, many farmers reported sheep deaths due to worms, despite anthelmintic treatment, and there is no information about AR in Eastern Canada. Haemonchus contortus has become the most problematic species in Nova Scotia. Closantel, a narrow spectrum drug that kills only H. contortus, was accessed through the Emergency Drug Release process. AR was investigated through Fecal Egg Count Reduction Tests on 13 farms over the grazing seasons of 2013 and 2014. Resistance to ivermectin, moxidectin, benzimidazoles, and levamisole was found on 5/6, 0/1, 6/7, and 0/8 farms, respectively. Post - treatment larval cultures showed that H. contortus was resistant to ivermectin and benzimidazoles, whereas closantel was effective against H. contortus and provided adequate worm control for several weeks. Levamisole was effective, but its short duration of activity necessitated more frequent treatments. Albendazole in combination with closantel was highly effective against all species, and drug combinations offer a means to extend the useful lifetime of both drugs. Seasonal monitoring showed a high peak in lamb infection levels in July - August of 2013 and 2014. Grazing lambs with and without ewes was investigated in 2014. Lambs grazed without ewes did not require treatment, whereas lambs grazed with ewes required treatment at the end of August. This grazing strategy may be an appealing and cost effective option to limit infections with H. contortus. To ensure the sustainability of sheep production in Nova Scotia, a transition to using anthelmintic combinations and incorporating alternative non-drug management strategies will be essential.