Effect of targeted selective treatement with closantel at lambing on Haemonchus burdens on Ontario sheep farms with anthelmintic resistance

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Haemonchus contortus is increasingly being recognised as a significant constraint to sheep production on pasture in Ontario; it is a significant cause of morbidity and mortality in lambs and adult sheep from the effects of profound anaemia and hypoproteinemia. Recent work in the province has demonstrated a high prevalence of resistance in this parasite to the two most commonly used anthelmintics in sheep, i.e. resistance to ivermectin and fenbendazole has been demonstrated on >90% of tested farms. Only ivermectin is approved for use in sheep, and benzimidazoles (i.e. fenbendazole and albendazole) are only available when used in an extra label manner. No other anthelmintics are available for use in sheep in Canada. As a result, farms with multiple drug resistance have no reasonable pharmaceutical alternatives for parasite treatment. Improved control strategies are therefore urgently required. This project will define the parameters that influence the infectivity of H. contortus on pasture and thus help enhance control programs for the parasite. In addition, via a proof of concept study, the ability of closantel (a narrow-spectrum anthelmintic with activity against H. contortus, not previously used in Canada) to control drug-resistant H. contortus infections will be assessed. Since ewes are the primary source of pasture contamination with H. contortus in the spring due to the periparturient egg rise induced by lambing and lactation, two treatment strategies for ewes at lambing will be evaluated, i.e. whole flock versus targeted selective treatment of ewes. Outcomes assessment will include ability of each treatment strategy to control infection and disease in the ewes and in their lambs, and to control contamination of grazing pastures - whilst not increasing the proportion of H. contortus showing resistance to closantel. The resultant data will help to generate novel strategies for controlling drug-resistant infections and for reducing anthelmintic use on sheep farms, thus contributing to more prudent use of anthelmintics in Ontario flocks. The data will also assist in the approval of a new anthelmintic for use in sheep in Canada.