Efficacy of closantel against ivermectin- and fenbendazole-resistant Haemonchus sp. in sheep in Ontario, Canada.
In Ontario, Canada, widespread resistance to ivermectin and fenbendazole, the only readily available ovine anthelmintics, has been documented, primarily in Haemonchus sp. In other parts of the world, closantel has been used to control such infections; however, the drug was not currently licensed for use in Canada and the USA. A randomized controlled trial was conducted on six client-owned farms in Ontario in 2013 and 2014 to determine the efficacy of closantel (Flukiver®5% Oral Suspension, Elanco Animal Health, 10mg/kg bodyweight) against ivermectin- and fenbendazole-resistant Haemonchus sp. infections in periparturient ewes and grazing lambs. Three farms were randomly assigned to treat all ewes, and three farms were randomly assigned to selectively treat individual ewes at lambing, using predetermined criteria. Fecal samples were collected from a minimum of 15 randomly selected ewes and 13 lambs per group on each farm at the time of treatment and approximately 14days later. Trichostrongyle-type fecal egg counts (FEC) were performed using a modified McMaster technique with a lower detection limit of 8.3 eggs per gram of feces (epg). Haemonchus-specific FECs were determined by multiplying FECs by the proportion of Haemonchus sp. identified from coproculture for each farm; Haemonchus-specific FEC reductions were calculated for each farm. Twenty grazing lambs had FECs conducted monthly, and when mean monthly FECs surpassed 200 epg, all lambs were randomly allocated to either closantel, positive control (ivermectin, fenbendazole, or levamisole) or negative control groups. Pre-treatment Haemonchus-specific mean FECs ranged from 27 to 3359 epg in ewes and 0-5698 epg in lambs. Efficacy of closantel against Haemonchus sp. ranged from 99% (95% CI: 97%-99%) to 100% in recently lambed ewes on all farms in both years (total n=274 ewes), and from 99% (95% CI: 98%-99%) to 100% in grazing lambs in both years on all but one farm (total n=171 lambs). On the latter farm, a whole flock treated farm, closantel efficacy in grazing lambs was 84% (95%CI: 81%-88%) in the first year, but 100% in the second year. Levamisole was effective against overall GIN in lambs on only two farms. Ivermectin and fenbendazole resistance continued to be present, particularly in Haemonchus sp. Closantel had excellent efficacy against Haemonchus sp. over the two year study period, regardless of treatment group, and therefore should be considered one viable component of sustainable integrated parasite control programs for farms with documented anthelmintic resistance and problems with haemonchosis.
Department of Population Medicine, Ontario Veterinary College, University of Guelph, 50 Stone Road E Guelph, Ontario N1G 2W1, Canada. Electronic address: twesters@uoguelph.ca.
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Westers
Department of Population Medicine, Ontario Veterinary College, University of Guelph, 50 Stone Road E Guelph, Ontario N1G 2W1, Canada.
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Jones-Bitton
Department of Population Medicine, Ontario Veterinary College, University of Guelph, 50 Stone Road E Guelph, Ontario N1G 2W1, Canada.
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Menzies
Centre for Veterinary Epidemiologic Research, Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island C1A 4P3, Canada.
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Van Leeuwen
Department of Population Medicine, Ontario Veterinary College, University of Guelph, 50 Stone Road E Guelph, Ontario N1G 2W1, Canada.
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Poljak
Department of Pathobiology, Ontario Veterinary College, University of Guelph, 50 Stone Road E Guelph, Ontario N1G 2W1, Canada.
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Peregrine
D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology*; D004341:Drug Evaluation / Q000662:veterinary; D005243:Feces / Q000469:parasitology; D005260:Female; D005273:Fenbendazole / Q000494:pharmacology; D041981:Gastrointestinal Tract / Q000469:parasitology; D006188:Haemonchiasis / Q000188:drug therapy / Q000469:parasitology / Q000662:veterinary*; D006189:Haemonchus / Q000187:drug effects*; D007559:Ivermectin / Q000494:pharmacology; D007978:Levamisole / Q000494:pharmacology; D009864:Ontario; D010270:Parasite Egg Count / Q000662:veterinary; D012458:Salicylanilides / Q000494:pharmacology*; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy* / Q000469:parasitology
D000871:Anthelmintics; D012458:Salicylanilides; D007978:Levamisole; D005273:Fenbendazole; D007559:Ivermectin; C023342:closantel
Anthelmintic resistance; Canada; Closantel; Fecal egg count reduction test; Gastrointestinal nematodes; Sheep