Prevalence of anthelmintic resistance on sheep farms in New Zealand

Authors: Pomroy WE, Moffat JR, Jackson R, Waghorn TS, Rhodes AP, Leathwick DM, West DM, Lawrence KE
Publication: New Zealand Veterinary Journal, Volume 54, Issue 6, pp 271-277, Dec 2006
Publisher: Taylor and Francis

Animal type: Livestock, Production animal, Ruminant, Sheep
Subject Terms: Abdomen, Alimentary system/gastroenterology, Animal production/wastage, Animal remedies/veterinary medicines, Anthelmintics, Parasite control, Parasites - internal, Biosecurity, Diagnostic procedures, Diet/rations/food, Disease surveillance, Epidemiology, Nematode, Farm/farm management, Survey, Management, Grazing, Evidence/information based methods, Nutrition/metabolism
Article class: Scientific Article

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Abstract: AIM: To establish the prevalence of anthelmintic resistance in parasitic nematodes on sheep farms in New Zealand.
METHODS: A cross-sectional prevalence study was conducted, using a standardised faecal nematode egg count (FEC) reduction (FECR) test (FECRT) for ivermectin, at a full (0.2 mg/kg) and half (0.1 mg/kg) dose rate, and albendazole, levamisole and albendazole-levamisole in combination, on 60 lambs (n=10 per group) on farms selected from throughout New Zealand. Farms that conformed with selection criteria were chosen at random (n=80) or with a history of suspected resistance to macrocyclic lactone (ML) anthelmintics (n=32). Resistance to an anthelmintic was inferred when there was <95% reduction in FEC 7–10 days after treatment. Larval cultures were performed for all control groups and for treated groups for which resistance was evident.
RESULTS: Of the farms randomly selected, 36% showed ≥95% FECR for all anthelmintics tested; resistance to ivermectin at 0.1 and 0.2 mg/kg liveweight was evident on 36% and 25% of these farms, respectively. Resistance to both ivermectin (0.2 mg/kg) and levamisole was evident on 8/80 (10%) farms, to ivermectin and albendazole on 10/80 (13%) farms, and to ivermectin, levamisole and albendazole on 6/80 (8%) farms. The prevalence of resistance to a half dose of ivermectin tended to be more prevalent on farms with a history of suspected ML resistance (p=0.06). Resistance to albendazole was seen across all the main parasite genera, and to levamisole in Nematodirus, Ostertagia (= Teladorsagia) and Trichostrongylus species. Resistance to ivermectin was dominated by Ostertagia spp, although Cooperia, Nematodirus and Trichostrongylus species were also implicated.
CONCLUSION: Anthelmintic resistance in parasitic nematodes of sheep is common in New Zealand. Not only was resistance to albendazole and levamisole common, but resistance to the ML, ivermectin, was at a higher prevalence than expected. Sheep farmers and advisors in New Zealand need to re-evaluate the way they manage parasites, and more research is urgently needed if the steady decline in anthelmintic susceptibility is to be halted.
KEY WORDS: Sheep, anthelmintic resistance, survey, macrocyclic lactone, ivermectin, benzimidazole, albendazole, levamisole

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