Benzimidazole resistance in equine small strongyles
Ryan, W.G.; Lumsden, G.G.; Smith, S.M.; Taylor, M.A.
Veterinary Record 121(21): 497
1987
ISSN/ISBN: 0042-4900 PMID: 3424635 Document Number: 303299
A group of 14 Exmoor ponies aged between 4 and 21 years that were grazing together on permanent pasture and had previously been treated regularly with various anthelmintics including fenbendazole were selected and matched according to faecal strongylate egg counts. The ponies were treated (day 0) with either fenbendazole 18.75% w/w paste at a dose rate of approximately 7.5 mg/kg body weight or with ivermectin 1.87% w/w paste at a dose rate of approximately 200 micro g/kg body weight. The geometric mean faecal egg counts of fenbendazole-treated and ivermectin-treated ponies were 654 and 615 epg, respectively on day -14, 1398 and 0 epg on day 7 and 332 and 0 epg on day 14. An in vitro hatch assay for anthelmintic sensitivity was carried out on eggs from faecal samples collected from 6 of the fenbendazole-treated ponies. The eggs were identified, by larval morphology, as Cyathostominae. The concentration of anthelmintic resulting in 50% egg mortality was greater than 0.1 ppm in 5 of the fenbendazole-treated ponies compared with 0.05 ppm in a control animal suggesting the presence of anthelmintic resistance.