In vitro field screening for anthelmintic resistance in strongyles of sheep and horses

Simplified in vitro field methods are described for the detection and assay of benzimidazole-resistance in sheep trichostrongylids and horse strongyles. Worm eggs are recovered from fresh faeces, within 1 hour of collection, by flotation in sugar solution. The separated eggs are then incubated for 20–24 hours at 27–30° C in solutions of thiabendazole in distilled water ranging from 0.1 to 1.1 p.p.m. for sheep trichostrongylids and from 0.05 to 0.5 p.p.m. for horse strongyles. Under controlled conditions, eggs from thiabendazole-susceptible individuals of both sheep and horse nematodes rarely hatch at thiabendazole concentrations of 0.1 p.p.m. Eggs from resistant individuals will hatch at 0.1 p.p.m. and above. Semi-quantitative estimates of the level of resistance can be determined by measuring the % egg-hatch at varying concentrations of thiabendazole. A field method for selecting test animals with low egg-counts, and an in vitro method for the culture of eggs or first-stage larvae to third stage for identification are described.