Benzimidazole-resistance in gastrointestinal nematodes of sheep and goats in Switzerland.

The occurrence of gastrointestinal nematodes, resi­stant against benzimidazoles, was investigated in twenty randomly selected sheep and goat farms in Switzerland using the larval hatch assay and the faecal egg count reductions test (FECRT). Benzimidazole-resistant tricho­strongyles were detected in fifteen farms (75 %). The degree of resistance was moderate to high in the majority of the farms, the mean reduction of the faecal egg counts after treatment was only 66 %. The LDso averaged at 0.44 µg/ml thiabendazole (cut off: 0.1 µg/ml). In those thirteen farms, in which both tests were carried out, the results were corresponding. A benzimidazole-resistant population of Haemonchus contortus (LDso 0.62 µg/ml) was isolated for the first time in Switzerland. This parasite was the domi­ nating resistant species in the majority of the farms. Al­though this survey is not representative for the very heterogenous sheep and goat husbandry systems in Switzer­land, the rate of farms which had to be classified as resi­stant is alarming. The frequent movements of sheep and goats most likely contribute considerably to the spread of resistant populations between flocks. In particular, the common practice of grazing animals from different farms an alpine pastures and the integration of untreated animals into new flocks represent major risk factors. lt is recom­mended, that the efficacy of benzimidazoles should be checked regularly with the FECRT in sheep and goat farms.