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

The occurrence of gastrointestinal nematodes, resistant against benzimidazo les, was investigated in twenty randomly selected sheep and goat farms in S witzerland using the larval hatch assay and the faecal egg count reductions test (FECRT). Benzimidazole-resistant tricho-strongyles were detected in f ifteen farms (75 %). The degree of resistance was moderate to high in the m ajority of the farms, the mean reduction of the faecal egg counts after tre atment was only 66 %. The LD50 averaged at 0.44 mu g/ml thiabendazole (cut off: 0.1 mu g/ml), In those thirteen farms, in which both tests were carrie d out, the results were corresponding. A benzimidazole-resistant population of Haemonchus contortus (LD50 0.62 mu g/ml) was isolated for the first tim e in Switzerland. This parasite was the dominating 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 sp read of resistant populations between flocks. in particular, the common pra ctice of grazing animals from different farms on alpine pastures and the in tegration of untreated animals into new flocks represent major risk factors . It is recommended, that the efficacy of benzimidazoles should be checked regularly with the FECRT in sheep and goat farms.