Prevalence of anthelmintic resistant nematodes in sheep and goats in Western France

Abstract

Twenty-three sheep farms and 15 dairy goat farms, located in western France and randomly selected, were surveyed for gastrointestinal nematodes resistant to anthelmintics. On each of the 23 sheep farms, 60 lambs were randomly distributed into four groups of 15: untreated control group, fenbendazole (benzimidazole) group (5 mg/kg BW), ivermectin group (0.2 mg/kg BW) and levamisole hydrochloride group (6.8 mg/kg BW). On each of the 15 goat farms, 30 adult goats were allocated into two groups of 15: untreated control group and fenbendazole group (10 mg/kg BW). Faecal egg counts and larval cultures were done 10 to 14 days after anthelmintic treatment. Furthermore, egg hatch assays conducted with thiabendazole were performed on pooled faeces of fenbendazole treated goats, results being expressed as Logarithmic Concentration 50 (LC 50). Faecal egg count reductions (FECR) less than 95% are indicative of anthelmintic resistance and LC 50 values above 0.1 μg/ml suggest benzimidazole (BZ) resistance. On this basis, in the sheep survey, anthelmintic resistance occurred on 83% of farms for benzimidazole and on 50% of farms for levamisole. Post-treatment larval cultures indicated Teladorsagia/ Trichostrongylus and Cooperia were the predominant resistant genera. A negative relationship was found between FECR with fenbendazole and the average number of annual BZ treatments given to ewe-lambs ( P<0.01). In contrast, no case of ivermectin resistant nematodes was seen in the 23 sheep farms. In goat survey, virtually all the 15 surveyed farms showed the occurrence of benzimidazole resistant nematodes according to the results of both FECR test and egg hatch assay. In post-treatment coprocultures, Teladorsagia/ Trichostrongylus was the predominant larval type on seven farms, and Haemonchus on two farms, while on the remaining farms there was a well-balanced mixture of all the genera including Oesophagostomum/ Chabertia. Although no relationship could be established between drenching practices and results of in vivo and in vitro tests, the frequent and sometimes exclusive use of BZ compounds associated with a permanent underdosing of these drugs achieved by using sheep dose rates are probably the main causes of the very high frequency of BZ resistance in nematodes on goats. The consequences of anthelmintic resistance on sheep and goat parasitism management are discussed.