The macrocyclic lactone eprinomectin has a null milk withdrawal period, and plays a key role in limiting infection by gastro-intestinal nematodes (GIN) in dairy sheep. Starting in 2019, a cluster of suspicions of lack of effectiveness of eprinomectin (EPN) was observed in the western area of the French Pyrenees. First, we conducted fecal egg count reduction tests in 47 dairy sheep farms of this area. They revealed that 29/47 (62 %) farms were facing resistance to eprinomectin. Post-treatment GIN species identification conducted in 21 of the 29 resistance cases showed Haemonchus contortus was the only resistant parasite to EPN treatment. A descriptive analyses of information on GIN and farm management was conducted, with regard to the spread of eprinomection resistance in these farms. We found that transhumance could be linked to the presence of eprinomectin resistance in GIN of the dairy sheep farms. Second, GIN monitoring was conducted during one year within 2 flocks grazing together during the summer transhumance. Ewes got infected with moderate levels of H. contortus in less than 2 months on pastures at 1400–1800 m of altitude. In addition, transfer of parasites between flocks seems to occur during summer grazing. It might contribute to change the type of helminthiosis farmers are used to face in their flocks, and to the transfer of resistant strains of parasites between flocks. Our study highlights that pre-transhumance anthelmintic treatment no longer guaranties low GIN levels during summer grazing, and that GIN infections have to be anticipated in the spring by means of integrated parasite management.
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