| Summary: | Abstract The aim of this study was to diagnose anthelmintic resistance (AR) of gastrointestinal nematodes to different anthelmintics in crossbred Dorper X Santa Inês sheep using the new research guidelines of the World Association for the Advancement of Veterinary Parasitology. After estimating and identifying an approximate number of 10,000 sheep in the study area, a total of 256 sheep at different reproductive stages, presenting faecal egg count per gram of faeces (FEC) ≥ 400 were used. The faecal egg count reduction test (FECRT) was carried out to evaluate the pre- and post-treatment of anthelmintics, including albendazole, closantel, disophenol, ivermectin, levamisole, monepantel, moxidectin and trichlorfon, using the new classification criteria to analyze the results of an FECRT, which defined three possible classifications: susceptible, resistant and inconclusive. The highest average FEC was observed when the ewes were lactating, while the lowest FEC was observed in ewe lambs. Resistance was observed for albendazole, closantel, disophenol, ivermectin, levamisole and moxidectin. Two drugs (trichlorfon and monepantel) proved to be highly effective in treating gastrointestinal nematodes, presenting CI varying from 98 to 100% and 97 to 100%, respectively. Gastrointestinal nematodes from sheep raised in the in the Rio Grande do Norte State, Brazil are susceptible to the active principles trichlorfon and monepantel.
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