Anthelmintic resistance in sheep in the semiarid region of Minas Gerais, Brazil.

Anthelmintic efficacy was evaluated among sheep that had become naturally infected with gastrointestinal nematodes in 17 flocks located in the semiarid region of Minas Gerais, Brazil. Feces were collected individually from 1021 hairy sheep to determine the number of eggs per gram of feces (EPG) and for coprocultures to identify nematode genera the nematodes. Only the animals that presented EPG counts greater than or equal to 200 were included in the study (totaling 381 sheep). The animals were divided into three treatment groups: albendazole, ivermectin and levamisole. Fourteen days after the administration of anthelmintics, fecal samples were taken from all animals. In each flock, the pre-treatment and post-treatment arithmetic mean EPG were used to calculate the efficacy (FECR) for each of the treatment groups and the lower 95% confidence limit. Data were analyzed with the "eggCounts 2.3" package in RStudio, using a Bayesian model for paired design. The anthelmintics were classified as being efficacious (when the FECR was both equal to or above 95% and the lower 95% confidence limit was equal to or above 90%) or as encountering anthelmintic resistance (when the FECR was below 95% and the lower 95% confidence limit was below 90%) or inconclusive (when none of the other criteria were fulfilled). Albendazole and ivermectin were not effective in any of the flocks. Levamisole was effective against gastrointestinal nematodes in 25% of the flocks studied. Haemonchus, Trichostrongylus and Oesophagostomum genera were identified in this study in a semiarid region of Minas Gerais, Brazil. The genus Haemonchus was the most prevalent, followed by Trichostrongylus and Oesophagostomum. After anthelmintic treatment, the most prevalent genus was Haemonchus, followed by Trichostrongylus; the genus Oesophagostomum was not detected. The highest percentage of Haemonchus larvae was observed after treatment with ivermectin, followed by albendazole and levamisole. This study revealed the existence of gastrointestinal nematodes in sheep that present multiple resistance to all three main classes of anthelmintic drugs.