Anthelmintic resistance profile of sheep in the central region of Rio Grande do Sul, Brazil

Abstract

ABSTRACT Gastrointestinal parasites, particularly Haemonchus contortus, cause significant economic losses in Brazilian sheep production. Rio Grande do Sul (RS) provides favorable conditions for both sheep farming and parasitic infections. This study evaluated the efficacy of various anthelmintics on 20 farms from different municipalities using various techniques. Helminth eggs (per gram) in feces were counted and observed using light microscopy to identify any Strongylid genera. Additionally, a fecal egg count reduction test was used to determine the anthelmintic resistance profiles. Samples were collected on the day of treatment, and then seven and 14 days after treatment. The drugs assessed were albendazole, closantel, disophenol, levamisole, and moxidectin, with some farms using combinations of these drugs. Effectiveness was defined as an egg count reduction of > 95%. The results showed multidrug resistance on most farms, with only 30% (6/20) exhibiting effective drugs when administered individually. Haemonchus sp. was the dominant genus, followed by Trichostrongylus and Cooperia. Drug combinations improved efficacy, achieving an average reduction of 90%, highlighting their potential as an alternative therapeutic strategy when single drug administration fails. Targeted anthelmintic combinations integrated with sustainable management practices are essential for mitigating the negative effects of drug resistance on sheep health and productivity in central RS.