Anthelmintic Resistance Surveillance in Haemonchus contortus Populations on Small Ruminant Farms in Oyo State, South West Nigeria

Haemonchus contortus, the barber's pole worm, is the most economically important gastrointestinal nematode of small ruminants globally, and the progressive spread of anthelmintic resistance severely compromises disease control in Nigerian sheep and goat production systems. This study conducts systematic anthelmintic resistance surveillance in H. contortus populations on small ruminant farms in Oyo State, South West Nigeria. An analytical cross-sectional study combined with in vivo and in vitro resistance testing was employed across 25 farms in Ogbomosho, Saki West, and Ibarapa North Local Government Areas. Larval development assays using thiabendazole, levamisole, and ivermectin at discriminating dose concentrations were performed on third-stage larvae cultured from pooled farm faecal samples. In vivo faecal egg count reduction tests were conducted on 200 sheep and goats treated with each drug class. Larval migration inhibition assay was used for ivermectin resistance phenotyping. Logistic regression modelled farm management factors associated with resistance phenotype. Evidence from 2020 to 2024 documents confirmed benzimidazole resistance in H. contortus on Nigerian small ruminant farms in southwestern states, with emerging ivermectin resistance signals. This study provides Oyo State-specific resistance prevalence data essential for regional anthelmintic stewardship guidelines. Findings will be submitted to the Oyo State Ministry of Agriculture and published in a veterinary parasitology journal. Keywords: Haemonchus contortus, anthelmintic resistance, small ruminants, Oyo State, resistance surveillance.