Anthelmintic resistance in gastrointestinal nematodes of small ruminants: current status, molecular mechanisms, and sustainable control strategies.
Gastrointestinal nematode (GIN) infections represent one of the most significant constraints to the health and productivity of small ruminants worldwide, leading to substantial economic losses and animal welfare concerns. Control of these parasites has historically relied on the extensive use of anthelmintic drugs; however, the rapid emergence and global spread of anthelmintic resistance (AR) have compromised their effectiveness. Resistance is now reported across all major classes of anthelmintics and is particularly prevalent in key species such as Haemonchus contortus. The development of AR is driven by complex mechanisms, including genetic mutations in drug target genes, enhanced drug efflux mediated by transport proteins, alterations in receptor function, and polygenic adaptations. Accurate detection of resistance remains challenging, with conventional methods such as fecal egg count reduction tests and in vitro assays offering limited sensitivity, while molecular diagnostics provide improved specificity but remain constrained by technical and economic factors. Sustainable control of GIN infections requires a shift toward integrated parasite management strategies. Approaches such as targeted selective treatment, maintenance of refugia, improved nutrition, genetic selection for host resistance, and alternative control methods including phytotherapy and biological control have shown promise in reducing reliance on anthelmintics. In conclusion, a multidisciplinary and sustainable approach is essential to mitigate the impact of AR and ensure long-term effectiveness of parasite control strategies in small ruminant production systems.