Benzimidazole resistance in Haemonchus contortus in small ruminants: molecular mechanisms and diagnostic approaches.

Benzimidazole resistance in Haemonchus contortus represents a widespread challenge in small ruminant production worldwide. Benzimidazoles remain one of the most commonly used drug classes, due to their broad spectrum of activity, affordability, and ease of administration. However, extensive and often inappropriate use of these compounds has led to the rapid emergence and global spread of resistance, particularly in H. contortus, a highly pathogenic and genetically diverse gastrointestinal nematode. Resistance to benzimidazoles is primarily associated with point mutations in the isotype-1 β-tubulin gene, most notably at codons 167 (F167Y), 198 (E198A), and 200 (F200Y), which result in modifications to the β-tubulin that prevent drug binding and compromise treatment efficacy. This review summarises the molecular mechanisms underlying benzimidazole resistance and provides an overview of currently available diagnostic approaches, including in vivo and in vitro assays, as well as molecular techniques such as real-time PCR, digital PCR, pyrosequencing, deep amplicon sequencing, and isothermal amplification methods. Despite regional variation in the distribution of resistance-associated polymorphisms, there is a consistent global predominance of the F200Y mutation. Finally, this review highlights the importance of integrating molecular surveillance with sustainable parasite control strategies to slow the further spread of benzimidazole resistance and preserve the long-term efficacy of available anthelmintics.