Albendazole resistance in naturally infected sheep with gastrointestinal nematodes in two northwest municipalities of Sinaloa, Mexico.

The main aim of this study was to evaluate the anthelmintic resistance to albendazole (ABZ) (from the benzimidazole [BZ] family) in sheep farms located in the northeast of Sinaloa, Mexico. The study was conducted on four sheep farms in the northwest of Sinaloa, Mexico. The faecal egg count reduction test (FECRT), along with endpoint and allele-specific polymerase chain reaction techniques, were used to determine the percentage and proportion of BZ resistance in gastrointestinal nematode (GIN) species before and after ABZ treatment. Genomic DNA (gDNA) from infective larvae (L3) was processed to identify GIN species and detect nucleotide changes at codons 167 and 200 of the β-tubulin gene. Results from the FECRT showed low ABZ efficacy, ranging from 0 % to 83 % across the four sheep farms. Haemonchus was the most prevalent nematode identified, with a high number of gDNA copies detected both before and after ABZ treatment, followed by Cooperia, Trichostrongylus, and Oesophagostomum, but only before ABZ treatment. Additionally, ABZ BZ] polymorphisms were confirmed at codon 200 (TTC/TAC) in Haemonchus contortus (H. contortus) across all four farms, with both gDNA fragments detected: a 550-bp fragment indicating susceptibility and a 250-bp fragment indicating a heterozygous population for BZ resistance. No BZ polymorphism was observed at codon 167 (TTC/TAC) in any population. In conclusion, sheep farms in the northwest of Sinaloa demonstrated resistance to BZ, with H. contortus being the main GIN species involved. These findings highlight the importance of conducting parasitological and molecular diagnostics for anthelmintic resistance, as detecting heterozygous populations may help improve control methods and preserve ABZ efficacy.