Preserving levamisole efficacy with targeted selective treatment: Long-term field evidence from small ruminant farms.

To demonstrate how targeted selective treatment (TST) can preserve anthelmintic (AH) effectiveness against gastrointestinal nematodes (GIN), this report describes the resistance status of levamisole (LEV) on three farms in Mexico and Brazil. We compiled historical results from Faecal Egg Count Reduction Tests (FECRT) on two university farms (UADY and PUCPR) and one commercial smallholder farm where LEV has been used in a TST scheme for over a decade. The results reveal a remarkable stability in the anthelmintic's efficacy over many years. On the UADY farm, where LEV has been the sole anthelmintic used since 2001, the faecal egg count reduction (%FECR) remained above 80% in all evaluations until dropping to 74% (95% CI: 49-91) in 2024, when a combination of LEV+benzimidazole proved to be 100% effective. Data from the PUCPR farm showed that LEV efficacy was sustained at a high level (97%) from 2014 to 2020 but decreased to 64% (95% CI: 3-87) in 2022. In 2023, combinations of LEV+moxidectin and LEV+closantel showed over 98% efficacy. Meanwhile, the commercial sheep farm demonstrated the greatest stability, sustaining an efficacy between 91% and 99% throughout the entire ten-year evaluation period (2015-2024). The historic data from the three farms suggest that TST schemes helps to maintain constant AH efficacy profiles over long periods.