Three-year evaluation of anthelmintic treatment efficacy against equine strongyles in France using FECRT

Reduced anthelmintic efficacy against strongyles in equids is a widespread concern, but up-to-date field data are scarce. The aim of this study was to evaluate, over a three-year period, the efficacy of the anthelmintic (AH) molecules available in France (fenbendazole, pyrantel, ivermectin and moxidectin) against strongyles in equids using faecal egg count reduction tests (FECRTs). A total of 104 complete FECRTs (i.e., required sample size reached) were conducted in 98 groups, involving 749 equids kept in 70 facilities. Fenbendazole-resistant populations were detected in 8/9 groups (89%) from 7/8 facilities (88%; upper 90% CI range: 30.5%-83%) and pyrantel-resistant populations in 15/22 groups (68%) from 12/17 facilities (71%; upper 90% CI range: 56%-97%). Regarding macrocyclic lactone efficacy, ivermectin-resistant populations were identified in 14/51 groups (27%) kept in 11/41 facilities (27%; upper 90% CI range: 85.6%-99.9%) and moxidectin-resistant populations in 5/22 groups (23%) kept in 4/18 facilities (22%; upper 90% CI range: 85.3%-99.7%). Resistance was mainly detected in the Normandy region and in breeding farms (25/70), but also in riding schools offering boarding services (3/70), experimental platforms (2/70), and a zoological park (1/70). In conclusion, this study provides the first evidence in France of resistance to all four available AH molecules and documents the spread of these resistances across the facilities. It also highlights the urgent need for regulatory changes requiring the use of AH only after a diagnostic assessment (e.g., targeted selective treatment based on faecal egg count results) in order to reduce the selective pressure exerted on parasite populations.