Assessing the benzimidazole resistance in equine strongyles by in vitro methods

The study aimed to conduct a survey on the occurrence of benzimidazole (BZ) resistance in strongyles by in vitro egg hatch test (EHT) and larval development test (LDT) and to identify the effective indicators of early resistance detection on horse farms with associated risk factors analysis appraisal. In total, 203 horses from 8 farms underwent the fecal sampling of which 77 horses were selected for in vitro testing. Simultaneously, 18 horses were chosen to analyse the results of in vitro tests compared to the in vivo fecal egg count reduction test (FECRT). The EHT indicated the presence of resistant strongyles in all farms with an exceeded threshold of 0.1 μg/ml thiabendazole (TBZ) in 43 horses (55.84 %). The percent egg hatch at 0.1 μg/ml TBZ in "resistant parasites" varies from 39.0 ± 6.0 % to 91.0 ± 1.0 %. The hatching range between 8.5 ± 1.5 % and 50.0 ± 6.0 % at a concentration of 0.1 μg/ml TBZ was detected wherein the recommended threshold was not exceeded. The analysis of the EHT results demonstrated that egg hatching at the 0.1 µg/ml TBZ concentration was identified as the most important predictor of the early detection of BZ resistance. In the LDT, the mean TBZ concentration which interrupted the development of 50 % and 99 % larvae to the infective stage (L3) from all horses was 0.0753 ± 0.0454 and 0.6798 ± 1.9144 μg/ml TBZ, respectively. Only cyathostomin L3 were found at TBZ concentrations ≥ 0.08 μg/ml and only in samples from 81.8 % of horses (36/44). A comparison of LDT results did not show a statistically significant agreement with EHT and FECRT. In conclusion, the in vitro EHT could be implemented as an indicator for early BZ resistance detection and showed that monitoring of hatching at selected concentrations could detect presence and estimate the proportion of the resistant parasite population on the horse farms.