Use of two in vitro methods for the detection of benzimidazole resistance in equine small strongyles ( Cyathostoma spp.)

Abstract

Ten stables were included in a study to evaluate two in vitro methods for the detection of anthelmintic resistance in cyathostomes by comparing a faecal egg count reduction test (FECRT) to a larval development assay (LDA) and an egg hatch assay (EHA). The LDA was used in seven stables and EHA in the last three. On the basis of FECR values, resistance to benzimidazoles was detected in eight of the ten small strongyle populations. Resistance to pyrantel pamoate and ivermectin was not detected. The mean concentrations that inhibited hatching in 50% of the eggs (EC 50), using thiabendazole (TBZ) in an EHA, were 1.02 μM in resistant populations and 0.37 μM in susceptible or suspected resistant ones. In the LDA, TBZ concentrations preventing 50% development by first/second stage larvae to the third larval stage (LC 50) were 3.8 times lower than EC 50 values in resistant worm populations. Mean LC 50 for morantel, levamisol, ivermectin monosaccharide and avermectin-B2 in small strongyle population ssusceptible to pyrantel and ivermectin was 8.0 μM, 0.99 μM, 15.6 nM and 2.93 nM, respectively. Data on pyrantel and ivermectin resistant populations could not be obtained as no resistant populations were detected. This study concludes that in vitro tests may be useful as a supplement to FECRT for the detection of benzimidazole resistance in cyathostomes, even if reference populations to be used as controls in the assays were not available. It is suggested that EC 50 values for TBZ > 0.6 μM in LDA and > 0.5 μM in EHA strongly indicate benzimidazole resistance in equine small strongyles.