Comparison of in vitro methods and faecal egg count reduction test for the detection of benzimidazole resistance in small strongyles of horses.

The objective of the study was to compare the in vitro egg hatch test (EHT), larval development test (LDT) and in vivo faecal egg count reduction test (FECR test) for the detection of benzimidazole resistance in equine strongyles. The presence of resistant or susceptible strongyle populations was determined in 25 stud farms using the in vivo FECR test and in vitro EHT. On the basis of the FECR values, resistance to fenbendazole was detected on 15 of the 25 farms (60%). The ED50 value (anthelmintic concentration producing 50% inhibition of hatching) for suspected resistant populations varied from 0.110 to 0.222 microg/ml thiabendazole (TBZ). Final LD50 values (anthelmintic concentration inhibiting development of 50% of eggs into L3 infective larvae) above 0.029 microg/ml TBZ in the in vitro larval development test on samples from 11 stud farms revealed the presence of populations of small strongyles suspected of being benzimidazole-resistant.