Moxidectin resistance in gastrointestinal nematodes in sheep

Abstract

This study focused primarily on the resistance of Haemonchus contortus, the most well known gastrointestinal nematode in sheep in the Netherlands, for moxidectin. The goal of this study was to determine the extent of the resistance problem in the Netherlands, focusing on sheep farms in Limburg, Noord-Brabant and Utrecht. To determine the extent of the occurrence of resistance at least 20 farms need to participate. FECRT (Faecal Egg Count Reduction Test) was used to assess the efficiency of moxidectin in these herds. During the first visit (T0) a mixed sample of faeces was collected and examined of the ewes and lambs. In case the mixed samples contained a EPG (Eggs per gram faeces) of ≥50, a second visit was planned. When visiting the second time individual faeces samples were taken (T1) and the sheep were wormed. About 10-14 days later a third visit was paid (T2) to take another individual faeces sample of the same sheep, in order to compare the EPG from this samples with the EPG of the samples taken at T1. Eventually 19 herds of ewes and 4 herds of lambs participated in the study. In case the decrease between T1 and T2 was less than 95% and the bottom line of the 95% confidence interval was less than 90%, resistance was demonstrated. When only one of the two criteria was met, the farm was suspected of resistance. Two herds of ewes and two herds of lambs showed a reduction of 95%. This study demonstrated the resistance for moxidectin in four herds. Two herds are suspected of resistance.