Evaluation of a larval development assay for the detection of anthelmintic resistance in Ostertagia circumcincta

Two experiments were carried out to evaluate a larval development assay for the detection of anthelmintic resistance in O. circumcincta. In Experiment I, the dose responses to levamisole (LEV), thiabendazole (TBZ) and ivermectin (IVM) of 8 isolates of O. circumcincta were measured 34 days after infection (DAI). Four of these isolates were shown to be resistant to 1 or more anthelmintics. With 2 exceptions, all isolates considered to be resistant had higher LD 50 values than the susceptible isolates for that anthelmintic. One exception was isolate RM8, which was considered to be resistant to all 3 anthelmintics based on faecal egg count reduction tests in goats, but the LD 50 value for LEV did not differ from that for the susceptible isolates. The other exception was an isolate comidered to be susceptible to TBZ which had a relatively high LD 50 value. In an unrelated trial that was prompted by this finding, this isolate was confirmed to be benzimidazole-resistant. Isolate RM8 and an isolate susceptible to all 3 anthelmintics (SK2) were used in the second experiment, which was conducted to monitor changes in the LD 50 values of LEV, TBZ and IVM over time following a single infection of 35 000 infective larvae in young sheep. Faecal samples were collected weekly from 24 to 115 DAI. With all 3 anthelmintics, the LD 50 values increased with time to a peak around 50–60 DAI, and then declined to levels similar to those observed soon after patency. This trend was consistent for both isolates. The highest mean LD 50 values for isolates SK2 for IVM and TBZ and RM8 for IVM and RM8, respectively, were 1.7 and 1.8 times, and 2.2 and 2.9 times higher than the initial mean LD 50 values. There was a clear distinction in LD 50 values between isolates at each sampling day for both IVM and TBZ. However, as a consequence of the changes in LD 50 values with time, the peak LD 50 values of IVM for isolate SK2 were higher than the minimum LD 50 values of isolate RM8. As there was no apparent difference in LEV efficacy between these 2 isolates, the data were pooled. The highest mean LD 50 value was 2.3 times higher than the initial LD 50 value.