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 assa y for the detection of anthelmintic resistance in O. circumcincta. In Experiment I, the dose responses to levamisole (LEV), thiabendazole (T BZ) and ivermectin (IVM) of 8 isolates of O. circumcincta were measure d 34 days after infection (DAI). Four of these isolates were shown to be resistant to 1 or more anthelmintics. With 2 exceptions, all isolat es considered to be resistant had higher LD50 values than the suscepti ble isolates for that anthelmintic. One exception was isolate RM8, whi ch was considered to be resistant to all 3 anthelmintics based on faec al egg count reduction tests in goats, but the LD50 value for LEV did not differ from that for the susceptible isolates. The other exception was an isolate considered to be susceptible to TBZ which had a relati vely high LD50 value. In an unrelated trial that was prompted by this finding, this isolate was confirmed to be benzimidazole-resistant. Iso late RM8 and an isolate susceptible to all 3 anthelmintics (SK2) were used in the second experiment, which was conducted to monitor changes in the LD50 values of LEV, TBZ and IVM over time following a single in fection 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 dec lined to levels similar to those observed soon after patency. This tre nd was consistent for both isolates. The highest mean LD50 values for isolates SK2 for IVM and TBZ and RM8 for IVM and RM8, respectively, we re 1.7 and 1.8 times, and 2.2 and 2.9 times higher than the initial me an LD50 values. There was a clear distinction in LD50 values between i solates at each sampling day for both IVM and TBZ. However, as a conse quence of the changes in LD50 values with time, the peak LD50 values o f IVM for isolate SK2 were higher than the minimum LD50 values of isol ate RM8. As there was no apparent difference in LEV efficacy between t hese 2 isolates, the data were pooled. The highest mean LD50 value was 2.3 times higher than the initial LD50 value. (C) 1997 Australian Soc iety for Parasitology.