Standardization of the larval migration inhibition test for the detection of resistance to ivermectin in gastro intestinal nematodes of ruminants.

Resistance to anthelmintics is an increasing problem in sheep, goat and cattle industries worldwide. For parasite management on farms reliable methods for the detection of resistance are required and it is important that tests give comparable, reproducible and reliable results in different laboratories. The ability of the larval migration inhibition test (LMIT), to detect ivermectin resistance in cattle and sheep nematodes has been evaluated through ring testing in up to six different laboratories in Europe, supported by an EU sixth Framework Project (PARASOL). Third stage larvae of Ostertagia ostertagi, Cooperia oncophora, and Haemonchus contortus with a known resistance status were obtained from faecal cultures of experimentally infected calves and sheep. Following a series of ring tests using identical protocols, reproducible results were obtained within and between participating laboratories. In all tests dose-response curves with R(2) values >0.90 were obtained by all laboratories. Resistance ratios of 8.3 and 8.4 were found when susceptible and IVM-resistant isolates of C. oncophora and H. contortus were compared and differences in the EC(50) values were highly significant (p<0.0001). Protocols for the LMIT and the preparation of ivermectin solutions are described in a supplementary file.