Evaluation of a laboratory test to detect resistance to closantel in Haemonchus contortus

Objective to evaluate a laboratory test for closantel resistance in Haemonc hus contortus. Procedure Field isolates of H contortus, known to be resistant to closantel , were tested in the assay. In addition, mixtures of closantel-susceptible and closantel-resistant laboratory reference strains were tested to develop a method of predicting the proportion of resistant worms in a sample from the field. Results The assay correctly identified as resistant all of the closantel-re sistant field isolates of H contortus. It also identified one isolate with an in vivo efficacy of 98% as having emerging resistance. Testing of the mi xtures of laboratory reference strains revealed that an isolate would be cl assified as resistant when it consists of about 25% or more resistant worms . Test samples that are not fully susceptible yet contain less than 25% res istant worms may be classified as emerging resistance. Conclusion The in vitro migration assay is a sensitive method of detecting closantel resistance in H contortus.