Evaluation of a laboratory test to detect resistance to closantel in Haemonchus contortus
J B Lloyd1, C C Fitzgibbon, I Barchia
1NSW Agriculture, Elizabeth Macarthur Agricultural Institute, Camden. joan.lloyd@agric.nsw.gov.au
Objective:
To evaluate a laboratory test for closantel resistance in Haemonchus 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-resistant field isolates of H contortus. It also identified one isolate with an in vivo efficacy of 98% as having emerging resistance. Testing of the mixtures of laboratory reference strains revealed that an isolate would be classified as resistant when it consists of about 25% or more resistant worms. Test samples that are not fully susceptible yet contain less than 25% resistant worms may be classified as emerging resistance.
Conclusion:
The in vitro migration assay is a sensitive method of detecting closantel resistance in H contortus.
