Evaluation of a laboratory test to detect resistance to closantel in Haemonchus contortus.
OBJECTIVE
To evaluate a laboratory test for closantel resistance in Haemonchus contortus.


METHODS
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.


CONCLUSIONS
The in vitro migration assay is a sensitive method of detecting closantel resistance in H contortus.
Lloyd
J B
JB
NSW Agriculture, Elizabeth Macarthur Agricultural Institute, Camden. joan.lloyd@agric.nsw.gov.au
Fitzgibbon
C C
CC
Barchia
I
I
D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology*; D004351:Drug Resistance; D006189:Haemonchus / Q000187:drug effects* / Q000302:isolation & purification; D007814:Larva; D021261:Parasitic Sensitivity Tests / Q000592:standards*; D011237:Predictive Value of Tests; D012458:Salicylanilides / Q000494:pharmacology*; D012756:Sheep
D000871:Anthelmintics; D012458:Salicylanilides; C023342:closantel