The indiscriminate repeated use of ivermectin has resulted in the emergence of ivermectin resistant field strains of Haemonchus contortus in sheep in South Africa. There is a need for a rapid and cost effective in vitro test to detect such strains. Infective H contortus larvae known to be resistant to ivermectin, were incubated in vitro in a diluted concentration of the drug. Subsequently their ability to migrate was compared with that of larvae of isolates known to be susceptible to ivermectin. The criterion used to determine resistance was the ability of the larvae to migrate out of gelled agar after exposure to ivermectin using diluted propylene glycol as the control compound. The results obtained by this technique confirmed the results obtained with critical controlled tests; significant differences between the ivermectin sensitive and resistant isolates were demonstrated, thereby confirming that this in vitro technique may be used as a screening method to identify ivermectin resistant isolates of H. contortus.
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