In vitro response of Haemonchus contortus larvae harvested during different times of the patent period to anthelmintics.

Three commercially available ruminant anthelmintics and an investigational drug were evaluated for effects on the motor function/motility of third stage (ensheathed) Haemonchus contortus. Helminth ova were collected at one, five, nine and 13 weeks during the patent period, cultured to the third larval stage and assayed for sensitivity to four different drugs. All four drugs (100 and 10 micrograms ml-1) significantly affected the motility of third stage H contortus larvae cultured from eggs passed at each of the times examined. However, the investigational drug (p-toluoyl chloride phenylhydrazone, 10 micrograms ml-1) had a significantly greater effect on the motility of larvae harvested at 13 weeks than those cultured at nine weeks (32 per cent difference). No other significant differences in the motility response during the patent period were observed.