ENHANCED ABILITY OF 3RD-STAGE LARVAE OF HAEMONCHUS-CONTORTUS TO WITHSTAND DRUG EXPOSURE FOLLOWING CHEMICALLY-INDUCED EXSHEATHMENT

Normal (ensheathed) and exsheathed third-stage larvae of Haemonchus co ntortus were exposed in vitro to various concentrations of levamisole or ivermectin. Exsheathment was induced by brief exposure to sodium hy pochlorite. When observed approximately 2 min after immersion in levam isole at 0, 5, 10, and 100 mu g/ml (3 trials), the mean percentage mot ility (to nearest whole number) of normal larvae was 84, 43, 37, and 1 5, respectively. However, the mean percent motility of exsheathed larv ae was 77, 78, 79, and 72, respectively. When observed 55 min after im mersion in levamisole at the same concentrations, the mean percent mot ility of normal larvae was 76, 4, 3, and 0, respectively, whereas that for exsheathed larvae was 72, 75, 68, and 0. When observed 45 min aft er initial exposure to ivermectin at 0, 8, 80, and 160 mu g/ml, the me an percent motility of normal larvae was 87, 6, 3, and 3, respectively , whereas the mean percent motility of exsheathed larvae was 94, 75, 2 9, and 14, respectively. Thus, both drugs were effective against both kinds of larva; but the time and concentration required for efficacy w ere markedly affected by the presence or absence of a sheath or by unk nown effects of the exsheathment process. For both levamisole and iver mectin, exsheathed larvae had a much greater ability than normal larva e to withstand drug exposure.