Reversal of order of ecdysis in Haemonchus contortus (Nematoda).

Exposure of Haemonchus contortus infective larvae to carbon dioxide under specific conditions is believed to activate a succession of events that ultimately control both exsheathment and subsequent development to the fourth stage. Rarely at pH 6, but always at pH 2, worms fail to exsheath yet develop to the fourth stage surrounded by cuticles two and three. Failure to exsheath is probably a consequence of inactive enzymes in exsheathing fluid, because exposure to EDTA, which inactivates exsheathing fluid, reduced exsheathment and yielded sheathed fourth-stage worms. Although exsheathment is blocked at pH 2, CO2 can activate the receptor for exsheathment as readily at pH 2 as at pH 6. Therefore the hypothesis that an early common pathway controls both exsheathment and development is not invalidated.