Desiccation survival of the infective larva of Haemonchus contortus.

The infective stage of Haemonchus contortus, the third-stage larva, retains the secondstage larval cuticle. Exsheathment takes place in the rumen of the host (Somerville, 1954), under the influence of a number of factors, chiefly unionized carbonic acid and dissolved gaseous carbon dioxide (Rogers, 1960). The ensheathed larva survives desiccation, but, if caused to exsheathe by appropriate treatment, it soon succumbs if dried. This aspect of sheath function is therefore clear; but the mechanism by which survival is improved is obscure. Rogers & Sommerville (1960) showed that the sheath is permeable to water and they therefore consider (1963) that it 'seems unlikely to prevent the larva from becoming desiccated'. The infective larva of the potato-root eelworm Heterodera rostochiensis survives prolonged desiccation inside the egg-shell, but it is very susceptible to drying once it has hatched (Ellenby, 19683). The free larva loses water much more rapidly than the enclosed larva; it was therefore suggested that the egg-shell, freely permeable when wet, becomes impermeable as it dries and that the reduction in the rate at which the larva dries helps it to survive. The hypothesis that the same sort of mechanism may operate in Haemonchus contortus is examined in the present work.