In vitro exsheathment of some nematode infective larvae

Abstract

Investigation of various factors which induce optimal in vitro exsheathment of the infective larvae of Dictyocaulus viviparus, Trichostrongylus colubriformis, and Haemonchus contortus reveal three distinct types of stimuli. Dictyocaulus viviparus larvae demonstrate an absolute requirement for the enzyme pepsin, which can be presented to the larvae in an inactive state. The exsheathing stimulus of the enzyme can be activated by placing larvae which had been previously immersed in an aqueous solution of pepsin into an HCl solution (pH 1–2). Trichostrongylus colubriformis shows a relative requirement for pepsin. Although it can be induced to exsheath in a buffer solution (pH 1.7) under 100% CO 2 gas, it undergoes more rapid and complete ecdysis when pepsin is present in the solution. Haemonchus contortus infective larvae are indifferent to pepsin. They undergo rapid and complete exsheathment in a variety of balanced salt solutions provided that the solutions are under a CO 2 gas phase. Although reducing agents were found to be highly effective in reactivating inactive rumen fluid as an exsheathing medium, they had no enhancing effect in the salt solutions when CO 2 gas was present at high concentrations.