Immune exclusion of Haemonchus contortus larvae in the sheep: effects on gastric mucin of immunization, larval challenge and treatment with dexamethasone.

The rapid expulsion of the gastric nematode Haemonchus contortus from primed sheep is a complex mechanism which probably involves immediate hypersensitivity reactions and mucus. Quantitative techniques were employed to determine the content and distribution of mucin in the gastric mucosae of sheep of varying immune status, following a single large challenge with H. contortus. Substantial depletion of both neutral and acidic mucin, following challenge, was noted in the gastric mucosae of naive sheep. When compared with normal controls, animals rendered immune by daily oral challenge had significant reductions in neutral mucin at the mucosal surface and increased quantities of acidic mucin deeper in the mucosa. Mucin distribution in immune sheep was not significantly altered 48 h after challenge. Treatment of immune animals with the corticosteroid dexamethasone, abrogated the protective response with gross depletion of mucus in challenged sheep, but had little measurable effect on the mucin profile of unchallenged animals, except for a slight increase in acidic mucin at the mucosal surface and the absence of any detectable mucin in the zone adjacent to the sub-mucosa. Immunity to H. contortus diminishes with time after immunization. Animals still immune 6 weeks after immunization were found to have mucin profiles which did not differ significantly from those of freshly immunized animals, whereas animals susceptible to re-infection 12 weeks after immunization had mucin profiles more closely resembling those seen in naive controls. Immunization by daily oral challenge was also accompanied by hyperplasia of the gastric mucosa which persisted for at least 12 weeks after immunization had ceased.