Immune exclusion of Haemonchus contortus larvae in sheep: dose dependency, steroid sensitivity and persistence of the response.

Naive and immune sheep, treated with anthelmintic, challenged directly into the abomasum and killed 48 hours later were used to examine the steroid sensitivity, persistence and dose dependency of immune mechanisms which result in the failure of incoming Haemonchus contortus larvae to establish (immune exclusion). Immune exclusion was almost totally abolished by corticosteroid treatment, the mean percentage of larvae establishing in the mucosa of immune sheep was 1.4 per cent of the challenge infection whereas in those treated with dexamethasone it was 24.4 per cent and in naive sheep 27.3 per cent. Immune animals challenged after seven and 42 days without antigenic experience excluded more than 90 per cent of the challenge larvae from their mucosae, whereas those challenged after 84 days without antigenic experience were as susceptible as naive animals. Immune exclusion was dose dependent, animals challenged with 10(6) and 10(5) larvae excluded 93 per cent and 82.5 per cent of the challenge dose whereas those challenged with 10(4) larvae failed to exclude larvae.