The effect of age on the response of sheep to vaccination with irradiated 'trichostrongylus colubriformis' larvae

Vaccination with irradiated 'Trichostrongylus colubriformis' larvae produced a high level of immunity, as judged by faecal egg counts and worm burdens following challenge with normal larvae, in nine of ten sheep aged 10 months. In lambs aged 3 months, vaccination was less effective. Some lambs developed partial immunity, b u t others did not respond. Serum levels of antibodies to 'T. colubriformis' acetylcholinesterase reflected the extent of antigenic exposure rather than the degree of immunity acquired, and there was no evidence that the unresponsiveness of the lambs was due to a deficiency in antibody production. Unresponsiveness was not associated with the numbers of circulating lymphocytes, monocytes or granulocytes, or with the numbers of mast cells, eosinophils and neutrophils at the site of infection. However, there were many globule leucocytes in the intestinal mucosa of adult sheep which were resistant to challenge infection. On the other hand, few of these cells were found in vaccinated lambs which generally gave a poor response to challenge.