ATTEMPTS TO GENERATE IMMUNITY AGAINST TRICHOSTRONGYLUS-COLUBRIFORMIS AND HAEMONCHUS-CONTORTUS IN YOUNG LAMBS BY VACCINATION WITH VIABLE PARASITES

The ability of young Merino lambs to achieve protective immunity follo wing vaccination via viable nematode infections was assessed. Lambs we re infected from 1 month of age by repeated continuous low dose (trick le) administration of Trichostrongylus colubriformis or Haemonchus con tortus infective larvae (L3), or by truncated infections with high dos es of viable T, colubriformis L3. After 7 weeks all groups were drench ed with anthelmintic and at 3 months of age they were re-infected with the homologous species. Protection was assessed by faecal egg counts at 3, 4, 5, 6 and 7 weeks after challenge, and worm count at 7 weeks a fter challenge. Young lambs were partially protected by 3 months of ag e against Trichostrongylus by trickle infection. This protection corre lated with local mast cell and T-cell priming, increased numbers of lo cal antigen-presenting cells and T-cells and increased worm-specific a ntibody titres in the intestine. However, there was no evidence that y oung lambs were capable of immunologically recognising H, contortus an tigens following trickle infection, nor did trickle infection signific antly protect young lambs against Haemonchus challenge. (C) 1998 Austr alian Society for Parasitology. Published by Elsevier Science Ltd.