The immune response of sheep to larval challenge with Ostertagia circumcincta and O-ostertagi

One of three groups of sheep was challenged twice-weekly with infective-sta ge larvae (L3) of the sheep parasite O, circumcincta, another with the catt le parasite O. ostertagi while the third received no larval challenge. Posi tive faecal egg counts (FEC) and a rise in plasma pepsinogen levels were ob served only in those animals given O. circumcincta. Anti-O. circumcincta L3 IgG titres were rapidly elevated during parasite challenge with either O. circumcincta or O. ostertagi. Throughout the experiment, no rise in anti-ad ult IgG titres or eosinophil numbers was observed in peripheral blood in an y group. On evidence of self-cure of the trickle-infection determined by a reduction in FEC, all groups were drenched and challenged with 15,000 O. ci rcumcincta L3. No effect of previous challenge on parasite establishment or FEC was observed, although egg viability was significantly reduced in both groups given prior challenge. Significant differences in adult female worm length were observed between groups. Those recovered from animals previous ly challenged with O. circumcincta were shorter than from those given O. os tertagi which were in turn shorter than those from previously unchallenged animals. In utero egg counts were significantly lower in worms from animals previously challenged with O. circumcincta than in those from unchallenged control animals. The results indicate that a level of immunity to O. circu mcincta can be conferred by exposure to O. ostertagi. (C)1999 Elsevier Scie nce B.V. All rights reserved.