How lambs control infection with Ostertagia circumcincta

Two of the most important questions for immunologists studying nematode inf ections are what effect does the host response have on the parasite and whi ch components of the host response are responsible for these effects. The n umber of nematodes and the mean length of adult female Ostertagia circumcin cta was measured in over 500 6-7 month old lambs of the Scottish Blackface breed. Quantitative genetic analyses indicated that there was substantial g enetic variation among lambs in faecal egg counts and in worm length but th e analyses provided no evidence for genetic variation in worm numbers. Sepa rate analyses have shown a strong relationship between decreased worm lengt h and decreased worm fecundity. Therefore, genetic resistance in lambs appe ars to be due to control of worm growth and not to control of worm numbers. The only immune response that is consistently associated with reduced worm length is the IgA response to fourth-stage larvae. The association is rema rkably strong. After allowing for the influence of worm number on worm leng th (density-dependence) essentially all of the variation among deliberately infected lambs in worm length can be accounted for, in a statistical sense , by variation in the strength and specificity of the local IgA response. T herefore, the IgA mediated suppression of worm growth and fecundity appears to be the major mechanism of resistance to O. circumcincta in lambs. (C) 1 999 Elsevier Science B.V. All rights reserved.