Lambs naturally infected with predominantly Ostertagia circumcincta show substantial variation in the concentration of nematode eggs in their faeces. The heritability increases with age indicating that genetic variation is due to an acquired and not an innate response. Necropsy analysis indicates that there is substantial genetic variation in mean adult female worm length but not in the number of larvae or adult worms. Variation in adult female worm length is associated with variation in fecundity. Therefore genetic resistance appears to operate through the control of worm fecundity. The only immunological responses that are consistently associated with differences in worm fecundity are the amount of parasite-specific IgA and the specificity of serum antibody. These two traits, together with density-dependent effects of worm number, account for the vast majority of variation in worm length and consequently of fecundity. In conclusion, in growing lambs resistance to infection with O. circumcincta appears to be remarkably simple; the major and perhaps the only mechanism is the IgA mediated control of worm fecundity.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 27: Reproduction; fish breeding; genetics and the environment; genetics in agricultural systems; disease resistance; animal welf, , 303–306, 1998
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