Mechanisms underlying resistance to nematode infection

Lambs show considerable genetic variation in faecal egg count following nat ural, predominantly Ostertagia circumcincta infection. This genetic variati on is acquired and not innate. Worm length is positively associated with wo rm fecundity. The genetic variation in faecal egg count is a consequence of genetic variation in worm length and hence worm fecundity, and not of gene ric variation in worm burdens. In contrast to lambs, mature sheep may be ab le to regulate both fecundity and worm numbers. In lambs, three factors acc ount for the majority of the variation in worm length: the strength of the local IgA response against fourth-stage larvae, the specificity of this res ponse against four molecules in particular, and the density-dependent influ ence of worm number. (C) 1998 Australian Society for Parasitology. Publishe d by Elsevier Science Ltd. All rights reserved.