Previous studies in deliberately infected sheep have shown an association between IgA activity against 4th-stage larvae of Teladorsagia circumcincta and parasite growth, development and fecundity. The purpose of this research was to determine if these results could be confirmed in naturally infected sheep and to explore the hypothesis that plasma IgA activity could help to identify resistant lambs with shorter adult nematodes. Plasma IgA activity was skewed with most animals having relatively low levels of IgA activity. Plasma IgA activity was repeatable and highly heritable. Animals with increased IgA activity had lower egg counts and shorter adult female T. circumcincta. Therefore, under conditions of natural parasite challenge, plasma IgA activity may help to identify lambs resistant to T. circumcincta.

Faecal egg counts and peripheral blood eosinophil counts were taken from Scottish Blackface lambs following natural, predominantly Teladorsagia circumcincta infection. Peripheral eosinophil concentrations were higher in animals with lower egg counts but only in lambs that were at least 3 months of age. The reduced egg counts were due to reduced fecundity of T. circumcincta; there was no association with the number of adult T. circumcincta. Associations with the number of parasites from other species of gastrointestinal nematodes appeared to be neutral or favourable. Estimated heritabilities for eosinophil concentrations in 4- and 5-month-old lambs were 0·48±0·16 and 0·43±0·17, respectively. Therefore, under defined circumstances, eosinophil concentrations may be a useful indicator of resistance to predominantly T. circumcincta infection.

Fructosamine concentrations reflect protein status and because infection with Teladorsagia circumcincta can induce a relative protein deficiency, we examined the usefulness of fructosamine concentrations as markers of the intensity of infection in naturally infected lambs. Fructosamine concentration was a heritable trait and variation in fructosamine concerntrations was associated with differences in body weight, and a variety of parasitological variables; animals with increased fructosamine concentrations grew more quickly, had increased faecal egg counts in one of the three study years, had decreased pepsinogen concentrations and decreased IgA activity against 4th-stage larvae of T. circumcincta. Fructosamine concentrations were also associated with variation in the subsequent acquisition of nematodes and in the length of adult female T. circumcincta; lambs with increased fructosamine concentrations had fewer nematodes but the mean length of adult female T. circumcincta was longer. Therefore fructosamine concentrations are potentially useful indicators of the severity of nematode infection and may predict magnitude of subsequent infection.