This study investigated traits that could be used to aid selective breeding for increased nematode resistance in sheep. Data were collected on approximately 1000 Scottish Blackface lambs over a 5-year period (1992–6). The animals were continually exposed to mixed nematode infection by grazing. Anthelmintic treatment was administered and blood samples were collected every 28 days from 4 to 24 weeks of age. Approximately 500 of the lambs were necropsied at 6 to 7 months of age and their worm burdens were recorded. The traits measured were the number of adult males, adult females, fifth-stage larvae and fourth-stage larvae for all species present. In addition, the mean length of adult females and the number of eggs in utero were estimated for
Teladorsagia circumcincta. Several indicator traits were also measured on some of the animals, including : faecal egg counts, immunoglobulin A activity and eosinophil count as markers of the local immune response, and plasma pepsinogen activity and fructosamine concentration as indicators of the pathological consequence of infection. The heritabilities for worm length, eggs in utero and worm burden were 0.53±0.17, 0.50±0.16 and 0.13±0.10 respectively. At a mean age of 22 weeks the heritabilities for fructosamine concentration, IgA activity, eosinophil count and pepsinogen activity were 0.39±0.16, 0.57±0.15, 0.35±0.15 and 0.56±0.16 respectively. Strong negative genetic correlations (< −0.6) were often observed between necropsy traits and eosinophil count, IgA activity and pepsinogen activity. Substantial genetic correlations were also observed between fructosamine concentration and worm length (0.67) and number of fourth-stage larvae (−0.80). The genetic and environmental correlations between indicators and necropsy traits were often opposite in sign. The indicator traits studied could aid selection for increased resistance to gastrointestinal nematodes in sheep.