The immune response against gastrointestinal nematodes in naturally infected Polish Wrzosowka sheep. I. The serum IgG response to Haemonchus contortus in ewes over three grazing seasons

Faecal nematode egg counts and IgG activity to Haemonchus contortus were followed in naturally infected Polish Wrzosówka ewes. The egg counts were overdis- persed; most individuals had relatively low egg counts but a small proportion had high counts. Egg production followed a different pattern each grazing season. Larvae cultured from eggs in 1996 were predominantly H. contortus and Trichostrongylus spp. In 1997 the recoveries were largely Trichostrongylus spp. In 1998, larval recoveries were mainly Trichostrongylus spp. and Teladorsagia spp. There were no discernible patterns in the composition of the nematode population within each grazing season. Egg counts in different months were moderately repeatable, indicating that animals tended to maintain their rankings over time. The repeatabilities rose during the grazing season but declined with increasing intervals between sample dates. IgG activity against a somatic extract of adult H. contortus was higher than activity against a preparation from excretory/secretory antigens but the two responses were very strongly correlated. The repeatability of IgG activity at monthly intervals was higher than the repeatability of faecal egg counts. Animals with higher than average IgG activity had significantly lower than average egg counts but only in the last two years of the study. A combination of egg counts and antibody responses may be better at identifying resistant animals than either method used in isolation, but more research is necessary to determine why the association between antibody and egg counts varies in different years.