Faecal samples were taken in 2 consecutive years from Scottish Blackface lambs on a commercial farm in central Scotland to examine variation among lambs in the number of nematode eggs in their faeces. Samples were taken at intervals of 4 weeks from 1 to 5 months of age. Lambs were treated with a broad-spectrum anthelmintic at each sample date. Ewes and lambs were naturally infected by grazing pasture contaminated with mixed, predominantly
Ostertagia circumcincta, nematode species. The ewes were removed from the lambs when the lambs were three months old. Only 3 of the 6 sets of samples taken prior to removal of the ewes fitted a negative binomial distribution but all sets of samples taken after this did so. The mean and the variance differed with the month and year of sampling. The amount of variation among lambs increased as the lambs grew from 3 to 5 months of age, possibly because of heterogeneity in the rate of development of protective responses. Counts taken in August and September, when the lambs were 4 and 5 months of age, were positively correlated with each other. In contrast, counts taken at 1 or 2 months of age were negatively correlated with counts taken when the lambs were 3–5 months old. Sex, date of birth, sire and dam were all associated with variation in faecal egg counts but the size and significance of their influence was dependent upon the date when faecal samples were taken. Four factors: sire, dam, sex and year accounted for over two-thirds of the variation among older lambs in faecal egg counts.