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.

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The impact of mixed, nematode infection upon a group of animals will depend upon the number of nematodes present, how they are distributed among hosts and whether individuals that are heavily parasitized with one species are more likely to be heavily parasitized with other species. A survey of over 500 six-month-old, Scottish Blackface lambs from a single farm in Southwest Strathclyde identified 7 different categories of nematodes in the abomasum and small intestine. There were considerable differences among years and among nematodes in the prevalence and mean intensity of infection. Ostertagia circumcincta was present in nearly all lambs and judged by prevalence and intensity is one of the most successful of all parasitic nematodes. Each category of nematodes had a skewed distribution; most animals had relatively few worms but a small proportion had many worms. The variance of the number of nematodes in each category were approximately equal to the square of the mean. The counts of adult O. circumcincta followed a negative binomial distribution, but the negative binomial distribution did not provide a good description of the observed values for the other species. These other species had a lower prevalence and possibly some sheep were not exposed to infection. There was no significant genetic variation among lambs in the number of nematodes present and therefore the differences among these lambs were unlikely to be a consequence of genetic differences in host susceptibility. Lambs with increased numbers of one species were more likely to be have increased numbers of the other species, but the correlations were weak and may reflect covariation in exposure to different parasites.

The number of Teladorsagia circumcincta 4th-stage larvae in naturally infected lambs from a single farm varied among lambs and among different years. Within each year the distribution of 4th-stage larvae among lambs was similar to that expected from a negative binomial distribution. The ratio of 4th-stage larvae to adult T. circumcincta was low in two years with a low mean intensity of infection but high in two years with a higher mean intensity of infection. The negative binomial distribution is defined by the mean and by k, a parameter that measures dispersion; k was low when mean infection intensity was low but higher when mean infection intensity was high. As k is an inverse index of overdispersion this indicated that the distribution of 4th-stage larvae was more overdispersed at low levels of infection. In a combined analysis, the number of adult T. circumcincta and the plasma IgA activity against 4th-stage larvae were both associated with increased numbers of 4th-stage larvae. There was a statistical interaction between the number of adults and IgA activity that moderated their combined effect.