THE PROCESSES INFLUENCING THE DISTRIBUTION OF PARASITIC NEMATODES AMONG NATURALLY INFECTED LAMBS

The impact of mixed, nematode infection upon a group of animals will d epend upon the number of nematodes present, how they are distributed a mong hosts and whether individuals that are heavily parasitized with o ne species are more likely to be heavily parasitized with other specie s. A survey of over 500 six-month-old, Scottish Blackface lambs from a single farm in Southwest Strathclyde identified 7 different categorie s of nematodes in the abomasum and small intestine. There were conside rable differences among years and among nematodes in the prevalence an d 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 variances of the number of nema todes in each category were approximately equal to the square of the m ean. 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 difference s among these lambs were unlikely to be a consequence of genetic diffe rences in host susceptibility. Lambs with increased numbers of one spe cies were more likely to be have increased numbers of the other specie s, but the correlations were weak and may reflect covariation in expos ure to different parasites.