The distribution of nematode egg counts and larval counts in grazing sheep and their implications for parasite control.

The frequency distribution of gastrointestinal nematode egg and lungworm larval counts was examined in 101 and 87 naturally infected ewes on two farms. The egg and larval outputs of the two flocks followed the negative binomial pattern of distribution (with k values below unity) at each time of sampling, which suggests highly overdispersed worm burdens. The results of the statistical analysis indicated that a relatively small part of the two flocks was responsible for the excretion of the majority of both gastrointestinal nematode eggs and Dictyocaulus filaria larvae. It is concluded that by eliminating "wormy" individuals of flocks either by selective breeding or by their selective anthelmintic treatment, effective control of parasite transmission can be achieved. Because of the phenomenon of nematode "clumping" it seems necessary to expand new methods for estimation of flock productivity caused by nematodes in livestock and to incorporate negative binomial parameter (k) in mathematical models of nematode population dynamics.