THE DISTRIBUTION OF NEMATODE EGG COUNTS AND LARVAL COUNTS IN GRAZING SHEEP AND THEIR IMPLICATIONS FOR PARASITE CONTROL

The distribution of nematode egg counts and larval. counts in grazing sheep and their implications for parasite control. International Journ al for Parasitology 24: 103-108. The frequency distribution of gastroi ntestinal nematode egg and lungworm larval counts was examined in 101 and 87 naturally infected ewes on two farms. The egg and larval output s of the two flocks followed the negative binomial pattern of distribu tion (with k values below unity) at each time of sampling, which sugge sts 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 b y eliminating ''wormy'' individuals of flocks either by selective bree ding or by their selective anthelmintic treatment, effective control o f parasite transmission can be achieved. Because of the phenomenon of nematode ''clumping'' it seems necessary to expand new methods for est imation of flock productivity caused by nematodes in livestock and to incorporate negative binomial parameter (k) in mathematical models of nematode population dynamics.