Characteristics of parasite populations in endemic trichostrongylidosis.

Abstract

Worm counts, faecal egg counts and erythrocyte losses are recorded for lambs which were exposed on a naturally infected pasture for different periods in different years. After each exposure period the lambs were kept under conditions in which no further infection could occur; at varying times, usually after the infections had had time to maturo, the lambs were killed and examined. A detailed analysis of the results is given. It is concluded that the negative binomial distribution of worms among sheep within a flock is not a reflexion of the pasture distribution of infective stages but is the result of a combination of Poisson distributions, these distributions being generated by genetic subsets within the flock. There is a remarkable correlation between the numbers of Haemonchus present and the numbers of worms of the other genera. It is suggested that there are host-mediated common elements in the biomass regulating parameters of mixed trichostrongyle infections and that there is a resource-allocating device. Three main hypotheses are proposed. The hypotheses are: (1) The dimensional constraints on the host-parasite system are the results of various kinds of pace-setters or biological clocks. In each annual cycle the parent worms programme the rate of development through each generation. (2) The rate of development of the worms is tied stochastically or deterministically to the rate of senescence and death. (3) The rate of development and death for each trichostrongyle is set at the start of each season by a host factor which acts on a comparable area of the genome in each trichostrongyle. Methods of testing these hypotheses are discussed and the work related to other systems.