Factors regulating tapeworm populations: estimations of the infection pressure and index of clustering from Taenia hydatigena before and after the removal of infected dogs.

A pasture was prepared so that an equilibrium of fresh and ageing eggs of Taenia hydatigena was present. This was done by placing dogs with patent infections on it for six months. Five months after their introduction, groups of lambs were grazed there for two, four, eight, 16 or 32 days. The dogs were then removed. Three and six months later, further groups of sheep were grazed there for a similar period. The infection pressures and the indices of clustering of the eggs were calculated by an examination of the lambs for cysts at autopsy. When the dogs were present, the daily rate of infection was about 60 per cent of the flock. This was reduced to 6.5 per cent then to 3.5 per cent within three and six months respectively after their removal. Very high cyst counts were obtained in some animals and the indices of dispersion diverged from a Poisson model during the period when the lambs were maintained on the pasture in the presence of the dogs. There was a trend for a reduction in the mean cyst counts and the index of dispersion tended to conform more closely to a Poisson model the longer the dogs were excluded. There was also a corresponding reduction in the survival rate of the cysts. It was suggested that an asymptote of less than 100 per cent in the prevalence rate may be accounted for by the acquisition of resistance without the persistence of infection and this was induced in some animals by the initial ingestion of ageing organisms which gradually formed the predominant type after the removal of the dogs from the pasture.