Epidemiology of parasitic protozoan infections in Soay sheep (Ovis ariesL.) on St Kilda

Abstract

The feral Soay sheep (Ovis ariesL.) population on Hirta, St Kilda, is host to a diverse component parasite community, but previous parasitological studies of the population have only focussed on the metazoan species. This paper reports the first epidemiological study of the protozoan species comprisingCryptosporidium parvum,Giardia duodenalisand 11 species ofEimeriain Soay sheep across 3 years of varying host population density. Prevalence and intensity of almost all species of protozoa significantly decreased with host age, with the exception ofE. granulosa, which increased in prevalence with host age. The prevalence ofC. parvumappeared to vary positively with host population density but that ofG. duodenalisdid not vary significantly with density. Most species ofEimeriashowed a distinct lag in infection level following the host population crash of 2002, taking up to 2 years to decrease. MixedEimeriaspecies intensity and diversity were highest in 2002, a year of low host density. Parasite diversity decreased with host age and was higher in males. There were 5 positive pair-wise associations between protozoa species in terms of prevalence. The results of this study highlight the potential for protozoal infection to shape the evolution of parasite resistance in wild host populations harbouring diverse parasite species.