| Popis: |
The isolated and un-managed Soay sheep population of Hirta, St. Kilda, which has been monitored intensively since 1985, is a model system for host-parasite studies. In hosts that died in the 2001/2 population crash, hitherto unknown and unexpected trends in the abundance of the main parasite species with host age were revealed. Specifically, previous studies in this system have failed to identify large Trichostrongylus axei and T. vitrinus burdens in the abomasum and small intestine respectively, which declined with host age, whereas Teladorsagia circumcincta burdens actually increased over the first few host age classes. Also male hosts had significantly higher burdens of Trichostrongylus species than females, with this genus making up a higher proportion of the strongyle-egg producing female adult nematode community in male hosts. These findings raise questions concerning previous interpretations of the main strongyle species contributing to strongyle egg counts. In living hosts sampled in late summer, the inventory of parasite species known to infect the sheep was increased by 40% with the identification of thirteen species of protozoa (Cryptosporidium parvum, Giardia duodenalis and eleven species of Eimeria) most of which have intracellular phases in their life cycles and likely, therefore, to exert contrasting selection on their hosts compared with extracellular helminth parasites. In general, protozoan burdens declined with host age and were higher in males than females; different species’ burdens appeared to lag the host population dynamics to different extents and only C. parvum varied positively with host population density. In living hosts sampled in late summer, simultaneous measures of strongyle, protozoan and ked (Melophagus ovina) intensity for individual hosts each explained independent variation in host body weight. No associations were detected between parasite diversity or intensity and the probability of survival through the 2001/2 population crash, possibly because of low statistical power. |
