Parasites and Immunology

In contrast to epidemics, such as influenza, the role that endemic parasites- parasites constantly present in a community- play in the population and evolutionary dynamics of their hosts remains contentious and poorly understood. By combining our knowledge of life history, pedigree and genetic data from the sheep with post-mortem surveys of parasites and disease, parasite egg counts from faeces collected in the field, and immunological assays from blood samples, the project has been able to provide new insights into the links between parasites, immunity, host fitness and population dynamics.

The first surveys of the parasite fauna of the Soay sheep on St Kilda took place in the 1960s. The sheep were found to have many of the species of parasites found in mainland domestic sheep, and this has been confirmed by subsequent studies through the 1990s and 2000s (see table below). Haemonchus contortis, a widespread and pathogenic parasite of mainland sheep, is a notable absence. Perhaps the biggest surprise in parasitological terms is the presence of a tapeworm, Taenia hydatigena, which requires a carnivore definitive host to complete its life cycle. No such carnivores are present on St Kilda, and researchers think the parasite population is maintained by regular deposition of parasites on St Kilda in the faeces of birds. A very recent addition to the list of parasites known to infect the Soay sheep is the trypanosome, Trypanosoma melophagia, which is transmitted through ecto-parasitic keds which are often found in large numbers on young Soay sheep.

By far the best studied of these parasites are the gastrointestinal nematode worms and, in particular, the relatively pathogenic Teladorsagia circumcincta and its close relatives. Post-mortem surveys on St Kilda in 1989 and 1990, led by Frances Gulland, suggested that T. circumcincta comprised around 75% of the gut nematodes found in both lamb and adult Soay sheep. A subsequent survey, by Barbara Craig and colleagues in 2002, found that two other species of strongyle nematodes, Trichostrongylus axei and Trichostrongylus vitrinus, were actually predominant in lambs, while T. circumcincta increased with age and predominated in adults. The reason for the discrepancy in findings remains to be determined – but it may well reflect temporal variation in the prevalence of different nematode species.

Post-mortem surveys of parasite burdens have allowed researchers a detailed glimpse of the diversity of the parasite fauna and suggested this might vary with both age and sex. However, as only natural mortalities can be used in such studies, other approaches are required to develop an understanding of the relationships between parasite burdens, immunity, growth, survival and reproductive success across the lifetimes of individual sheep. Since the late 1980s, Jill Pilkington MBE and co-workers have collected egg counts of different parasites from faecal samples taken from Soay sheep at capture and in the field. Counts of the eggs shed by strongyle worms (including Teladorsagia and Trichostrongylus species) into sheep faeces do seem to correlate well with the number of adult worms found in the guts post-mortem and provide an important longitudinal window onto variation in nematode burden.

Studies led by Ken Wilson using these faecal egg counts (or FECs) show marked temporal and spatial variation within the population. Within years, FECs start to increase in spring, particularly in female sheep that show a clear spike around the time of lambing: the “peri-parturient rise” that is widely observed in mainland domestic animals. Eggs counts then remain high through the summer, starting to decline around September and falling through the autumn. Among-year variation in FECs is also marked, and can be in part explained by variation in the number of lambs grazing the study area in a given year: newly infected lambs lack immunity and are thought to represent the major source of eggs shed onto the pasture. There are also differences in FECs of sheep resident in different parts of the Village Bay study area. Although the reasons for this spatial variation are not known, it is paralleled by variation in the number of larval parasites found on pasture in different areas.

Work by Ken Wilson and colleagues also demonstrated marked differences between the sexes in FECs across the lifespan. The first eggs are detectable in the faeces of lambs at around 45 days old and the subsequent increase in FECs is much more dramatic in male than in female lambs. Outside of the peri-parturient period, males continue to have higher worm burdens than females – and the reasons for this difference remain an important topic for current and future research. This research also showed that as males age, their egg counts decline until around two years of age and in females FECs also decline until around 4 or 5 years of age. These declines probably reflect the acquisition of immunity. Recent work led by Adam Hayward showed that FECs then start to increase as animals age, possibly as a result of the deterioration of immune function in later life. In females, this rate of increase in worm burdens with age is exacerbated when individuals experienced harsh over-winter conditions earlier in their lifetimes. Furthermore, old females give birth to lambs that themselves exhibit higher egg counts than do lambs from females in their prime, suggesting parasitological effects of ageing may span generations.

A fundamental question has been how parasites impact the survival and reproductive performance of the Soay sheep. A series of experimental treatments with anthelmintic boluses (which should clear gut nematodes for several months) suggest the effects on survival are context-dependent. An experiment by Frances Gulland and colleagues in the summer of 1988, just prior to a severe population crash, found that treated animals survived until later in the winter but were just as likely to die as untreated controls. A second experiment in 1991, which also involved a follow-up bolus to increase the treatment duration, preceded a winter with lower mortality and showed that treatment improved survival in female lambs and male yearlings, although not in females yearlings. Treated yearling males also showed more rapid body and horn growth compared to controls. However, another experiment in the summer of 2001 conducted by Owen Jones and colleagues had no discernable effect on the over-winter survival of sheep.

Numerous researchers have also demonstrated associations between August FECs, August body mass and subsequent winter survival among Soay sheep. Heavier sheep tend to have lower worm egg counts, and FEC only seems to predict survival independently of body mass in lambs. A study by Adam Hayward and others revealed that, in adults, the relationship between FECs and annual survival and reproduction is complex, and depends on both the age and body mass of the animal. Interestingly, analyses by Barbara Craig and co-workers suggest that in adult sheep high worm egg counts and high coccidia (Eimeria spp; see table above) oocyst counts independently predicted low August body mass. This suggests gut nematode infections are telling only part of the story – and Barbara’s research into the protozoan parasites infecting the sheep suggests their diversity may vary over time and with age and sex. The way protozoans interact with gut nematodes to influence host immunity and demography remains to be determined. Furthermore, we still do not know whether bacterial or viral pathogens common in domestic flocks are present in Soays on St Kilda. A small-scale survey in 2001 suggested they are not and a larger, in-depth serological survey lead by Andrea Graham is currently underway.

One pattern is abundantly clear from the parasitological research conducted to date on St Kilda: individuals vary considerably in their susceptibility to infection by parasites, judging by post-mortem surveys and faecal egg count data. Analyses lead by Judith Smith in the late 1990s and Dave Coltman in the early 2000s suggested that observed variation in FECs has a genetic basis. One might expect natural selection to remove any gene variants that were associated with higher infection rates or parasite burdens. So why is there still genetic variation for FECs? Several hypotheses exist, and central amongst these are explanations for why genetic variation in immunity and immune genes might be maintained in natural systems. One idea is that because parasites evolve so fast and the nature of infections changes so often, variation in the immune repertoire is essential and natural selection accordingly maintains that variation. A second idea is that because mounting an effective immune response is energetically costly, it draws resources away from other important functions like growth and reproduction, and the net result is that there may be no single ‘optimal’ strategy of investment in immunity because the fitness costs cancel the benefits in many contexts.

Testing these ideas in natural systems is rather challenging, because linking genetic variation in immunity with parasite burdens and/or fitness is no mean feat in wild populations. However, research in the Soay sheep has been able to achieve just that. In the early 1990s, Frances Gulland and co-workers found a link between a protein polymorphism (Ada) and both FECs and survival in the sheep, and a few years later Steve Paterson and others provided evidence of associations between alleles in a highly variable region of important immune genes, the major histocompatability complex or MHC, and parasite burdens and fitness in young Soay sheep. At around the same time, analyses by Dave Coltman suggested that variation in FECs had a genetic basis and that polymorphisms at loci controlling production of the cytokine interferon gamma were correlated with both FECs and concentrations of antibodies against T. circumcincta in blood.

Andrea Graham and colleagues then utilised the large number of blood plasma samples collected during the August catches between 1997 and 2007 to investigate immunological variation in more detail. They measured anti-nuclear antibodies (or ANA), which bind to material commonly found inside mammalian cells and are associated both with a healthy immune response and – at very high concentrations – with autoimmune disease. Over the 11 year period studied, high levels of ANA were heritable and associated with improved over-winter survival in crash years but with reduced annual reproductive performance. This supports the idea that reproductive costs might cancel out the survival benefits of a strong immune response. Since 2010, blood samples collected in August have been preserved in new ways to allow different assays of immunity to be performed back in the lab in Edinburgh. Early results from this research, part of a collaboration between Tom McNeilly at the Moredun Research Institute and Dan Nussey and Rose Zamoyska in Edinburgh, have shown that in female sheep the proportions of different types of T cells, which are central to acquired immunity, show dramatic changes with age. Ongoing work aims to link measures of antibodies, T cell phenotype and function with parasite burdens, fitness and population dynamics.