ome rather complex. Some of the variation in parasite loads we observe is predictable. For example, in mammals and some other taxa, males tend to be more heavily infected than females, perhaps due to differences in immune function (Potdin 1996a, Schalk and Forbes 1997, McCurdy et al. 1998). Parasite loads tend to increase with age and may plateau in older animals, though if acquired immunity is important (or there is parasite-induced host mortality) then they may tdtimately decline again, so reducing the degree of parasite aggregation. Genetic differences in susceptibility to infection may also be important, though their extent and direction are much more difficult to predict. Other factors that may contribute to the observed heterogeneities in worm burdens are the condition of the host (which may be a function of parasite load), host behaviour, parasite genetics and seasonality. Comparative studies of aggregation suggest that the infection process and the habitat of the host may make

Throughout their circumpolar distribution mountain hares Lepus timidus show unstable population dynamics characterised by regular and sometimes dramatic changes in abundance. The periodicity, amplitude and degree of cyclicity are different in different regions. The reasons for these fluctuations and geographic differences are not fully understood. In Fennoscandia there is experimental and correlative evidence that some mountain hare populations are limited by predators, but the experiments needed to conclusively demonstrate the role of predators, or of other potential factors have not been undertaken. In Scotland the rigorous control of predators means that the role of predators is largely dismissed, but this has not been experimentally tested. The most promising line of enquiry suggests that intestinal parasites have the potential to destabilise some mountain hare populations. There is paucity of literature from Asia and central Europe and no firm conclusions could be drawn. Time-series analysis of hunting bag records from Scotland largely confirmed the dominance of weak cycles with a mean periodicity of around 9 years found in earlier

Mountain hare Lepus timidus populations show unstable dynamics and since hares carry a significant helminth infection and host--parasite interactions are known to be destabilising, they have been proposed as a possible causal mechanism for the observed instability. We assessed the prevalence, intensity of infection and aggregation of the helminth parasites Graphidium strigosum and Trichostrongylus retortaeformis recovered from 589 mountain hares culled from 30 Scottish sporting estates in 1999 and 2000. Graphidium strigosum showed low prevalence and intensity of infection and was highly aggregated. In contrast, T. retortaeformis showed high prevalence and intensity of infection and a low degree of aggregation. Differences in body condition of the hares were best explained by a model including sex and month of collection and interaction terms for sex-month and intensity of infection of T. retortaeformis-month. The low degree of aggregation of T. retortaeformis and the significant negative effect of intensity of infection on body condition are in accordance with the hypothesis that the host--parasite interaction is the causative destabilising mechanism for mountain hare dynamics.

this paper with red grouse (LagoZA lagoZA scoZAwW )

An important epidemiological consequence of aggregated host -- parasite associations occurs when parasites are vectors of pathogens. Those hosts that attract many vectors will tend to be the focus of transmission. But to what extent, and can we identify characteristics of these key hosts? We investigated these questions with respect to the host -- tick relationship of the yellow-necked mouse, Apodemus flavicollis,a critical host in the maintenance of the zoonotic disease, tick-borne encephalitis. Transmission of the virus occurs when ticks feed in a `co-feeding' aggregation. Thus, the number and frequency of co-feeding groups provides an estimate of the potential rate of virus transmission. We recorded the spatio-temporal variations in co-feeding on a population of rodents in conjunction with recording individual host characteristics. Using Lorenz curves, we revealed conformation of tick-borne encephalitis transmission potential to the 20/80 Rule, where the 20% of hosts most infested with ticks were accountable for 80% of transmission potential. Hosts in the transmission cohort were identified as the sexually mature males of high body mass. Therefore control efforts targeted at this group would substantially reduce transmission potential compared to non-targeted control of the population, which resulted in a linear reduction in transmission potential. Focusing on the `wrong' functional group would have little impact upon transmission potential until a considerable proportion of the population had been subject to control. However, individuals can change their functional status over time making it difficult to predict the contribution of these individuals to future transmission.

www.elsevier.com/locate/ijpara The role of sex in parasite dynamics: Model simulations on transmission of Heligmosomoides polygyrus in populations of yellow-necked mice, Apodemus flavicollis