infection resistance;

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

We investigated the role of host sex in parasite transmission and questioned: `Is host sex important in influencing the dynamics of infection in free living animal populations?' We experimentally reduced the helminth community of either males or females in a yellownecked mice (Apodemus flavicollis) population using an anthelmintic, in replicated trapping areas, and subsequently monitored the prevalence and intensity of macroparasites in the untreated sex. We focussed on the dominant parasite Heligmosomoides polygyrus and found that reducing parasites in males caused a consistent reduction of parasitic intensity in females, estimated through faecal egg counts, but the removal of parasites in females had no significant influence on the parasites in males. This finding suggests that males are responsible for driving the parasite infection in the host population and females may play a relatively trivial role. The possible mechanisms promoting such patterns are discussed. Keywords Gastrointestinal nematodes, macroparasite dynamics, sex-biased parasitism.

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.

By sampling individual rainbow trout, Oncorhynchus mykiss, at a fish farm we showed that Argulus coregoni were aggregated within their host population. The relative significance of susceptibility and exposure generating the observed pattern was tested using experimental infections. We examined, whether rainbow trout developed protective resistance mechanisms against the louse following a challenge infection and if there was variation between individual trout in their susceptibility to A. coregoni metanauplii. Fish were exposed to 20 A. coregoni for 5, 25, 50, 85 or 120 min and the numbers attaching recorded. Three weeks later, developing argulids were removed and the experiment repeated with a standardized exposure of 20 metanauplii. Prior exposure of fish with A. coregoni did not reduce the total infection intensity compared to naïve fish, but fish gained infection more rapidly. We suggest that there is no protective acquired resistance of pre-exposed rainbow trout to subsequent Argulus exposure. The possibility that an immunosuppressive mechanism by argulids was acting enabling the higher attachment rate could be refuted since control individuals, not previously exposed to lice, gained the infection at a similar rate as the fish challenged twice. Our results do not indicate clear differences in susceptibility among individual fish but the transmission of metanauplii on fish seemed to be opportunistic and non-selective. Our results support the view that variation in exposure time, rather than differences in susceptibility of individual hosts, might be the key factor in generating the aggregated distribution of Argulus on their hosts.

shift and epidemiology in a seasonal

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