Infection and host population dynamics : the use and assessment of generic indices of health

Pathogens may be important for host population dynamics, as they can be a proximate cause of morbidity and mortality. While a number of studies have attempted to assess the role of pathogens in host dynamics, none has evaluated the influence of host health status on hostinfection dynamics. The present thesis addresses this issue and 'investigates whether opportunistic infection is facilitated by an underlying impoverished condition. To this end, haematological parameters are used as proxies of health status and the host system utilised is the field vole, Microtus agrestis. The data originated from field vole populations from Kielder Forest (Northumberland, UK), which were longitudinally sampled monthly for two years, and on each trapping session a blood specimen from every individual was examined. To allow comparisons with results collected under controlled conditions, samples from a captive colony were also analysed. Chapter 3 describes physiological dynamics using haematological parameters. There was marked seasonal variation of all cell types not explained by other variables evaluated, which delimits three well characterised 'physiological' seasons. Immunocompetence appeared lowest in winter, but red blood cells were high and indices of infection were at their lowest levels. Spring was characterised by a fall in red blood cell counts and peaks in indicators of infection. At the beginning of the last season, summer-autumn, red blood cell counts recovered, the immunocompetence increased and the indicators of infection decreased. Poor body condition appeared to affect the innate immune response and the immunocompetence, suggesting that the capacity to fight infection is dependent on host condition. The first pregnant females of the year were those in better condition. All the haematological parameters were affected by high population densities: counts were lower at times of high host densities, or following high host densities. Chapter 4 evaluated the significance oflarge granular mononuclear cells that were a common finding in breeding females. They might be related to the NK cell lineage as other similar cell types associated with pregnancy, but in males, these cells seem to be a sign of response to antigen. Chapter 5 examined if death is preceded by disturbed haematological parameters. Individuals with lower indicators of condition were more likely to disappear. In winter, individuals showing elevated indicators ofinflammatory response experienced decreased survival. / Using haematological parameters, it was demonstrated in Chapter 6 that poor condition preceded infection and that infection was followed by poor condition, which suggests a potential vicious circle. Chapter 7 investigated whether body growth is associated with the condition ofindividuals or their immune response. It was found that juveniles experiencing a chronic inflammatory response presented an inferior growth, and that during a year of not covered metabolic demands, an acute inflammatory response resulted in less growth for all ages. Chapters 8 and 9 evaluated if poor condition precedes higher probability of infection with specific pathogens; and if such infections are followed by a disruption in haematological parameters. The probability of imminent cowpox virus infection was greater in individuals with poor body condition and low red blood cell counts. Immunosuppressed individuals were more likely to suffer from high intensities of infection by Trypanosoma micron, and such high intensity infections produced greater impact on indicators of condition. The results presented are consistent with the hypothesis that host condition might be the underlying ultimate cause of the impact that pathogens exert on population dynamics. However, once let in, pathogens themselves seem to have the capacity to further affect populations, making hosts more vulnerable and thus increasing the transmission of infection.EThOS - Electronic Theses Online ServiceGBUnited Kingdo