title: The role of multiple host species in shaping the transmission dynamics of Bartonella parasites within natural rodent communities.
Creators: name: Withenshaw, Susan
Publisher: name: University of Liverpool Repository
Year: 2015

description: Diseases caused by parasites are responsible for immense human and animal suffering, declines in biodiversity, and substantial economic losses across the globe. It is therefore important to understand how parasites spread through and persist within natural populations, so that control interventions that aim to reduce individual infection risk can be designed and implemented appropriately. Crucially, most parasites exist within multi-host communities, and often appear to infect multiple sympatric species, all of which potentially play a role in parasite persistence. However, certain species may contribute disproportionately to transmission and be nearly completely responsible for the persistence of a parasite within a community. Identifying such “key hosts” therefore offers a means to appropriately target control interventions to maximise success. However, assessing host species contributions to parasite transmission within multi-host communities is a challenging task, and much insight can be gained from studies of model host-parasite systems. In this thesis, the transmission dynamics responsible for the persistence of several endemic Bartonella parasites (bacterial flea-borne haemoparasites) are investigated within wild sympatric populations of wood mice (Apodemus sylvaticus) and bank voles (Myodes glareolus) in northwest England. Bartonella infections were first identified to species-level according to an existing method based on length polymorphism of a fragment of the 16S-23S internal transcribed spacer region (ITS). Broad patterns of prevalence suggested that some species were host generalists while others were host-exclusive, indicating that different transmission dynamics underlie the persistence of each Bartonella species within the rodent community. Attempts to identify key transmission hosts for each Bartonella species based on the effect of past host population densities on infection risk proved inconclusive. However, finer-scale characterisation of Bartonella infections, using DNA sequencing, found that Bartonella species that appear to be generalists actually comprise a complex of genetic variants, the majority of which are host-specific, suggesting that transmission between host species is uncommon and limited to a relatively few host-shared variants. Furthermore, detailed characterisation of the flea community infecting wood mice and bank voles found that these Bartonella vectors were host-generalists, and that at least two flea species were able to transfer between individuals of different host species. This suggests that a lack of between-species transmission is likely to arise through different compatibility between host species and Bartonella variants, rather than as a result of current ecological encounter barriers (e.g. through differential Bartonella-flea or flea-rodent specificity). The results of an experimental manipulation of between-species transmission within these wild communities support the notion that between-species transmission of Bartonella parasites is uncommon. Across three woodland sites, bank voles were treated with a veterinary insecticide to remove their fleas and therefore reduce the rate of transmission of Bartonella from treated bank voles to the rest of the rodent community. Following treatment, risk of bank vole infection with bank vole-exclusive Bartonella variants was reduced, but there was no affect on the risk of bank vole infection with host-shared variants, nor risk of infection in wood mice with either wood mouse-exclusive or host-shared variants. Importantly, these treatment effects were best identified by grouping the parasites on a ‘functional’ (i.e. host-exclusive versus host-shared variants) rather than a taxonomic (i.e. Bartonella species) basis. Together, these findings highlight the importance of characterising parasite infections to as fine a scale as possible, and the value of using a combination of observation, genetic and experimental approaches to understand parasite transmission within complex natural multi-host systems.