Eco-epidemiology of a complex system: unraveling the link between Lyme disease and biodiversity

Background/Question/Methods

In the eastern US, most human cases occur in the North, despite the vector – the blacklegged tick- being found throughout the eastern U.S.  Variation in the abundance of infected ticks also exists among Lyme disease endemic regions, as well where the blacklegged tick is actively expanding.  A central question in Lyme disease ecology is how does biodiversity affect human disease risk?

To explain this pronounced North-South gradient in Lyme disease, medical entomologists conventionally hypothesize that nymphal ticks (the epidemiologically riskiest life stage) exhibit a different host-seeking behavior in the South such that the risk of humans being bitten by an infected nymph is negligible.  An alternative hypothesis has been that juvenile ticks in the South disproportionately feed on incompetent hosts such as lizards, rather than on reservoir competent hosts; resulting in few infected nymphs.  This hypothesis has been broadened into a more general biodiversity-dilution hypothesis, whereby increased host biodiversity is thought to reduce the density of infected ticks. 

We used a modeling approach to explore the consequences of biodiversity on the relative abundance of infected nymphs.  We modified a previously published model that investigates transmission between a seasonally dynamic reservoir host population and a seasonally dynamic vector tick population. 

Results/Conclusions

Our model shows that either reduction or amplification of the density of infected nymphs can occur with increased host biodiversity, depending on the biological details of the modeled host community.  If competition exerted by alternative incompetent hosts reduces the density of reservoir hosts, dilution may occur.  Similarly, if alternative hosts feed juvenile ticks at a much higher rate such that they divert juvenile ticks away from reservoir hosts, dilution may occur.   Finally, if alternative hosts kill juvenile ticks that attempt to feed on them, for example, through grooming or through an immune response, dilution can also occur.  If alterative hosts, however, allow a greater proportion of juvenile ticks to feed successfully, then amplification may occur.

In addition to these model results, empirical results for a multisite (n=9) study conducted to test the biodiversity- dilution hypothesis will be discussed in the context of the model as well as of other studies addressing the relationship between biodiversity and Lyme disease.