Experimental Evidence for Opposing Effects of High Deer Density on Tick-Borne Disease Prevalence and Hazard

Abstract Identifying the mechanisms driving disease risk is challenging for multi-host pathogens, such as Borrelia. burgdorferi s.l., the tick-borne bacteria causing Lyme disease. Deer are tick reproduction hosts but do not transmit B. burgdorferi s.l., resulting in potentially opposing effects on transmission. Here, we use a deer exclosure experiment to test three hypotheses for how high deer density shapes B. burgdorferi s.l. prevalence in ticks: (H1) high transmission on rodents due to higher tick densities; alternatively, (H2) low B. burgdorferi s.l. prevalence because more ticks feed on deer rather than transmission-competent rodents (dilution effect); (H3) ecological cascades, whereby lower vegetation decreases rodent abundance thus reducing transmission. Although we found support for all three mechanisms, prevalence was reduced almost 3-fold in high deer density plots compared to exclosures, suggesting that the dilution (H2) and cascade (H3) mechanisms outweighed the increased opportunities for transmission (H1). High deer density led to lower vegetation and fewer rodents, providing evidence for an ecological cascade. However, Lyme disease hazard (density of infected ticks) was increased 5-fold at high deer densities due to an 18-fold rise in tick density. This demonstrates that reproduction hosts like deer can drive up vector-borne disease hazard at high densities, despite simultaneously reducing pathogen prevalence.