A framework for reconciling competing effects of competitors on disease transmission

Biodiversity is disappearing at an alarming rate, and at the same time, infectious diseases increasingly spill over from wildlife to humans. Therefore, it is urgent to understand how species diversity affects infectious diseases in wildlife. In some ecosystems, increasing diversity of competitor species decreases disease risk, termed the 'dilution' effect because diversity dilutes out infection. However, in other systems, a more diverse community increases disease risk, termed the 'amplification' effect. When to expect dilution or amplification is a topic of debate among disease ecologists. This project aims to reconcile the competing effects of biodiversity using Sin Nombre hantavirus (SNV) in deer mice as a model. This system displays aspects of both dilution and amplification at the same time. This project lays out a framework for examining potential mechanisms of dilution and amplification and how they can interact to potentially be synergistic or antagonistic. This will facilitate moving beyond the long-standing debate of which pattern (dilution or amplification) is more prevalent, clarifying when different mechanisms may be dominant and help explain the different patterns seen in various studies. It will also benefit society by advancing prevention efforts for hantavirus pulmonary syndrome (HPS). This human illness caused by SNV, has affected more than 700 people (killing more than 250) since its discovery in 1993. Other broader Impacts of this research include outreach and work with potentially affected communities, with particular focus on K-12 outreach. Three mechanisms determine if increasing competitor diversity will lead to dilution or amplification of a directly transmitted pathogen. Dilution will occur if increased species diversity leads to 1) decreased host density, 2) decreased contact rates, or 3) decreased probability of transmission given contact (transmissibility). Conversely, amplification could occur if increased diversity does the opposite. This project develops a framework to disentangle these component dilution and amplification effects, centered around a mathematical model that combines SIR (Susceptible-Infected-Recovered) dynamics with a Lotka-Volterra competition coefficient. Aim 1 is to determine how increased small mammal diversity / composition affects mouse population demography and subsequent SNV prevalence, by performing Bayesian population analyses on longitudinal data. Aim 2 is to determine how increased small mammal diversity affects the transmission rate, through changes in transmissibility (via stress, co-infections, immunity, and viral shedding) and/or contact rates between deer mice, using a combination of long-term monitoring of physiology in natural populations, and experimental manipulation in the field. Aim 3 is to synthesize the findings from Aims 1 and 2 into a general model of how competitors can differentially affect all three components of transmission and scale up to determine the variety of observed patterns in disease prevalence within and across systems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.