Studies have shown that in the Southwest US, Sin Nombre hantavirus (SNV) prevalence in deer mice is lower in communities with higher small mammal diversity– the so-called ‘dilution effect’. The mechanism driving this relationship has not been understood. Dilution will occur if increased species diversity leads to 1) decreased deer mouse density (if there is density-dependent transmission) or 2) decreased transmission rate. With this study, we aim to examine the relative importance of deer mouse density and transmission rate in determining SNV prevalence in deer mice in communities of varying small mammal diversity (by Simpson’s diversity index). We have longitudinal datasets from CDC-funded studies of variable lengths at 10 sites in Arizona, New Mexico, and Colorado, from 1994-2006, and 7 sites in Montana from 1994-2015. These sites span a range of small mammal diversities and deer mouse population dynamics, allowing the opportunity to examine important questions about ecological drivers of SNV prevalence. We fit our established SI (Susceptible-Infected) epidemiological model to these sites, which predicts SNV prevalence based on deer mouse densities. We estimate parameters for each site and compare to small mammal diversity across sites.
Results/Conclusions
Our model predicts infection dynamics well for each site given mouse population density. We found that transmission is indeed density-dependent and that increased small mammal diversity leads to decreased deer mouse density, which leads to decreased SNV prevalence, but with time lags that make the relationship hard to detect (but which can be predicted using our mechanistic model). We found that the transmission rate varied between sites. In contrast to the dilution effect, we found that transmission rate was positively correlated with small mammal diversity- a component ‘amplification effect’. In conclusion, the decrease in SNV prevalence in deer mice in diverse small mammal communities (‘dilution effect’) is driven by reduced deer mouse density, and once this is taken into account (with a mechanistic model), there is a positive effect of small mammal diversity on transmission rate (a component ‘amplification effect’ )—both dilution and amplification are occurring at the same time in the same system! Experimental studies are needed to examine potential drivers of the increase in transmission rate.