Movement patterns and spatial epidemiology of a prion disease in mule deer population units

Spatial patterns of disease occurrence across a landscape are likely products of both the ecological processes giving rise to underlying epidemics and the physical pathways of disease spread. Spatially explicit epidemic models often rely on assumptions about system boundaries and processes for spread that may not faithfully represent true patterns of host or vector distribution and movements. As a foundation for future modeling and parameter estimation, we evaluated potential influences of distribution and movements of mule deer (Odocoileus hemionus) on the spatial epidemiology of chronic wasting disease (CWD) in north-central Colorado. We used cluster techniques to define mule deer population units based on location data, and then used these as the sampling unit for subsequent analyses. We found marked differences in prevalence between population units that appeared at least partially related to deer movements. Migration (mean migration rate = 44%) rather than dispersal movements (less than or equal to2% dispersal rate) appeared the most likely mechanism for disease spread among population units. Analysis of exchange matrices coupled with prevalence differentials between population units indicated that a single source of CWD was unlikely in north-central Colorado. Using anthropogenic boundaries (such as counties or game management units) to define sample units rather than population units could have obscured the potential role of deer movement in the spatial epidemiology of CWD. Using population units or subpopulations as the sample unit and including movements at this scale are broadly applicable approaches for spatial epidemiology.