Differentiation of springtime vegetation indices associated with summer anthrax epizootics in west Texas, USA, deer

Anthrax outbreaks in white-tailed deer, Odocoileus virginianus, are frequent in west Texas, USA, particularly across the Edwards Plateau. However, the outbreak severity varies among years. We summarize the outbreak history in white-tailed deer at a ranch north of Del Rio, Texas, from 2001 to 2010 and compare mortality rates to remotely sensed vegetation indices derived from Moderate Resolution Imaging Spectroradiometer satellite data. It has long been posited that the occurrence of mid- to high-latitude epizootics is associated with hot, dry summer conditions preceded by a wet spring, with cases occurring after summer rain events. Here we employed vegetation green-up indices as a proxy for such environmental conditions. Annual trajectories of vegetation indices identified a clear pattern of early green springs with dry summers in severe outbreak years. In contrast, later, less intense spring green-up with greener summers were associated with enzootic mortality years, when few cases occurred. There was a statistically significant difference in the annual timing and intensity of spring green-up from vegetation indices for epizootic and enzootic years. Years with epizootics have early, intense spring conditions, whereas enzootic years have low-intensity green-up. These results suggest that early green-up signatures may be useful in identifying epizootic climatic conditions ahead of the summer anthrax period. Such analyses are required to ultimately develop an early warning system for wildlife managers and veterinary public health officials.