The Spatiotemporal Relationship between Climate and Valley Fever in the Southwestern United States

Valley fever, formally coccidioidomycosis, is an infectious disease endemic in the southwestern United States and contracted by the inhalation of airborne Coccidioides spp. fungal spores. Incidence throughout the region has been increasing and susceptible individuals who contract the disease may suffer from short term, flu-like illness to severe, long term morbidity. The relationship between climatological and environmental precursors and increases in valley fever incidence remains poorly understood. Furthermore, previous studies have been largely limited to the single-county-level and climate data extrapolated from point sources.

This study analyzes valley fever incidence on the county-level throughout the southwestern United States (AZ, CA, NM, NV, UT) and its relationship to climate drivers by compiling the first robust valley fever incidence database and analyzing spatial and temporal dynamics. The climate driver datasets used to interpret the valley fever incidence include precipitation, soil moisture, surface air temperature, vegetation status derived from normalized difference vegetation index, and aerosol optical depth. We also analyzed anthropogenic factors that may influence local environmental conditions, including indicators of soil disturbance from construction statistics and agricultural data on the spatial distribution of croplands and the timing of plowing and harvest.

Our constructed climatological map of valley fever incidence from 2000-2014 provides evidence that areas deemed endemic by the Centers for Disease Control and Prevention should be extended to include a larger portion of California’s Central Valley and the southwestern United States. The seasonal dynamics of incidence differ greatly by county across the southwestern United States. There is a peak in winter incidence in counties that receive the majority of their annual precipitation from Pacific storm systems, and a more uniform seasonal pattern of incidence in counties that receive significant precipitation during summer as part of the North American monsoon. Our data suggest that from 2010 to 2011, a substantial increase in incidence throughout the region is correlated with preceding climate conditions, with drying trends leading to increases in incidence. We plan to use the results from our analysis to create a species distribution model; this model will be forced with climate change scenarios to forecast where valley fever may become prevalent and what new communities may be at risk.