Discerning Relationships between Coccidioidomycosis (Valley Fever), Root Zone Soil Moisture, and El Nino Southern Oscillation in California and Arizona

Abstract:

The soil-borne fungal disease Coccidioidomycosis (Valley Fever) is prevalent across the southwestern United States (US). Previous studies have suggested that the occurrence of this affliction is associated with an anomalously wet or dry soil moisture state. Spatial analysis of soil moisture can be problematic if based on sparse in situ measurements. However, a root zone (0 to 40 cm depth) soil moisture product Soil MERGE (or SMERGE version 2.0) provides continuous coverage across the contiguous US for four decades (1979 to 2019). This product was developed by merging output from the North American land data assimilation system (Noah model version 2.8) with surface satellite retrievals from the European Space Agency Climate Change Initiative (Combined product version 3.3). In this study, average SMERGE anomalies were aggregated on a county basis and a monthly time scale between 2001 to 2018. Coccidioidomycosis cases in Arizona (AZ; Arizona Department of Health Services) and California (CA; US Centers for Disease Control) were normalized based on county population on an annual basis using US Census Bureau American Community Survey data. A detailed statistical analysis on a seasonal basis of two counties (Kern County CA; Maricopa County AZ) in each state with the greatest number of normalized cases was made. Highest correlations were noted when soil moisture was lagged by twelve months and normalized cases by twenty-four months relative to the El Nino Southern Oscillation (ENSO) index. Wavelet Transform Coherence (WTC) analysis between SMERGE - Coccidioidomycosis normalized cases and ENSO was executed for all seventy-four CA and AZ counties. To facilitate analysis these counties were grouped into seven regions based on US National Weather Service forecast areas. Greater ranked correlation values between ENSO and SMERGE - Coccidioidomycosis normalized cases were noted for periodicities of six-to-seven years based on WTC analysis between the years 2005 to 2014. Interestingly, higher correlation values were not localized to the counties with the highest number cases but were widespread across both CA and AZ. ENSO has been previously demonstrated to strongly influence the hydroclimate of the southwestern US. This study provides a tangible example of how oceanic-atmospheric teleconnections can impact human health.