Understanding soil properties conducive for Coccidioides ssp. presence in the United States

Coccidioides immitis and C. posadasii are closely related fungal pathogens that cause coccidioidomycosis, a respiratory disease also known as Valley fever. In general, Coccidioides are regarded to grow in arid to semi-arid soils in North and South America. If a person inhales these spores, they can become sick with Valley fever. The soil properties conducive for the presence of Coccidioides are not currently well defined, including whether there are differences in the soil properties conducive for each species. Recent efforts, especially over the last decade, to collect soil samples positive for Coccidioides now provide the data to begin examining these questions. We compiled Coccidioides spp. occurrence data from both previous studies and studies published on the National Center for Biotechnology Information (NCBI) database to examine the generalized soil properties associated with the presence of the pathogen. We analyzed 13 different soil properties from the California Soil Resource Lab at University of California Davis database derived from USDA-NCSS soil data and one measure of ecoregions from the US Environmental Protection Agency. Comparing the two species, we found that C. immitis was present in soils with a statistically significant higher water holding capacity and silt content than C. posadasii. Additionally, C. immitis was found in soils with significantly higher soil organic matter and calcium carbonate content than C. posadasii. This may suggest that C. immitis is more likely to grow in wetter and more productive soils compared to C. posadasii. Understanding the soil properties conducive for each Coccidioides species will allow us predict areas prone to their presence, enabling the creation of higher resolution risk maps for Valley fever and preventative messaging to at-risk populations.