Temporal surveillance of Coccidioides in soils and aerosols at a single location in Arizona

Valley fever is a lung infection caused by the inhalation of infectious spores from the fungi Coccidioides spp., a genus of soil dwelling fungal pathogen endemic to the arid regions of the southwestern United States, Mexico, Central and South America. Valley fever can exclusively be acquired through environmental reservoirs and is non-communicable from host to host. Very few Valley fever studies have focused on detecting Coccidioides spores in airborne respirable particles, which is the vector to infection. This study looks at the presence of Coccidioides in the air, soil, and burrow systems at a highly positive site in Mesa, Arizona. Monthly soil samples were taken from 14 animal burrows and 2 – 40 meter transects. Aerosol samples were collected for 24 hours every 6 days, following the Environmental Protection Agency sampling schedule. Two types of filter media were used for aerosol sampling: quartz fiber filters were used to determine gravimetric PM₁₀, key ions, and organic and elemental carbon, and cellulose filters were used to analyze key elements. Meteorological data, including relative humidity, wind speeds, wind direction, and temperature were collected from a nearby weather station. Temporal soil sampling showed that C. posadasii stayed present at the site during the entire duration of the study however, temporal fluctuations of fungal burden occurred with decreases in detection occurring in the early spring and mid-summer months. Spatial variation was also detected, with certain burrow systems maintaining a high fungal burden throughout the year while others transiently housing the pathogen. We also showed that the pathogen was detected in rodent burrows significantly more frequency than in our surface soil transects. The temporal patterns of positivity for all the burrows were consistent over three years of sampling. Coccidioides were detected in ~68% of aerosol samples. Bulk PM₁₀ did not have a statistically significant relationship with presence of Coccidioides, however, there was a statistically significant relationship between the amount of crustal material in the aerosols and presence of Coccidioides. Crustal material was reconstructed using the primary elements that make up earth’s crust (Al, Si, Fe, Ca, and Ti). Previous studies often link the presence of Coccidioides in the air with bulk PM₁₀ concentrations; however, we found that looking at bulk PM₁₀ concentrations gives an incomplete story. Additionally, there were statistically significant relationships with presence of Coccidioides and meteorological parameters, such as relative humidity and wind speed. This study emphasizes the importance of dust entrainment in the aerosolization and transport of Coccidioides.