Association between wildfires and coccidioidomycosis incidence in California, 2000–2018: a synthetic control analysis

Abstract

Background: The frequency and severity of wildfires in Western U.S. has increased over recent decades, motivating hypotheses that wildfires contribute to incidence of coccidioidomycosis, an emerging fungal disease in the Western U.S. with sharp increases in incidence observed since 2000. While coccidioidomycosis outbreaks have occurred among wildland firefighters clearing brush, it remains unknown whether fires are associated with increased incidence among the general population. Methods: We identified 19 wildfires occurring within California’s highly endemic San Joaquin Valley between 2003 and 2015. Using geolocated surveillance records, we applied a synthetic control approach to estimate the effect of each wildfire on the incidence of coccidioidomycosis among residents that lived within a hexagonal buffer of 20 km radii surrounding the fire. Results: We did not detect excess cases due to wildfires in the 12 months (pooled estimated percent change in cases: 2.8%; 95% CI: -29.0-85.2), 13-24 months (7.9%; 95% CI: -27.3-113.9), or 25-36 months (17.4%; 95% CI: -25.1-157.1) following a wildfire. When examined individually, we detected significant increases in incidence following three of the 19 wildfires, all of which had relatively large adjacent populations, high transmission prior to the fire, and a burn area exceeding 5,000 acres. Discussion: We find limited evidence that wildfires drive increases in coccidioidomycosis incidence among the general population. Nevertheless, our results raise concerns that large fires in regions with ongoing local transmission of Coccidioides may be associated with increases in incidence, underscoring the need for field studies examining Coccidioides spp. in soils and air pre- and post- wildfires.

What this study adds: Amid the growing intensity of wildfires in California and rising coccidioidomycosis incidence, this study fills a gap in the literature by examining potential associations between wildfire activity and coccidioidomycosis transmission in populations residing around wildfire burn areas. Our findings suggest that public health efforts to mitigate the impact of wildfires on coccidioidomycosis risk should continue to focus on prevention and worker protections during wildland firefighting. While we do not find evidence that increasing wildfire severity and frequency play an important role in driving observed increasing trends in coccidioidomycosis incidence in the state, our findings underscore the need for focal study of Coccidioides spp. in soils and air pre- and post-fires in populous regions with known local transmission.

Description

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Keywords

Coccidioidomycosis, Coccidioides, Valley fever, Wildfires, Synthetic Control, Drought, Climate Change, California

Citation

Phillips, S., Jones, I., Sondermyer-Cooksey, G., Yu, A. T., Heaney, A. K., Zhou, B., Bhattachan, A., Weaver, A. K., Campo, S. K., Mgbara, W., Wagner, R., Taylor, J. W., Lettenmaier, D. P., Okin, G. S., Jain, S., Vugia, D., Remais, J. V., & Head, J. R. (2023). Association between wildfires and coccidioidomycosis incidence in California, 2000–2018: a synthetic control analysis. Environmental Epidemiology, 7(4), e254. https://doi.org/10.1097/ee9.0000000000000254

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