Recent and Forecasted Increases in Coccidioidomycosis Incidence Linked to Hydroclimatic Swings, California, USA

Abstract

In 2023, California reported near-record high coccidioidomycosis cases after a dramatic transition from drought to heavy precipitation. Using an ensemble model, we forecasted 12,244 cases statewide during April 1, 2024-March 31, 2025, a 62% increase over cases reported 2 years before and on par with case counts for 2023.

Keywords: California; United States; Valley fever; climate change; coccidioidomycosis; ensemble modeling; forecast; fungi; hydroclimate; infectious disease; precipitation variability.

Figure 1

Statewide monthly coccidioidomycosis cases, January 1, 2000–March 31, 2025, in study of recent and forecasted increases in coccidioidomycosis incidence linked to hydroclimatic swings, California, USA. Black dots indicate confirmed cases reported during 2000–2022, times symbols indicate the provisional cases reported during January 1, 2023–December 31, 2024, the green line represents the ensemble model fit to the observed case data (R² = 0.87), and the purple line indicates the ensemble model predicted (April 1, 2023–March 31, 2024) and forecasted (April 1, 2024–March 31, 2025) cases. Shading represents 90% prediction intervals.

Figure 2

Regional monthly coccidioidomycosis cases, January 1, 2015–March 31, 2025, in study of recent and forecasted increases in coccidioidomycosis incidence linked to hydroclimatic swings, California, USA. A) Bay Area; B) Central Coast; C) Eastern California; D) Northern California; E) Northern San Joaquin Valley; F) Southern Coast; G) Southern Inland; H) Southern Sacramento Valley; I) Southern San Joaquin Valley. Black dots indicate confirmed cases reported during 2015–2022, green line represents the ensemble model fit to the observed case data, and the purple line indicates the ensemble model predicted (April 1, 2023–March 31, 2024) and forecasted (April 1, 2024–March 31, 2025) cases during April 1, 2023–March 31, 2025. Shading represents 90% prediction intervals.

Figure 3

Forecasted regional monthly coccidioidomycosis cases during January 1, 2023–March 31, 2025, under varying future climates in 2024–2025 in study of recent and forecasted increases in coccidioidomycosis incidence linked to hydroclimatic swings, California, USA. A) Bay Area; B) Central Coast; C) Eastern California; D) Northern California; E) Northern San Joaquin Valley; F) Southern Coast; G) Southern Inland; H) Southern Sacramento Valley; I) Southern San Joaquin Valley. The baseline climate scenario represents the 50th percentile of precipitation during 1981–2023 and extrapolated monthly average temperatures assuming a 42-year linear trend. The 20th and 80th percentile precipitation scenarios assume the baseline temperature scenario, and the 3°F warmer or cooler temperature scenarios assume the baseline precipitation scenario (i.e., 50th percentile).