What causes valley fever?

Causes of Valley Fever

Valley fever is caused by inhaling airborne spores of the dimorphic fungi Coccidioides immitis or Coccidioides posadasii, which are soil-dwelling organisms endemic to arid regions of the southwestern United States, Mexico, and parts of Central and South America. 1

Causative Organisms

• The disease results from inhalation of arthroconidia (fungal spores) from two species: Coccidioides immitis and Coccidioides posadasii, both of which reside in soil and are dimorphic fungi 1, 2
• These fungi are soil-dwelling organisms that thrive in arid desert environments 2

Geographic Distribution

• In the United States, most infections occur in highly endemic regions including southern Arizona, central and other areas of California, southern New Mexico, and west Texas 1
• Approximately 97% of U.S. cases are reported from Arizona and California alone 3
• The recognized geographic distribution is expanding to include new areas such as eastern Washington, Oregon, and Utah 2
• The fungus is also found in parts of Mexico, Central America, and South America 2, 4

Mechanism of Transmission

• Acquisition occurs via inhalation of arthroconidia that become airborne after soil disturbance 2
• Natural events that disperse spores include earthquakes, dust storms, and fires 2
• Human-related activities that aerosolize spores include military maneuvers, recreational activities, agriculture, and construction 2
• Dust storms are particularly important in dispersing Coccidioides spores, making dust exposure a key risk factor 5, 6

Environmental Conditions Favoring Fungal Growth

• The fungus thrives under specific climatic conditions: moist, warm soil followed by dry conditions with strong winds 6
• Coccidioides tolerates hot, dry periods better than competing soil organisms and proliferates during wet periods following droughts 6
• Recent increases in cases have been linked to hydroclimatic swings, particularly wet winters following drought years 7

Epidemiological Trends

• The incidence of reported coccidioidomycosis has increased substantially, from 2,265 cases in 1998 to 22,401 in 2011 1
• The estimated number of infections per year has risen to approximately 150,000, with half to two-thirds being subclinical 1
• California saw a near-record 9,054 provisional cases in 2023, with forecasts predicting continued high incidence 7

Clinical Timing

• Symptoms typically appear 1-3 weeks after exposure to the fungal spores, representing the incubation period 1, 8, 3