What is the cause of Valley fever?

Valley Fever: Cause and Pathophysiology

Valley fever (coccidioidomycosis) is caused by inhaling spores (arthroconidia) of the soil-dwelling fungi Coccidioides immitis or Coccidioides posadasii. 1

Causative Organisms

• Valley fever is specifically caused by two species of fungi: Coccidioides immitis and Coccidioides posadasii 1, 2
• These fungi reside in soil and are dimorphic, meaning they exist in different forms depending on their environment 2
• Once inhaled by humans, the arthroconidia transform into spherules in the lungs, which grow and rupture to release more endospores 3

Geographic Distribution

• Most infections in the United States occur in endemic regions including southern Arizona, central and other areas of California, southern New Mexico, and west Texas 1
• The fungi are found in arid desert soils of the southwestern US, parts of Mexico, and Central and South America 2
• The geographic distribution of Coccidioides is expanding, with cases being diagnosed in new areas such as eastern Washington, Oregon, and Utah 2, 4

Transmission Mechanisms

• Transmission occurs exclusively through inhalation of airborne spores that become airborne after both natural events (earthquakes, dust storms, fires) and human activities (military maneuvers, recreational activities, agriculture, construction) 2
• No person-to-person or zoonotic transmission occurs 5
• Climate factors play a role in fungal growth and spore dispersal - the fungi thrive in moist, warm soil and are dispersed during dry, windy conditions 6

Risk Factors for Infection

• Living in or traveling to endemic areas is the primary risk factor 5
• The incidence of infection is increasing, likely due to both climate and population changes 2
• Certain populations have higher risk for severe or disseminated disease, including persons with cellular immunodeficiencies and non-Caucasian races (especially African Americans and Filipinos) 2

Clinical Manifestations

• Approximately 60% of infections are asymptomatic 2
• About one-third of infected individuals develop pulmonary illness, which is a leading cause of community-acquired pneumonia in highly endemic areas 2
• The most common clinical presentation is a self-limited acute or subacute community-acquired pneumonia that becomes evident 1-3 weeks after infection 1
• A common symptom triad called "desert rheumatism" includes fever, erythema nodosum, and arthralgia, often with respiratory symptoms 3
• In rare cases (0.5-2%), the infection can disseminate to extrapulmonary locations such as skin, bones/joints, and the central nervous system 2

Epidemiological Impact

• The estimated number of infections per year has risen to approximately 150,000, with half to two-thirds being subclinical 1
• In regions where coccidioidomycosis is endemic, incidence of reported cases increased substantially from 2,265 cases in 1998 to 22,401 in 2011 1
• Valley fever has been identified as the cause of 17% to 29% of all cases of community-acquired pneumonia in some endemic regions 5

Understanding the cause and transmission of Valley fever is essential for proper diagnosis, treatment, and prevention strategies in endemic regions and for travelers to these areas.