Valley fever (coccidiomycosis)

January 2007     View PDF
Reprinted from University of Arizona’s Valley Fever Center for Excellence, US

Valley Fever is primarily a disease of the lungs that is common in the southwestern United States and northwestern Mexico. It is caused by the fungus Coccidioides immitis, which grows in soils in areas of low rainfall, high summer temperatures and moderate winter temperatures. These fungal spores become airborne when the soil is disturbed by winds, construction, farming and other activities. In susceptible people and animals, infection occurs when a spore is inhaled. Within the lung, the spore changes into a larger, multicellular structure called a spherule. The spherule grows and bursts, releasing endospores which develop into spherules. Valley Fever symptoms generally occur within three weeks of exposure. Valley Fever is not a “contagious” disease, meaning it is not passed from person to person. Second infections are rare.

Valley Fever derives its name from its discovery in the San Joaquin Valley of California, where it was also referred to as “San Joaquin Valley fever”, “desert fever”, or “desert rheumatism”. The medical name for Valley Fever is coccidioidomycosis (often shortened to “cocci”, pronounced KOK-SEE), meaning a fungal infection (“mycosis”) caused by the fungus Coccidioides.

Valley Fever is prevalent in the San Joaquin and Central Valleys of California, and in the hot, desert regions of southern Arizona (especially in the Phoenix and Tucson areas), southern Nevada, southern Utah, southern New Mexico, western Texas (especially around El Paso), and Mexico (in the states of Sonora and Chihuahua). In addition, Coccidioides immitis is found in semi-arid and arid soils in Central and South America.

Estimates indicate that in the United States more than 4 million people live in areas where the Valley Fever fungus is prevalent (or “endemic”) in the soils. About 80% of these people live in southern Arizona, which includes the Phoenix and Tucson metropolitan areas. Residents of Phoenix, Arizona and Bakersfield, California have shown positive skin-test reaction rates of 30–40%, meaning that about one-third of residents tested have had Valley Fever sometime in the past. Among those who have never had Valley Fever, the chance of infection is about 3% per year, but the longer one resides in an endemic area, the greater the risk. In the southwestern U.S., there are approximately 100,000 new infections each year.

People working in certain occupations, such as construction, agricultural work, work involving disturbance of desert soils, and archaeology, have an increased risk of exposure and disease. The fungal spores of Coccidioides immitis are often found in abundance in the soil around rodent burrows, Indian ruins and burial grounds. In these settings, infections are more likely to be severe because of intensive exposure to a large number of spores. Many infections, however, occur in persons without occupational risks. Exposure to windstorms or recently disrupted soils may increase the chances of infection.

Valley Fever infections are more likely to occur during certain seasons. In Arizona, the highest prevalence of infections occurs June through July and from October through November. In California, the risk of infection is highest from June through November, without the late summer break.

Many domestic and native animals are susceptible to the disease, including dogs, horses, cattle, sheep, burros, coyotes, rodents, bats and snakes. Dogs are especially susceptible and often need long-term therapy with antifungal medication.

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