What is Valley Fever?

What is Valley Fever?

Valley Fever (coccidioidomycosis, or cocci) is caused by the soil-dwelling fungus, Coccidioides immitis. The tiny seeds, or spores, become wind-borne and are inhaled into the lungs, where the infection starts.

When soils containing the fungus are disturbed and dust is raised, spores may be inhaled with the dust. Dust disturbing activities include, the wind, construction, farming, among others.

Once inside the lung, the spore transforms itself into a larger, multicellular structure called a spherule. The spherule continues to grow and will eventually burst, releasing endospores which develop into new spherules, and then repeats the cycle (Figure 1).

Valley Fever is a sickness of degree. About 60 percent of the people who breathe the spores do not get sick at all. For some it may feel like a cold or flu. For those sick enough to go to the doctor, it can be serious, with pneumonia-like symptoms, requiring medications and bed rest.

Of all the people infected with Valley Fever, one or more out of 200 will develop the disseminated form, which is devastating, and can be fatal. These are the cases in which the disease spreads beyond the lungs through the bloodstream - typically to the skin, bones and the membranes surrounding the brain, causing meningitis.                                                                                       

The Endemic Area

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 rheumatism".  Valley Fever is prevalent in the San Joaquin and Central Valleys of California,  and in the hot, desert regions of southern Arizona (this includes the major metropolitan areas of Phoenix and Tucson), southern Nevada (including Las Vegas), southern Utah, southern New Mexico, western Texas (including El Paso), and Mexico (especially in the states of Sonora and Chihuahua). In  addition, Coccidioides immitis is found in semiarid and arid soils in Central and South America (Figure 2).

Who gets it?

Other 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 percent of these people live in southern Arizona, which includes the Phoenix and Tucson metropolitan areas. If you live in an endemic area, you may have had Valley Fever without even knowing it. In some endemic areas, it estimated that as much as half the population has been infected. Among those who have never had Valley Fever, the chance of infection is about three percent 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 per year.

Particular occupations, including construction, agricultural work, any work involving disturbance of arid soils and archaeology, have been shown to have an increased risk of exposure and disease. Particular soil compositions have proven to have a higher composition of fungus causing Valley Fever, Coccidioides immitis. These include the soils around rodent burrows, Indian ruins and burial grounds. In these environments infections can be more severe due to the intensive exposure to a large number of spores. Wind storms have also demonstrated the capacity to distribute the spores by making them airborne. This allows the fungus to infect people that may live and work many kilometers from the originating soil(s).

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 anti-fungal medication.

Valley Fever and Climate

Although the relationship between the weather and the density of C. immitis in the soil may never be understood in detail, the following relationship seems plausible.  During periods of drought , in Arizona for example spring and fall,  the number of organisms competing with C. immitis decreases.  C. Immitis, itself does not thrive, but remains viable though dormant during the arthroconidia phase of its' lifecycle.  When the rains return, the arthroconidia begin to germinate and multiply to higher density than usual because of the lack of competing organisms.  Once the soil dries again, the arthroconidia become ariborne and potentially infectious (Figure 3).

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What is Valley Fever?

Frequently Asked Questions

Current Research

Valley Fever Related Events


Directory of Related Links

Information provided by the Valley Fever Vaccine Project of the Americas and the Valley Fever Center for Excellence