Coccidioidomycosis is an infection caused by the dimorphic fungus Coccidioides immitis. Usually, the primary pulmonary infection is self-limited, but disseminated and even fatal disease can occur. The infection is endemic in the Western Hemisphere in an area contained between 40 degrees of latitude north and south. In the United States, this area includes the deserts of the southwestern states: California, Arizona, New Mexico, and western Texas. This area corresponds to the Lower Sonoran life zone, which is characterized by arid to semiarid climate, hot summers and short winters, and limited rainfall with few freezes. The organism grows in the superficial layers of the soil to a depth of 20 cm. Arthroconidia are aerosolized during wind storms or when soil is disturbed, as by construction work, archaeologic digs, earthquakes, or farming. Prolonged droughts followed by rains increase the rate of primary infections, which are more likely to occur in the winter following germination of the organism in the soil after a rainy season. In endemic areas, infections occur in individuals without any predisposing condition at a rate estimated to be around 2% to 4% per year. On occasion, the infection has been transmitted through fomites, such as soil from potted plants.

In the Southwest, C. immitis results in 100,000 infections per year. It is seen also in other parts of the country in individuals who have lived or traveled to the endemic areas of the
Southwest. Although the infection rate in endemic areas has continued to drop, the rapid population growth in these areas continues to make coccidioidomycosis a significant health problem. While health care workers in endemic areas learn to suspect and identify the disease using the proper diagnostic tools, a high index of suspicion remains necessary, especially with increased exposure of larger numbers of travelers from nonendemic areas to the Southwest.

The organism’s life cycle exhibits two distinctive phases: a saprophytic or vegetative phase and a parasitic phase. On culture media and in nature, the organism grows as a mycelium with branching, septate hyphae. After 5 to 7 days, the mycelia develop rectangular spores (arthroconidia, arthrospores), which are easily airborne. The arthrospores measure 2 to 8 μm, allowing them to reach the alveolar spaces when aerosolized. In the host tissues, the arthroconidia begin the parasitic phase of the life cycle. They enlarge and develop into spherules, which are double-walled structures of 20 to 100 μm diameter. The spherules undergo internal segmentation, leading to the formation of endospores that measure 2 to 5 μm. When the spherules rupture in the surrounding tissues, they release the endospores, leading to the formation of more spherules. The spherule is noninfectious. As a result, person-to-person transmission of coccidioidomycosis does not occur except in rare situations wherein the organism is allowed to revert to its mycelial form, such as growth on a plaster cast from a draining wound. Alternately, when the spherules rupture into the environment, the released endospores can lead to formation of hyphae, thus repeating the cycle in nature.

The organism grows readily on standard laboratory media, producing visible mycelial colonies in 3 to 4 days. During the second week, the colonies develop cobweb-like aerial hyphae that can be aerosolized and are infectious. Identification of the colonies is achieved by animal inoculation, demonstration of the spherule stage, demonstration of specific exoantigens, or identification of the organism using specific genetic probes.

Acquisition of C. immitis infection usually occurs via the respiratory route, which may result from infection with as little as a single spore. In most patients, the infection remains localized to the lungs and the hilar nodes. In a few (0.5%), dissemination to extrapulmonary sites occurs via the lymphatics or the bloodstream. Rarely, infection can be acquired by skin puncture with a contaminated object.

The pulmonary disease is manifested by a bronchopneumonia that may involve any lobe. As the disease progresses, cell-mediated immune responses, which are essential for control of the disease process, become defective, resulting in an ineffective type 2 helper T-cell response.

C. immitis can spread beyond the lung, a process that usually occurs within weeks or months after initial infection. Endogenous reactivation of primary disease also may occur, especially in immunosuppressed children. Extrapulmonary dissemination may occur after primary infection in normal hosts and after primary infection or reactivation in immunocompromised hosts. Once dissemination occurs, cell-mediated immune responses often are impaired, especially in severe infections. Dissemination may occur to any part of the body and usually involves the meninges, bones, joints, skin or soft tissues, or lymph nodes. The tissue reaction in disseminated disease is primarily granulomatous, with the lesions containing abundant giant cells and histiocytes. Usually, spherules are seen readily within the macrophages.

The rates of dissemination beyond the lungs vary considerably and are higher in Filipinos, Hispanics, blacks, pregnant women, and infants. Immunosuppressed hosts, especially those with human immunodeficiency virus (HIV) infection, are prone to severe pulmonary disease and dissemination.