Coccidioidomycosis, commonly known as Valley Fever, is caused by dimorphic soil-dwelling fungi of the genus Coccidioides. Genetic analysis has demonstrated the existence of two species, C. immitis and C. posadasii. These species are indistinguishable with regard to the clinical disease they cause and their appearance on routine laboratory media. Thus, the organisms will be referred to simply as Coccidioides for the remainder of this chapter.
Coccidioidomycosis is confined to the Western Hemisphere between the latitudes of 40°N and 40°S. In the United States, areas of high endemicity include the southern portion of the San Joaquin Valley of California and the south-central region of Arizona. However, infection may be acquired in other areas of the southwestern United States, including the southern coastal counties in California, southern Nevada, southwestern Utah, southern New Mexico, and western Texas, including the Rio Grande Valley. Outside the United States, coccidioidomycosis is endemic to northern Mexico as well as to localized regions of Central America. In South America, there are endemic foci in Colombia, Venezuela, northeastern Brazil, Paraguay, Bolivia, and north-central Argentina.
The risk of infection is increased by direct exposure to soil harboring Coccidioides. Because of difficulty in isolating Coccidioides from the soil, the precise characteristics of potentially infectious soil are not known. Several outbreaks of coccidioidomycosis have been associated with soil from archaeologic excavations of Amerindian sites both within and outside of the recognized endemic region. These cases often involved alluvial soils in regions of relative aridity with moderate temperature ranges. Coccidioides was isolated at depths of 2–20 cm below the surface.
In endemic areas, many cases of Coccidioides infection occur without obvious soil or dust exposure. Climatic factors appear to increase the infection rate in these regions. In particular, periods of aridity following rainy seasons have been associated with marked increases in the number of symptomatic cases. The number of cases of symptomatic coccidioidomycosis has increased dramatically in south-central Arizona, where most of the state's population resides. The factors causing this increase have not been fully elucidated; however, an influx of older individuals without prior coccidioidal infection into the region appears to be involved. Other variables, such as climate change, construction activity, and increased awareness and reporting, may also be factors. A similar increase in the incidence of symptomatic cases has recently been observed in the southern San Joaquin Valley of California.
Pathogenesis, Pathology, and Immune Response
On agar media and in the soil, Coccidioides organisms exist as filamentous molds. Within this mycelial structure, individual filaments (hyphae) elongate and branch, some growing upward. Alternating cells within the hyphae degenerate, leaving barrel-shaped viable elements called arthroconidia. Measuring ∼2 by 5 μm, arthroconidia may become airborne for extended periods. Their small size allows them to evade initial mechanical mucosal defenses and reach deep into the bronchial tree, where infection is ...