Coccidioidomycosis, commonly known as Valley fever (see “Epidemiology,” below), 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 (hence the sobriquet “Valley fever”) 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). The recent acquisition of cases well outside the recognized areas, including in eastern Washington state and in northeastern Utah, suggests that the endemic region may be expanding. 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. In the United States, 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. When found, Coccidioides is isolated 2–20 cm below the surface; it is not found in soil at greater depths, nor is it usually isolated from cultivated soil.
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. Overall, the incidence within the United States increased substantially over the past decade, with nearly 43 cases per 100,000 residents of the endemic region in 2011. Most of that increase occurred in south-central Arizona, where most of that state’s population resides, and in the southern San Joaquin Valley of California, a less populated region. The factors causing this increase have not been fully elucidated; however, an influx of older individuals without prior coccidioidal infection appears to be involved. Other variables, such as climate change, construction activity, and increased awareness and reporting, may also be contributors. Health care providers should consider coccidioidomycosis when evaluating persons with pneumonia who live in or have ...