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 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). Cases where infection was acquired well outside the recognized endemic areas, including in eastern Washington state and in northeastern Utah, have been recently described, suggesting 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 environmental sites, the precise characteristics of potentially infectious soil are not known. In the United States, several outbreaks of coccidioidomycosis have been associated with soil from archeologic 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 at greater depths, nor is it usually isolated from cultivated soil.
In endemic areas, most cases of coccidioidomycosis occur without obvious soil or dust exposure, and it is presumed that infection occurs through inhalation of airborne fungal particles. Climatic factors may 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. From 2011−2017, there were 95,371 cases of coccidioidomycosis reported in the United States. During this time, there has been a general increase in the incidence of disease. In California, this increase has occurred both within the established endemic area of the San Joaquin Valley and in the areas contiguous to it. The factors associated with this increase have not been elucidated but likely include an influx of older individuals without prior coccidioidal infection into endemic areas, construction activity, increased reporting, and changing climatic conditions.
PATHOGENESIS, PATHOLOGY, AND IMMUNE RESPONSE