Coccidioides is a haploid ascomycete found in soil of semiarid regions ofthe southwestern US, Mexico, and Central and South America [21, 22]. As a saprobe, this dimorphic fungus generates mycelia in its saprobic phase that produce enterothal-lic arthroconidia. Aerosolized arthroconidia can be inhaled and then enter the parasitic phase with differentiation into large spherules. At least in vitro, spherula-tion is induced by elevated temperature (34-41 °C) and CO2 concentration (1020%); at low temperature arthrospores produce hyphae. Spherules divide internally into endospores and rupture with maturation to release thousands of endo-spores which in turn are capable of generating new endosporulating spherules [23]. Coccidioidomycosis is characterized by progressive pulmonary disease and/ or dissemination to virtually any body site with the exception of the gastrointestinal tract. Meningitis develops in 30-50% of cases of disseminated disease and, if left untreated, is fatal. Coccidioidomycosis can affect previously healthy individuals and, depending on the inhaled dose, genetic predisposition, and immune status of the host, the mycosis ranges from self-limiting infection to disease with high morbidity and mortality [24]. Two very closely related species cause coccidioidomycosis: Co. immitis and Co.posadasii [25]. No sexual phase is known in either species. The genomes of Co. immitis and Co. posadasii are currently being sequenced by the Broad Institute and The Institute for Genomic Research (TIGR), respectively (see Table 18.2).

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