Epidemiologic, Clinical, and Diagnostic Aspects of Coccidioidomycosis

MYCOLOGY The Coccidioides genus is considered dimorphic. Dimorphism is characterized by production of filamentous (mycelial) forms by certain fungi during their saprophytic phase in the environment or when incubated at lower temperatures on media. Conversion to yeast-like cellular forms occurs during their parasitic phase when invading an animal host or when incubated at elevated temperatures (11). Dimorphism in the genera Blastomyces, Histoplasma, Paracoccidioides, and Sporothrix, as well as in a single species of Penicillium (P. marneffei), is temperature dependent (thermal dimorphic). Dimorphism in Coccidioides is also characterized by the production of septate hyphae and thick-walled arthroconidia (2 by 4 m) that are formed along the length of the hyphae (enteroarthric development) during the saprophytic phase (6). The arthroconidia are commonly separated by empty, thin-walled, brittle cells (disjunctors) formed by autolysis of alternate conidia along the hyphae. Thus, arthroconidia are easily released into the air by soil disruption and wind. Upon inhalation into lungs or on rare occasion after percutaneous implantation into tissue, each arthroconidium transforms into a new multinucleated, spherical structure called the spherule. This structure increases in size and forms a thick outer wall. As it grows, the spherule begins to divide internally by invagination and formation of cleavage furrows, culminating in the production of numerous uninucleated endospores. Spherules vary from 60 to over 100 mi n diameter, while endospores remain 2 to 5 m in size. Mature spherules may contain 800 to 1,000 endospores, which are released into tissue upon the spherule’s rupture. In its parasitic phase, each endospore grows into a new spherule (1, 11). This phase of Coccidioides is influenced by more complex factors, including presence of phagocytic cells and increase in CO2, rather than temperature (10). Until the 1990s, the genus Coccidioides was thought to have only one species. No teleomorph (sexual form) has been described. Molecular phylogenetic methods have since shown that there are definite differences between isolates originating from diverse geographic locations, such as California (CA), Arizona (AZ), Texas (TX), Mexico, and South America (SA). The strains fall into two phylogenetic clades representing separate evolutionary species. One group (Group II) contains isolates originating from CA, while a second group (Group I) contains isolates from outside of CA (non-CA), inclusive of those from AZ, TX, Mexico, and SA. These clades have now been given species rank (4). The CA group (Group II) retained the species name of C. immitis, while the non-CA group (Group I) was named C. posadasii Fisher, Koenig, White, et Taylor, sp. nov. (2, 4).