Coccidioides posadasii

Coccidioides posadasii is a pathogenic fungus that, along with Coccidioides immitis, is the causative agent of coccidioidomycosis, or valley fever in humans. It resides in the soil in certain parts of the Southwestern United States, northern Mexico, and some other areas in the Americas, but its evolution was connected to its animal hosts. Coccidioides posadasii and C. immitis are morphologically identical, but genetically and epidemiologically distinct. C. posadasii was identified as a separate species other than C. immitis in 2002 after a phylogenetic analysis. The two species can be distinguished by DNA polymorphisms and different rates of growth in the presence of high salt concentrations: C. posadasii grows more slowly. It also differs epidemiologically, since it is found outside the San Joaquin Valley. Unlike C. immitis, which is geographically largely limited to California, C. posadasii can also be found in northern Mexico and South America.