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Coccidioides posadasii was first discovered in Argentina in 1890 by Alejandro Posadas, after whom the fungus is named. The doctor had a patient by the name of Domingo Ezcurra, who had the first documented case of coccidioidmycosis, also known as Valley Fever, a serious fungal infection caused by C. posadasii and its close relative, Coccidioides immitis. Posadas followed Ezcurra’s case until the latter’s death, after which his head was preserved and put on display in an anatomy museum, where it is still studied today.
C. posadasii is a dimorphic pathogen, meaning it has two major morphological forms in its life cycle. In its vegetative state, it lives in the soil as a saprobe, using its thin, filamentous hyphal form to digest organic materials. The mycelium is septate, and when cultured, the fungus is initially moist and smooth and then becomes suede-like and greyish-white and brown. When the soil begins to dry out and nutrients run low, C. posadasii’s inner cell walls thicken and alternate compartments are broken down to form arthroconidia, also known as arthrospores. These spores form by means of mitosis and are anywhere from rectangular to barrel-shaped, approximately three by five micrometers in size. The arthroconidia are eventually are dispersed by wind or other soil disturbances. This allows them to either be dispersed to another location, where the hyphal form is reformed, or to be inhaled by humans, cows, rodents, and other potential host organisms.
It is at this point that the fungus reverts to its parasitic phase and its second major morphological form. C. posadasii is a thermal dimorph, meaning that heat triggers its change in form. Thus, the host organism’s body temperature (usually around 37 degrees Celsius) will turn the spores into spherules. These structures, which have membranous walls rich in lipids and are anywhere from 20 to 80 micrometers in diameter, in turn septate and produce around 200 to 300 uninucleate endospores. These endospores are released when the sphorule breaks open and can be carried around the body by the bloodstream or the lymphatic system to infect other tissues. Susceptible tissues include the lungs, skin, heart, pericardium, bones, and central nervous system.
No method of sexual reproduction in C. posadasii has been found, which makes them mitosporic, also called imperfect, fungi. The absence of a sexual phase made classifying the fungus very difficult at first. It was only when Sigler and Carmichael noticed the similarities between the arthroconidia of C. posadasii and the aleurioconidia of Malbranchea Sacc. This indicated that the fungus probably belonged in the order Onygenaceae, a classification that was later confirmed by molecular methods. Although there is no evidence of sexual reproduction, there are “sexes” within the species. Individuals within coccidioides strains contain one of two alleles at a specific locus, each anywhere from 8.1 to 9 kb in length and made up of five or six genes.