Coccidioides immitis/posadasii complex

Recently Coccidioides immitis has been recognised as 2 species: C. immitis and C. posadasii (Fisher et al. 2002). The two species are morphologically identical and can be distinguished only by genetic analysis and different rates of growth in the presence of high salt concentrations (C. posadasii grows more slowly). C. immitis is geographically limited to California’s San Joaquin Valley region, whereas C. posadasii is found in the desert regions of the USA southwest, Mexico and South America. The two species appear to coexist in the desert regions of the USA southwest and Mexico.

Colonies of C. immitis/posadasii on Sabouraud’s dextrose agar at 25C are initially moist and glabrous, but rapidly become suede-like to downy, greyish white with a tan to brown reverse, however considerable variation in growth rate and culture morphology has been noted.   Microscopy shows typical single-celled, hyaline, rectangular to barrel-shaped, alternate arthroconidia, 2.5-4 x 3-6 µm in size, separated from each other by a disjunctor cell.   This arthroconidial state has been classified in the genus Malbranchea and is similar to that produced by many non-pathogenic soil fungi, e.g. Gymnoascus species.

Culture of Coccidioides immitis.

Arthroconidia of C. immitis.

WARNING: RG-3 Organism. Cultures of C. immitis/posadasii represent a severe biohazard to laboratory personnel and must be handled with extreme caution in an appropriate pathogen handling cabinet. C. immitis is a dimorphic fungus, existing in living tissue as spherules and endospores, and in soil or culture in a mycelial form. Culture identification by the exoantigen test is now the method of choice.

MIC data is limited.  Antifungal susceptibility testing of individual strains is recommended.

Clinical significance:

Coccidioidomycosis is initially, a respiratory infection, resulting from the inhalation of conidia, that typically resolves rapidly leaving the patient with a strong specific immunity to re-infection. However, in some individuals the disease may progress to a chronic pulmonary condition or to a systemic disease involving the meninges, bones, joints and subcutaneous and cutaneous tissues.C. immitis/posadasii is a soil inhabiting fungus endemic in south-western U.S.A., northern Mexico and various centers in South America. Several cases have now been diagnosed in Australia, all in patients with a history of travel to endemic areas.

Mycosis: Coccidioidomycosis

Further reading:

Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.