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Identification of Viable Histoplasma capsulatum

Category Microbiology Laboratory
Test Identification of Viable Histoplasma capsulatum

Histoplasma capsulatum (teleomorph: Ajellomyces capsulatus) is a microfungus that is endemic to most of the United States, with particularly high concentrations in the Mississippi, St. Lawrence and Ohio River valleys. The fungus thrives in matter with high nitrogen content, in particular bird manure (i.e. pigeon or starling roosts). Local outbreaks are usually correlated to the disturbance of roosting sites (i.e. a building renovation), which can produce large numbers of airborne fungal spores.

Histoplasmosis (an infection by Histoplasma capsulatum) can be acute, chronic or disseminated. Acute infections are most common in otherwise healthy individuals and it is estimated that more than 95% of these cases will clear up on their own with little more than flu-like symptoms. Chronic histoplasmosis is a disease that can reactivate years, and even decades, after the initial exposure as the host’s immune system weakens. Disseminated histoplasmosis is commonly associated with those individuals with impaired immune systems (such as AIDS patients), and occasionally otherwise healthy individuals under the age of two (2) years. Disseminated histoplasmosis can be life-threatening and is considered a serious fungal infection.

The detection and quantitation of Histoplasma capsulatum faces many problems. First, and foremost, H. capsulatum can not be identified by classical nonviable testing. The spores of H. capsulatum are too similar to certain other fungi to allow an accurate identification. This means all H. capsulatum investigations must be by viable cultures. The analysis is further complicated by the fact that H. capsulatum is a very slow growing fungus. Under ideal laboratory conditions, the fungus takes between six to eight weeks to grow to maturity. These facts dissuade most IAQ investigators from testing for this organism. Due to this long growth time, special media must be used for any type of bioaerosol sampling, as you must both promote the growth of H. capsulatum while simultaneously suppressing the growth of more rapidly growing fungi (i.e. Aspergillus, Penicillium, etc.).

So when should an IAQ investigator look for Histoplasma capsulatum, and how should it be done? Histoplasma capsulatum should be suspected in any situation which involves the removal of a significant amount of bird manure, especially in the geographical locations mentioned above. The most common situations involve the renovations of exposed areas in the upper floors of buildings (attics, unfinished upper floors, etc.) where birds have been known to roost for extended periods of time. Another common situation, though unusually from the IAQ standpoint, are areas where chickens have roosted for significant periods of time (i.e. old farm renovations). Sampling is most appropriately done with an Andersen-style impactor utilizing specific media. The most common media for this test are BHI-CC (Difco) and Mycosel (Difco, BD), both of which are available from EMSL with advanced notice. Swab samples may also be submitted, but bulk material should not be submitted as it poses an elevated biosafety risk to the analytical lab.

Analytical Code: M120

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