Where the fungus lives
Histoplasma capsulatum is primarily found in the temperate regions of the world. It’s very common in the Ohio and Mississippi river valleys of North America, where many people have already been exposed to the fungus. 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.
It thrives in damp soil that’s rich in organic material, especially the droppings from birds and bats. For that reason, it’s particularly common in chicken and pigeon coops, old barns, caves and parks.
Birds themselves aren’t infected with histoplasmosis — their body temperature is too high — but they can carry H. capsulatum on their feathers, and their droppings support the growth of the fungus. Birds commonly kept as pets, such as canaries and parakeets, aren’t affected. And although bats, which have a lower body temperature, can be infected, you can’t get histoplasmosis directly from a bat or from another person.
Histoplasmosis is caused by the reproductive cells (spores) of the fungus Histoplasma capsulatum. The spores are extremely light and float into the air when dirt or other contaminated material is disturbed.
Histoplasmosis and your lungs
Because the spores of H. capsulatum are microscopic in size, they can easily enter your lungs and settle in the small air sacs. There, the spores are trapped by macrophages — immune system cells that attack foreign organisms. The macrophages carry the spores to lymph nodes in your chest, where they continue to multiply. This may lead to inflammation, scarring and calcium deposits. In cases of heavy infection, the lymph nodes may become so enlarged that they obstruct your esophagus or your lungs’ airways.
Most often, however, you’re not likely to have noticeable signs and symptoms, and the infection clears on its own without treatment. But if your immune system isn’t able to eliminate the spores, they can enter your bloodstream and travel to other parts of your body. In that case, you may develop a variety of severe problems that can be fatal if not diagnosed and treated quickly.
Even if you’ve had histoplasmosis in the past, you can still get the infection again. However, if you contract histoplasmosis again, the illness will likely be milder than the initial infection.
Histoplasma capsulatum and Cryptococcus neoformans are considered facultative pathogens, meaning that they can infect and cause disease in healthy humans (though most commonly infections occur in the immune compromised or suppressed). This situation differs from most common indoor fungi in that these organisms are capable of causing a specific and diagnosable disease. These fungi are of particular interest in indoor environment testing due to the fact that they are both commonly associated with areas that have significant amounts of bird droppings.
How histoplasmosis infection occurs
You get histoplasmosis when you breathe spores of the fungus Histoplasma capsulatum into your lungs
The detection and quantitation of Histoplasma capsulatum faces many problems. 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 bio aerosol 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).
Cryptococcus neoformans is typically recovered from environmental samples as nondescript yeast. This yeast is endemic throughout the world with different strains predominating in different locales. This yeast thrives in high nitrogen content material and is often associated with bird droppings. Outbreaks are typically associated with some kind of roosting site disturbance.
Cryptococcosis (an infection with Cryptococcus neoformans) is a trademark secondary infection in AIDS patients and most other cases are in patients with some other predisposing condition (such as lupus erythematosus, lymphoma, or leukemia). The infection is started by the inhalation of airborne fungal material and with ensuing infection in the lungs. The disease usually spreads to the brain and meninges, often proving to be fatal.
The main problem with the detection of Cryptococcus neoformans is that it is nondescript yeast upon primary isolation. The differentiation of C. neoformans from other yeasts is based on biochemistry rather than direct observation and as such requires viable cultures of the organism. The analysis for C. neoformans can be performed in the typical 6-10 day turnaround for viable cultures, but the test requires special media that do not allow other fungi to be reliably recovered or identified. Acceptable matrices include both swabs and bulk material are acceptable, as well as Andersen-style plates. For Andersen sampling the IAQ investigator should use Birdseed Agar (BBL) for the isolation medium, as it provides the quickest and most selective recovery of possible C. neoformans.
Both Histoplasma capsulatum and Cryptococcus neoformans are pathogens of significant consequence that may occur and should not be overlooked in an IAQ investigation that involves guano contamination.
While they are not commonly tested for because of the reasons cited above, that does not mean that they should not be a factor in an evaluation of a site.
Although theses tests are more specialized and potentially more time consuming than traditional mold tests, they may be the only correct option in determining the agent responsible for occupant complaints.
If you encounter a situation requiring testing for Histoplasma capsulatum or Cryptococcus neoformans, call American Environmental Laboratories at 314-664-2800.