Diseases of the Respiratory System - Respiratory diseases caused by fungi

Two fungal diseases affecting respiration are of great importance in particular regions of the United States. They are both caused by types of fungi capable of growing within mammalian tissue, thereby infecting and destroying it. Both cause chronic diseases very similar to tuberculosis and may lead to death, though that is a far less common outcome—even when untreated—than in tuberculosis.

The spores of the fungi are inhaled from the air, and the response of the body is similar to that in tuberculosis in that most people become merely infected (the spores being contained by body defenses) while a few develop progressive disease. The body also produces antibodies, and consequently skin tests similar to the tuberculin test can identify infected individuals.

Histoplasmosis (named for a fungus called histoplasma ) organisms are prevalent in the Midwest, generally in the areas of the Ohio, Mississippi, and Missouri rivers. Largely unknown prior to World War II, histoplasmosis has been studied extensively since. Local epidemics have brought it to public attention on several occasions. The fungus grows readily in soil containing large amounts of bird (chickens, pigeons, starlings) or bat excrement. One of the better ways to assure exposure is to clean out an old chicken coop. The concentration of organisms may reach such high levels in bat caves that entry by spelunkers may prove fatal. In contrast to tuberculosis, the amount of exposure seems to play a very important role in determining the extent of the disease. There also seem to be few cases of late breakdown (the rule in tuberculosis). Most people develop the active disease, if at all, at the time of their initial exposure.

Coccidioidomycosis (for Coccidioides fungus) also generates in the soil, in this case in California, the southwest United States, and Mexico. It grows best in hot, dry soil. The common names for this disease are desert rheumatism or valley fever . Infection and disease occur in a similar pattern to that of histoplasmosis. Skin testing of large population groups for both these fungi in the appropriate geographical areas indicates that the majority of exposed individuals quite adequately contain the initial infection and never develop any illness or active disease.

Because Americans travel into infected regions, these diseases are being seen more frequently in people who do not live where the fungi are found. Both conditions, fortunately, are often self-limited, even when active disease develops. For more severe cases there is a drug, amphotericin-B , which is quite effective; however, because it is also quite toxic to the patient, it must be given in progressive doses starting with a small initial dose, to permit the body's tolerance to build up. It is hoped that less toxic agents will be found in the future that will be just as effective against the fungus.