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Histoplasmosis is one of the most frequently occurring fungal infections in the mid-west but it is estimated that approximately 90% of the cases go unrecognized. Histoplasmosis is caused the by the fungal organism, Histoplasma capsulatum. Infections with this fungus have been noted on every continent, except Antarctica, but it is very common in the Ohio, Mississippi, and Missouri river valleys.

Histoplasma is a dimorphic fungus, which means it has two forms, and appears as a light colored, fuzzy appearing colony in the environment but changes to a single cell yeast when it grows in the human body. People can become infected when they inhale parts of the fungus called conidia. These small fungal particles can be suspended in the air and when inhaled will travel down into the smallest chambers of the lung, called alveoli, and there the conidia are engulfed by white blood cells and transform into the yeast phase in these cells. Histoplasma capsulatum prefers to grow in slightly acidic, moist soils, in temperate climates with average rainfall. This makes the forests, fields, and river bottoms of Missouri an ideal home for this fungus. Fungal spores are often suspended in the air by a disturbance, such as plowing a field or construction projects that require excavation. This fungus is also found commonly in areas with high concentrations of manure, such as chicken coops, bird roosts and caves. The fungus is not carried by the birds but has been found in the digestive tract of bats. Cave explorers are at increased risk of coming into contact with Histoplasma.

Most infections are mild and many resolve without treatment. Symptoms can include fever, chills, fatigue, muscle aches, headache, and a non-productive cough. Some infections result in the formation of granulomas which are small clusters of white blood cells that organize into multiple small masses that can be seen on chest X-rays. While most histoplasma infections are mild and involve the lung, serious infections are also seen, and can involve other areas of the body such as the lymph nodes, skin, gastrointestinal tract, and other organs such as the eye, liver, and spleen. These serious infections usually occur in patients with a weakened immune system. Some cases of histoplasmosis will occur years after the patient was initially exposed.

Histoplasma infections are treated with a class of antibiotic called an antifungal. The most common antifungal used is an oral medication called itraconazole. In serious infections, an intravenous medication called amphotericin B is utilized. The length of therapy can range from weeks to months depending upon the severity of the disease. In a few cases, surgical removal of an infected portion of the lung is all that is needed and the patient does not require any antifungal therapy.

It is important to realize that while we have a fungus among us, that it seldom results in serious disease and with appropriate precautions we can still enjoy our rich Missouri outdoor heritage.