Histoplasmosis (cont.)

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What causes histoplasmosis?

Histoplasmosis is caused by a dimorphic (two forms, mycelia and yeast) fungus named Histoplasma capsulatum. The genus name is misleading; the fungus has no capsule, but early investigators mistakenly thought it did. The mycelial phase consisting of mycelia fragments and spores can be inhaled and may reach the lung alveoli. Macrophages (human phagocytic cells of the immune system) surround and engulf (phagocytosis) H. capsulatum, which then changes inside the macrophages to the yeast form in about 15-18 hours. In most cases, the macrophage response kills the yeast. When macrophages fail to kill all the yeast, a variation of the disease develops because the yeast form multiplies and invades other cells. The larger the number of mycelia and spores the person is exposed to, the more likely the person will develop symptomatic disease.

What are the symptoms and signs of histoplasmosis?

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About 90% of infections caused by H. capsulatum are asymptomatic (produce no symptoms). Occasionally, a few asymptomatic patients will show small scars in lung X-rays. Symptomatic people often develop fever, chills, dry cough, malaise, sweats, and abdominal pains about three to 14 days after exposure. If the disease progresses, symptoms such as weight loss, fatigue, dyspnea, chest pain, and reduced or loss of vision may occur. A sign of progression are patchy infiltrates seen on chest X-rays, usually in the lower lung fields. Other symptoms that can occur, especially in patients that are immunosuppressed, are mouth ulcers, fevers, headaches, confusion, seizures, encephalopathy, and infrequently, death.

Medically Reviewed by a Doctor on 4/11/2014

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