Histoplasma capsulatum, a dimorphic fungus that grows as a mold in soil and as a yeast in animal and human hosts.
Through inhalation of spores (conidia) from soil (often soil contaminated with bat guano or bird droppings); not transmitted from person to person.
Distributed worldwide, except in Antarctica, but most often associated with river valleys. Activities such as spelunking, mining, construction, excavation, demolition, roofing, chimney cleaning, farming, gardening, and installing heating and air-conditioning systems are associated with histoplasmosis. Activities that expose people to areas where bats live and birds roost also increase risk. Outbreaks have been reported associated with travel to many countries in Central and South America, most often associated with visiting caves.
Incubation period is typically 3–17 days for acute disease. Ninety percent of infections are asymptomatic or result in a mild influenzalike illness. Some infections may cause acute pulmonary histoplasmosis, manifested by high fever, headache, nonproductive cough, chills, weakness, pleuritic chest pain, and fatigue. Most people spontaneously recover 2–3 weeks after onset of symptoms, although fatigue may persist longer. Dissemination, especially to the gastrointestinal tract and central nervous system, can occur in people who are immunocompromised.
Culture of H. capsulatum from bone marrow, blood, sputum, and tissue specimens is the definitive method. Demonstration of the typical intracellular yeast forms by microscopic examination strongly supports the diagnosis of histoplasmosis when clinical, epidemiologic, and other laboratory studies are compatible. EIA on urine, serum, plasma, bronchoalveolar lavage, or cerebrospinal fluid is a rapid diagnostic test commercially available in the United States.
Treatment is not usually indicated for immunocompetent people with acute, localized pulmonary infection. People with more extensive disease or persistent symptoms beyond 1 month are generally treated with an azole drug, such as itraconazole, for mild to moderate illness or amphotericin B for severe infection.
People at increased risk for severe disease should avoid high-risk areas, such as bat-inhabited caves.