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 that expose people to soil disruption or areas where bats live and birds roost, such as construction, excavation, demolition, farming, gardening, and caving, can increase risk of histoplasmosis. 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. High-dose exposure can lead to severe pulmonary disease. Dissemination, especially to the gastrointestinal tract and central nervous system, can occur in people who are immunocompromised.
Several methods are available to diagnose histoplasmosis.
Although the gold standards remain culture and histopathologic identification, antigen or antibody testing are commonly used.
Rapid Histoplasma antigen testing by EIA on multiple specimen types (for example, urine, serum, plasma, bronchoalveolar lavage, or cerebrospinal fluid) is available at multiple US laboratories. Antigen testing is most sensitive in severely ill patients.
Antibody testing by EIA, immunodiffusion (ID), and complement fixation (CF) can be used to detect subacute and chronic forms of histoplasmosis. Antibodies to Histoplasma typically become detectable in serum 4–8 weeks after infection. A small proportion (<5%) of people living in histoplasmosis-endemic areas have positive serology by CF or ID. Testing a single serum specimen can aid in diagnosis, but testing serial specimens offers greater specificity (detection of seroconversion and increases in antibody titer). An antibody response may be absent in immunocompromised people.
Other endemic mycoses (such as blastomycosis, paracoccidioidomycosis, and talaromycosis [formerly penicilliosis]) can lead to false-positive antigen and antibody tests for H. capsulatum.
Culture of H. capsulatum from bone marrow, blood, sputum, and tissue specimens is the definitive method but may take weeks to grow. DNA probe is sometimes used to confirm H. capsulatum in culture.
Demonstration of the typical intracellular yeast forms in tissue by microscopic examination strongly supports the diagnosis of histoplasmosis when clinical, epidemiologic, and other laboratory studies are compatible. Molecular diagnostics, such as PCR on tissue specimens, are increasingly available to support microscopic findings, although the performance of these tests may vary.
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. Patients with acute respiratory distress may benefit from steroids as well as antifungal treatment.
People at increased risk for severe disease should avoid high-risk areas, such as bat-inhabited caves.
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