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Histoplasma capsulatum is a soil saprophyte that grows well in soil enriched with bird droppings.
The usual disease is self-limited, affects the lungs, and is asymptomatic.
Chronic cavitary pulmonary disease, disseminated disease, and meningitis may occur and can be fatal, especially in young children and in immunosuppressed patients.
Aiding in the diagnosis of active histoplasmosis
Complement fixation (CF) titers > or =1:32 indicate active disease. A rising CF titer is associated with progressive infection.
Positive immunodiffusion test results supplement findings of the CF test. The simultaneous appearance of both H and M precipitin bands indicates active histoplasmosis. The M precipitin band alone indicates early or chronic disease or a recent histoplasmosis skin test.
Patients infected with Histoplasma capsulatum demonstrate a serum antibody with a rising titer within 6 weeks of infection. A rising titer is associated with progressive infection. Specific antibody persists for a few weeks to a year, regardless of clinical improvement.
Recent histoplasmosis skin tests must be avoided because the test causes a misleading rise in complement fixation titer, as well as an M precipitin band, in approximately 17% of patients having previous exposure to Histoplasma capsulatum.
Cross-reacting antibodies sometimes present interpretive problems in patients having blastomycosis or coccidioidomycosis.
Mycelial by CF: negative (positives reported as titer)
Yeast by CF: negative (positives reported as titer)
Antibody by immunodiffusion: negative (positives reported as band present)
Kaufman L, Kovacs JA, Reiss E: Clinical immunomycology. In Manual of Clinical and Laboratory Immunology. Fifth edition. Washington DC, ASP Press, 1997