Histoplasmosis

Histoplasmosis is a respiratory disease acquired by inhaling the spores of a fungus found in soil, especially where there are bird or bat droppings.

Lung, Histoplasmois

Pathophysiology

The fungus lives in the environment, particularly in soil that contains large amounts of bird or bat droppings. In the United States, Histoplasma mainly lives in soil in the central and eastern states, especially areas around the Ohio and Mississippi River valleys.

Geographic distribution

The fungus also lives in parts of

1. Central and South America,
2. Africa,
3. Asia, and
4. Australia.

Histoplasmosis distribution

Transmission

People can get histoplasmosis after breathing in the microscopic fungal spores from the air, often after participating in activities that disturb the soil. Although most people who breathe in the spores don’t get sick, those who do may have a fever, cough, and fatigue. Many people who get sick will get better on their own without medication. In some people, such as those who have weakened immune systems, the infection can become severe, especially if it spreads from the lungs to other organs.

Chest X-ray acute pulmonary histoplasmosis

Histoplasma capsulatum var. capsulatum (near-worldwide distribution) and Histoplasma capsulatum var. duboisii (in Africa).

Reservoir and endemic areas

Soil, particularly when heavily contaminated with bird or bat droppings. Endemic areas include the central and eastern United States, particularly areas around the Ohio and Mississippi River Valleys, 2 as well as parts of Central and South America, 3 Africa, 4 Asia, 5 and Australia.

Sequelae

Can include pericarditis, broncholithiasis, pulmonary nodules, mediastinal granuloma, or mediastinal fibrosis. 1,9 In persons who develop progressive, chronic, or disseminated disease, symptoms may persist for months or longer. Mortality is high in HIV-infected persons who develop disseminated histoplasmosis.

Histoplasmosis - Old necrotizing fibrocalcific granulomas

Diagnosis

Histoplasma antigen detection in urine and/or serum is the most widely used and most sensitive method for diagnosing disseminated histoplasmosis and acute pulmonary histoplasmosis following exposure to a large inoculum. 1 Other methods include antibody tests, culture, and microscopy.

• Antigen detection: Enzyme immunoassay (EIA) is typically performed on urine and/or serum, but can also be used on cerebrospinal fluid or bronchoalveolar lavage fluid.
• Antibody tests: Because development of antibodies to Histoplasma can take two to six weeks, antibody tests are not as useful as antigen detection tests in diagnosing acute histoplasmosis or in immunosuppressed persons, who may not mount a strong immune response.
  • Immunodiffusion (ID): Tests for the presence of H (indicates chronic or severe acute infection) and M (develops within weeks of acute infection and can persist for months to years after the infection has resolved) precipitin bands; ~80% sensitivity.
  • Complement Fixation (CF): Complement-fixing antibodies may take up to 6 weeks to appear after infection. CF is more sensitive but less specific than immunodiffusion.
• Culture: can be performed on tissue, blood, and other body fluids, but may take up to 6 weeks to become positive; most useful in the diagnosis of the severe forms of histoplasmosis. A commercially available DNA probe (AccuProbe, GenProbe Inc.) can be used to confirm.
• Microscopy: for detection of budding yeast in tissue or body fluids, low sensitivity, but can provide a quick proven diagnosis if positive.
• Polymerase Chain Reaction (PCR): PCR for detection of Histoplasma directly from clinical specimens is still experimental, but promising.

Treatment

Mild to moderate cases of acute pulmonary histoplasmosis will often resolve without treatment; however, treatment is indicated for moderate to severe acute pulmonary, chronic pulmonary, disseminated, and central nervous system (CNS) histoplasmosis. Antifungal agents proven to be effective are amphotericin B (including liposomal and lipid formulations) and itraconazole (for mild-to-moderate infections and step-down therapy).

Histoplasmosis Resources
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