Histoplasmosis

Histoplasmosis, also known as Darling's disease,^[1] is a disease caused by the fungus Histoplasma capsulatum. Its symptoms vary greatly, but the disease primarily affects the lungs. Occasionally, other organs are affected—this form of the disease is called disseminated histoplasmosis, and it can be fatal if untreated.

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Pathogen

H. capsulatum is found throughout the world and is endemic in certain areas of the United States, particularly in states bordering the Ohio River valley and the lower Mississippi River. (Positive histoplasmin skin tests occur in as many as 80% of the people living in areas where H. capsulatum is common, such as the eastern and central United States.) H. capsulatum grows in soil and material contaminated with bat or bird droppings. The fungus has been found in poultry house litter, caves, areas harboring bats, and in bird roosts (particularly those of starlings). The fungus is thermally dimorphic. In the environment it grows as a brownish mycelium, whereas at body temperature (37°C in humans) it morphs into a yeast. The inoculum is represented principally by microconidia that once inhaled into the alveolar spaces germinate and then transform into budding yeast cells.

Symptoms

If symptoms of histoplasmosis infection occur, they will start within 3 to 17 days after exposure; the average is 12-14 days. Most affected individuals have clinically-silent manifestations and show no apparent ill effects.^[2]The acute phase of histoplasmosis is characterized by non-specific respiratory symptoms, often cough or flu-like. Chest X-ray findings are normal in 40-70% of cases.^[2] Chronic histoplasmosis cases can resemble tuberculosis; disseminated histoplasmosis affects multiple organ systems and is fatal unless treated.

While histoplasmosis is the most common cause of fibrosing mediastinitis, this remains a relatively rare disease. Severe infections can cause hepatosplenomegaly, lymphadenopathy, and adrenal enlargement. Lesions have a tendency to calcify as they heal.

Ocular histoplasmosis damages the retina of the eyes. Scar tissue is left on the retina which can experience leakage, resulting in a loss of vision not unlike macular degeneration.

Treatment

Antifungal medications are used to treat severe cases of acute histoplasmosis and all cases of chronic and disseminated disease. Typical treatment of severe disease first involves treatment with amphotericin B, followed by oral itraconazole.^[3] In many milder cases, simply itraconazole is sufficient. Asymptomatic disease is typically not treated. Past infection results in partial protection against ill effects if reinfected.

Prevention

It is not practical to test or decontaminate most sites that may be contaminated with H. capsulatum, but the following precautions can be taken to reduce a person's risk of exposure:

• Avoid areas that may harbor the fungus, e.g., accumulations of bird or bat droppings.
• Before starting a job or activity having a risk for exposure to H. capsulatum, consult the NIOSH/NCID Document Histoplasmosis: Protecting Workers at Risk. This document contains information on work practices and personal protective equipment that will reduce the risk of infection.

Histoplasmosis in Popular Culture

• Johnny Cash included a reference to the disease, even correctly noting its source in bird droppings, in the song "Beans for Breakfast".^[4]

• Bob Dylan was hospitalized due to histoplasmosis in 1997, causing the cancellation of concerts in the United Kingdom and Switzerland.^[5]

• In the episode Family, episode 21 of season 3 of the television show 'House, M.D.' a patient was diagnosed with histoplasmosis.^[6]

References

1. ^ Histoplasma capsulatum, cause of histoplasmosis in humans and other animals, Tom Volk's Fungus of the Month for January 2000. Retrieved on 2007-06-24.
2. ^ ^{a b} Silberberg P (2007-03-26). Radiology Teaching Files: Case 224856 (Histoplasmosis). Retrieved on 2007-07-27.
3. ^ Histoplasmosis: Fungal Infections: Merck Manual Home Edition. Retrieved on 2007-06-24.
4. ^ Beans for Breakfast lyrics
5. ^ CNN - Bob Dylan hospitalized with Histoplasmosis
6. ^ House episode "Family"

Note: The original version of this article is adapted from the U.S. CDC public domain document at http://www.cdc.gov/ncidod/dbmd/diseaseinfo/histoplasmosis_g.htm