histoplasmosis

histoplasmosis

[his-toh-plaz-moh-sis]
histoplasmosis: see fungal infection.
Histoplasmosis, also known as Darling's disease, is a disease caused by the fungus Histoplasma capsulatum which was discovered in 1905. 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. Histoplasmosis is common among AIDS patients because of their lowered immune system.

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. 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. 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.

Cause

Histoplasma 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 bird or bat droppings (guano). 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. Histoplasmosis is not contagious, but is contracted by inhalation of the spores from disturbed soil or guano.

Diagnosis

Histoplasmosis can be diagnosed by samples containing the fungus taken from sputum, blood, or infected organs. It can also be diagnosed by detection of antigens in blood or urine samples by ELISA or PCR. It can also be diagnosed by a test for antibodies against Histoplasma in the blood. Histoplasma skin tests indicate whether a person has been exposed, but do not indicate whether they have the disease.

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.

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. In many milder cases, itraconazole is sufficient. Asymptomatic disease is typically not treated. Past infection results in partial protection against ill effects if reinfected.

Presumed ocular histoplasmosis syndrome and age-related macular degeneration (AMD) can be treated by the drug Avastin. Ophthalmologists are using Avastin "off-label" to treat AMD and similar conditions since research indicates that vascular endothelial growth factor (VEGF) is one of the causes for the growth of the abnormal vessels that cause these conditions. Some patients treated with Avastin had less fluid and more normal-appearing maculas, and their vision improved. Avastin injections into the affected eye have been used by retina specialists since early 2005. Thus there is no long term 10 to 15 year follow up data for possible late complications.

History

Histoplasma was discovered in 1905 by Darling, but was only discovered to be a widespread infection in the 1930s. Before then, many cases of histoplasmosis were mistakenly attributed to tuberculosis, and patients were mistakenly admitted to tuberculosis sanatoriums. Some patients contracted tuberculosis in these sanatoriums (reference: Mandell, Bennett and Dolin).

Society and Culture

  • Johnny Cash included a reference to the disease, even correctly noting its source in bird droppings, in the song "Beans for Breakfast".
  • Bob Dylan was hospitalized due to histoplasmosis in 1997, causing the cancellation of concerts in the United Kingdom and Switzerland.
  • In episode 21 of season 3 of the television show House, M.D. a patient was diagnosed with histoplasmosis.

Additional images

References

Note: The original version of this article is adapted from the U.S. CDC public domain document at

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