Histoplasmosis
Medical Author: Charles Davis, MD, PhD
Medical Editor: Melissa Conrad Stöppler, MD
What is histoplasmosis?
Histoplasmosis is a disease, usually affecting the lungs, caused by the
Histoplasma capsulatum fungus. Although many people that are or have been infected
with H. capsulatum do not appear ill, some people in the acute phase of the
disease have a dry cough, fever, and chest pains and do feel ill. There are
several types of histoplasmosis (acute, chronic, and disseminated, all with
subtypes).
H. capsulatum was first described by Samuel Darling in 1906 within human tissue cells
(histiocytes). In 1932, Katharine Dodd and Edna Tompkins made the first diagnosis of
histoplasmosis in an infant. Since the 1930s, H. capsulatum has been found
worldwide, but the majority of cases are found in river valleys in temperate
regions of the world and in equatorial Africa (in Africa, H. capsulatum has a
variant thick-walled yeast form termed H. duboisii). Often an outbreak occurs in
a group of people after a visit to a certain area like a cave that contains bat droppings. In the U.S., histoplasmosis is endemic in the Ohio, Missouri, and
Mississippi river valleys.
Histoplasmosis is the most common endemic fungal infection diagnosed in the U.S.
with about 250,000 new cases per year. H. capsulatum can occur in high
concentrations in some sources (for example, bird and bat feces). Histoplasmosis
has also been named Ohio River Valley fever and bird-fancier's disease.
Histoplasmosis can affect other mammals like dogs and cats, but these animals do
not transfer the disease to humans or to other animals. Dogs, cats, and other
mammals develop symptoms mainly due to lung infections with H. capsulatum that
mimic human histoplasmosis.
What causes histoplasmosis?
Histoplasmosis is caused by a dimorphic (two forms, mycelia and yeast) fungus
named Histoplasma capsulatum. The genus name is misleading; the fungus has no
capsule, but early investigators mistakenly thought it did. The mycelial phase
consisting of mycelia fragments and spores can be inhaled and may reach the lung
alveoli. Macrophages (human phagocytic cells of the immune system) surround and
engulf (phagocytosis) H. capsulatum, which then changes inside the macrophages
to the yeast form in about 15-18 hours. In most cases, the macrophage response
kills the yeast. When macrophages fail to kill all the yeast, a variation of the
disease develops because the yeast form multiplies and invades other cells. The
larger the number of mycelia and spores the person is exposed to, the more
likely the person will develop symptomatic disease.
Next: What are the symptoms and signs of histoplasmosis? »
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