Histoplasmosis: Symptoms, Pathogenesis, Treatment and Prevention

Histoplasmosis: Symptoms, Pathogenesis, Treatment and Prevention

What is Histoplasmosis?

  • Histoplasmosis is primarily a disease of the lungs caused by Histoplasma capsulatum var. capsulatum, a facultative, parasitic fungus that grows intracellularly.
  • The incubation period for symptomatic patients is approximately 10 days; symptoms include flulike responses such as fever, cough, headaches, and muscle aches.
  • H. capsulatum appears as a small, budding yeast in humans and on culture media at 37°C (98.6°F).
  • At 25°C (77°F) it grows as a mold, producing small microconidia (1 to 5µm in diameter) that are borne singly at the tips of short conidiophores.

Conidiospores arranged in chains at the end of a conidiophore

  • Large macroconidia (8 to 16µm in diameter) are also formed on conidiophores.

Mycelia, microconidia, and chlamydospores as found in the soil

  • In humans, the yeastlike form reproduces by budding. The mycelial form of the fungus is spotted in soils over the world and is localized in areas that have been infected with bird or bat body wastes.

Yeastlike budding of H. capsulatum

  • The microconidia can become airborne when contaminated soil is disturbed. Microconidia are thus most prevalent where bird droppings-especially from starlings, crows, blackbirds, cowbirds, sea gulls, turkeys, and chickens-have accumulated.
  • It is noteworthy that the birds itself are not contaminated due to their high body temperature; their body wastes just supply the nutrition to the fungus. Only bats and humans shows the disease and harbor the fungus.

Pathogenesis of Histoplasmosis

  • Infection ensues when microconidia are inhaled. Histoplasmosis is not, however, transmitted from person to person.
  • Histoplasmosis is found worldwide, with Africa, Australia, India, and Malaysia being endemic regions.
  • Within the United States, histoplasmosis is endemic within the Mississippi, Kentucky, Tennessee, Ohio, and Rio Grande river basins. Between 50 and 80% of the people who reside in parts of these areas have antibodies against the fungus.
  • It has been calculated that in endemic places of the United States, about 500,000 peoples are contaminated annually: 50,000 to 200,000 become ill; 3,000 require hospitalization; and about 50 die.
  • The total number of infected individuals may be over 40 million in the United States alone.

  • Histoplasmosis is a common disease among poultry farmers, spelunkers (people who explore caves), and bat guano miners (bat guano is used as fertilizer).
  • H. capsulatum grows within macrophages, the fungus can be disseminated to many organs (though rarely) as the infected macrophages circulate throughout the body.
  • Lesions may appear in the lungs and show calcification; thus, the disease may resemble tuberculosis.
  • Most infections resolve spontaneously. Laboratory diagnosis is accomplished by complement fixation tests and isolation of the fungus from tissue specimens.
  • Most individuals with this disease exhibit a hypersensitive state that can be demonstrated by the histoplasmin skin test.

Treatment of Histoplasmosis

Currently the most effective treatment is with amphotericin B, ketoconazole, or itraconazole.

Prevention of Histoplasmosis

  • Biting insects, fleas, ticks, and mites can harbor microorganisms that can be subsequently passed to humans.
  • In some cases, the arthropod vector is a transient or accidental carrier of the organism.
  • In other cases, the vector is an intermediate host in which the microorganism undergoes developmental changes.

Reference and Sources

  • https://www.researchgate.net/publication/11825016_Pathogenesis_of_Histoplasma_capsulatum
  • https://cmr.asm.org/content/24/2/247
  • https://www.researchgate.net/publication/11825017_Cell-mediated_immunity_to_Histoplasma_capsulatum
  • https://www.academia.edu/5626922/human_disease_caused_by_fungi_and_protozoa

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