Histoplasmosis

Histoplasmosis is a fungal disease caused by the thermally dimorphic fungus Histoplasma capsulatum and other phylogenetically related Histoplasma species.

Histoplasma spp. exist as two morphological forms: an environmental mould at 25 to 30 °C and a pathogenic yeast at mammalian body temperature of 37 °C (1,2):

• in the environment, Histoplasma spp. moulds grow in nitrogen-rich soils, such as those enriched with bird droppings or bat guano, and produce aerosolised conidia (asexual, non-motile fungal spores) that serve as the infectious propagules
• following inhalation by the human host, conidia are rapidly phagocytosed by alveolar macrophages and transition to the yeast form
• Histoplasma yeasts survive within the macrophages, replicate and disseminate via the reticuloendothelial system to cause multi-organ infection
• less than 5% of individuals exposed to a low-level inoculum of H capsulatum will develop clinical symptoms, up to 75% of individuals exposed to a high-level inoculum will (2)

Histoplasmosis has a world-wide distribution. However it is commonly seen in the USA especially in Ohio and the Mississippi river valley (1,2):

• though historically considered endemic only to focal regions of North, Central, and South America (primarily the Ohio and Mississippi River valleys, parts of Mexico, and areas of Brazil and Colombia), Histoplasma spp. are now recognised to have a global presence, with autochthonous cases documented in Africa, South and Southeast Asia, and pockets of Europe and Oceania
  • autochthonous cases are defined as those cases occurring in individuals with no history of travel outside their country of birth or residence
• in the UK, histoplasmosis remains a relatively rarely encountered disease though imported cases are increasingly reported
  • no definitively proven autochthonous cases of histoplasmosis have ever been documented, and Histoplasma spp. have never been recovered from environment

Risk factors for Histoplasma spp. exposure include any activities which disturb contaminated soil and increase aerosolisation and inhalation of conidia by the human host.

Many exposures result in asymptomatic infection (1,2):

• however, H capsulatum can cause acute or chronic pulmonary disease as well as infections involving other organ systems
• disseminated disease occurs primarily among immunocompromised individuals and can involve widespread systemic manifestations

PCR in the diagnosis of histoplasmosis (2):

• sensitivity ranging from 86% to 98% and specificity approaching 100%

Management of histoplasmosis depends on the presence and degree of clinical symptoms and underlying immune status.

Reference:

1. Brown L et al. Histoplasmosis in the United Kingdom: Diagnosis and management in a non-endemic setting.Clinical Infection in Practice August 7th 2026.
2. Jenks JD, Koirala J. Histoplasmosis. [Updated 2026 Jul 15]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-.