Histoplasmosis is a systemic disease caused by the thermally dimorphic fungus Histoplasma capsulatum. H. capsulatum is distributed worldwide and endemic to the Ohio and Mississippi River Valleys in the United States, where the CDC estimates 60% to 90% of people who live in the region have been exposed to the fungus at some point during their lifetime, and to certain regions of Central and South America. Medicare records from 2007 to 2016 have found cases of histoplasmosis have now spread to 47 states and Washington, D.C.
Histoplasmosis is most common among patients positive for HIV or otherwise have a compromised immune system. It is especially a problem in regions of the world where antiretroviral therapy (ART) is not widely available, as ART prevents HIV- infected people from reaching the stage where they are especially vulnerable to histoplasmosis and other opportunistic infections. In Latin America, histoplasmosis is one of the most common opportunistic infections among people living with HIV. There is a 30% mortality rate among HIV/AIDS patients also diagnosed with Histoplasmosis.
H. capsulatum is typically transmitted by the inhalation of microconidia by the host, deposition in the alveoli, and rapid conversion to a parasitic yeast form in host tissues. Infection is typically asymptomatic. Among immunocompetent hosts in endemic areas, 95-99% of the primary infections are not recognized or detected . Because the symptoms of histoplasmosis have significant clinical overlap with other diseases, the definitive diagnosis of histoplasmosis requires either isolation of H. capsulatum from a clinical specimen or direct visualization of the yeast form in clinical specimens.
These procedures may require invasive measures to obtain tissues and cultures may take up to six weeks to reveal fungal growth. In contrast, enzyme immunoassays (EIAs) detect H. capsulatum polysaccharide antigen in bodily fluids including urine and blood. Such tests provide rapid results and reasonable specificity and sensitivity and may be used to supplement culture and microscopic examination to diagnose histoplasmosis.