What is histoplasmosis?

Histoplasmosis is a systemic infection. It is caused by inhaling spores of the Histoplasma capsulatum fungus, which is present worldwide but 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 Americas.  Additional studies are revealing that the extent of histoplasmosis is wider than traditionally thought with cases in Africa, Australia, and Asia.  Histoplasmosis is most dangerous to infants and people who have HIV/AIDS or an otherwise weakened immune system. In Latin America, histoplasmosis is one of the most common opportunistic infections among people living with HIV, where 30% of HIV/AIDS patients diagnosed with histoplasmosis die from it.  It is especially a problem in regions of the world where antiretroviral therapy (ART), which prevents HIV infected people from reaching the stage where they are especially vulnerable to histoplasmosis and other opportunistic infections, is not widely available.

The fungus H. capsulatum is commonly found in bird or bat droppings. When contaminated soil is disturbed the microconidia become airborne. Infection occurs when microconidia is inhaled. It is then deposited in the alveoli and quickly converted to a parasitic yeast form in host tissues. Infection is usually asymptomatic.  

How is histoplasmosis diagnosed?

Among immunocompetent hosts in endemic areas, 95-99% of the primary infections are not recognized or detected. Because the symptoms of histoplasmosis are so similar to other diseases, the definitive diagnosis of histoplasmosis requires either isolated H. capsulatum from a clinical specimen or direct visualization of the yeast form in clinical specimens. These procedures may require invasive medical procedures to obtain sample tissues, and cultures may take up to six weeks to reveal fungal growth. In contrast, enzyme immunoassays (EIAs or ELISAs) are based on the detection of the H. capsulatum polysaccharide antigen in bodily fluids including urine and blood. Such tests provide rapid turnaround time and reasonable specificity and sensitivity and may be used to supplement culture and microscopic examination to diagnose histoplasmosis.   

Click here to learn more about the OIDx Histoplasma Capsulatum ELISA test.

1. Darling, S. A Protozoan General Infection Producing Pseudotubercules In the Lungs and Focal Necrosis In the Liver, Spleen, and Lymph Nodes. J. Am. Med. Association. 1906, 46, 1283-1285.
2. Manos, N.; Ferebee, S.; Kerschbaum, W. Geographic variation in the prevalence of histoplasmin sensitivity. Dis. Chest. 1956, June, 29(6), 649-68.
3. Wheat, J. Histoplasmosis: recognition and treatment. Clin. Infect. Dis. 1994, 19 Suppl 1:S19-S27
4. Haddad, N.; Powderly, W. The changing face of mycoses in patients with HIV/AIDS. The AIDS reader. 2001, 11, 365-8, 75-8.
5. Colombo, A.; Tobon A.; Restrepo, A.; Queiroz-Telles, F.; Nucci, M. Epidemiology of endemic systemic fungal infections in Latin America. Med. Mycol. 2011, Nov, 49(8), 785-98.
6. Goodwin, R.; Loyd, J.; Des Prez, R. Histoplasmosis in normal hosts. Medicine (Baltimore). 1981, 60(4), 231-266.
7. Guimaraes, A.; Nosanchuk, J.; Zancope-Oliveira, R. Diagnosis of histoplasmosis. Braz. J. Microbiol. 2006, 37, 1-13.
8. Falci, D.; Hoffman, E.; Paskulin, D.; Pasqualotto, D. Progressive disseminated histoplasmosis: a systematic review on the performance of non-culture-based diagnostic tests. Braz. J. of Infectious Diseases. 2017, 21(1), 7-11.