Histoplasma capsulatum
Histoplasma capsulatum is a dimorphic fungus found in areas of endemicity around the world, notably in river basins.
Histoplasma capsulatum lives in soil, especially where there are high quantities of bird or bat droppings; when disturbed, its conidia become airborne and can be breathed. Frequently, infections are asymptomatic, but in immunocompromised people, histoplasmosis can become widespread, resulting in significant morbidity and death. (1)
Epidemiology and distribution
Histoplasmosis is the most prevalent endemic mycosis in the United States. In the U.S., an estimated 60% to 90% of persons who reside in the Ohio and Mississippi River basins have been exposed to the fungus at some point in their lives.
One study of histoplasmosis patients hospitalized in the United States found the crude death rate to be around 5% for children and 8% for adults. (2) There are areas of endemicity on every continent in the world. Italy, North, Central, and South America, West Africa (Congo and Zimbabwe), South Africa, Southeast Asia (India, China, Malaysia, Taiwan), and Australia have all recorded published instances. Potentially, more than half of the world’s population resides in H. capsulatum’s endemic zone
Risk factors
Histoplasmosis can cause severe illness right away or redevelop years after the first exposure if a person’s immune system is weakened by cancer, chemotherapy, immune-suppressing drugs, or HIV infection. Risk factors for chronic pulmonary histoplasmosis include exposure to soil or particles contaminated with droppings of chickens, bats, or blackbirds, living in, or travelling to
central or eastern United States, pre-existing chronic obstructive pulmonary disease (COPD), and a compromised immune system.
Symptoms
Acute pulmonary histoplasmosis
Acute respiratory illness is marked by respiratory symptoms, fatigue, fever, chest pains, and a dry cough. Patients may be asymptomatic and have granulomatous disease on a chest x-ray. These granulomas might be caseating or noncaseating. Approximately 6% of individuals develop acute pericarditis during primary infection, likely due to the granulomatous inflammatory response developed in mediastinal lymph nodes close to the pericardium. (3)
Chronic Pulmonary histoplasmosis
Chronic cavitary pulmonary histoplasmosis (CCPH) is characterized by low-grade chronic symptoms, persistent cavitation, and the development of pulmonary fibrosis and progressive respiratory insufficiency after acute pulmonary histoplasmosis. The susceptibility for cavitation in the higher lobes, particularly the apical and apical-posterior segments, is comparable to that seen in pulmonary tuberculosis (TB) and chronic pulmonary aspergillosis (CPA). (3, 4)
Disseminated histoplasmosis
If the infection cannot be cleared, as seen in immunocompromised people with impaired cellular immunity, the organism reproduces intracellularly and spreads throughout the body via lymphatic and hematogenous circulation, culminating in a condition known as disseminated histoplasmosis. Most patients who acquire disseminated histoplasmosis are immunocompromised or are elderly. In addition to various symptoms and signs such as lung illness, skin lesions, fever, and encephalopathy, disseminated histoplasmosis appears most conspicuously in the digestive system, from mouth to anus. Without treatment, widespread histoplasmosis typically kills within a few weeks. As a result, precise diagnosis and therapy are vitally necessary. (5)
Diagnosis and treatment
Histoplasmosis diagnosis is frequently difficult and requires a high index of suspicion, a fungal culture of respiratory samples, antibody testing, and a suitable radiological image. Culture is still the gold standard for diagnosis, although it can take up to 8 weeks and has low sensitivity. Similar to CPA, antibody detection may assist in distinguishing CPH from TB and has been widely used in the diagnosis of histoplasmosis. (6) Morbidity from CPH is caused by the slow advancement of cavities and the gradual loss of lung function, particularly if it is not detected and
treated. Once the diagnosis has been established, itraconazole treatment for at least 12 months is advised. (7)
References
1. www.cdc.gov. (2019). About Histoplasmosis | Types of Diseases | Histoplasmosis | Fungal Disease | CDC. Available at: https://www.cdc.gov/fungal/diseases/histoplasmosis/definition.html.
2. Chu, J.H., Feudtner, C., Heydon, K., Walsh, T.J. and Zaoutis, T.E. (2006). Hospitalizations for Endemic Mycoses: A Population-Based National Study. Clinical Infectious Diseases, 42(6), pp.822–825. doi:10.1086/500405.
3. Akram, S.M. and Koirala, J. (2020). Histoplasmosis. [online] PubMed. Available at: https://www.ncbi.nlm.nih.gov/books/NBK448185/.
4. Baker, J., Kosmidis, C., Rozaliyani, A., Wahyuningsih, R. and Denning, D.W. (2020). Chronic Pulmonary Histoplasmosis—A Scoping Literature Review. Open Forum Infectious Diseases, 7(5). doi:10.1093/ofid/ofaa119.
5. CA, M., PV, B., R, R. and Nair, L.R. (2013). Disseminated histoplasmosis. Indian Journal of Orthopaedics, 47(6), pp.639–642. doi:10.4103/0019-5413.121601.
6. Baker, J., Kosmidis, C., Rozaliyani, A., Wahyuningsih, R. and Denning, D.W. (2020). Chronic Pulmonary Histoplasmosis—A Scoping Literature Review. Open Forum Infectious Diseases, 7(5). doi:10.1093/ofid/ofaa119.
7. www.who.int. (n.d.). Guidelines for diagnosing and managing disseminated histoplasmosis among people living with HIV. [online] Available at: https://www.who.int/publications/i/item/9789240006430