Histoplasmosis is a pulmonary and hematogenous disease caused by Histoplasma capsulatum; it is often chronic and usually follows an asymptomatic primary infection. Symptoms are those of pneumonia or of nonspecific chronic illness. Diagnosis is by identification of the organism in sputum or tissue or use of specific serum and urine antigen tests. Treatment, when necessary, is with amphotericin B or an azole.
(See also Overview of Fungal Infections.)
Histoplasmosis occurs worldwide, including parts of Central and South America, Africa, Asia, and Australia.
In the United States, the endemic area for histoplasmosis includes
The Ohio–Mississippi River valleys extending into parts of northern Maryland, southern Pennsylvania, central New York, and Texas
Bat cave–associated outbreaks have occurred worldwide and in the United States have been reported in Florida, Texas, and Puerto Rico.
H. capsulatum is a dimorphic fungi that grows as a mold in nature or in culture at room temperature but converts to a small (1 to 5 micrometers in diameter) yeast cell at 37° C and during invasion of host cells. Infection follows inhalation of conidia (spores produced by the mycelial form of the fungus) in soil or dust contaminated with bird or bat droppings. Risk of infection is greatest when tree or building removal generates airborne spores (eg, at construction sites in areas habituated by birds or bats) or when exploring caves.
Risk factors for severe histoplasmosis include
Heavy, prolonged exposure
Age ≥ 55 years
Infancy
Compromised T-cell–mediated immunity (eg, in those who have HIV/AIDS or an organ transplant or who are taking immunosuppressants such as corticosteroids or tumor necrosis factor inhibitors)
Initial infection occurs in the lungs and usually remains there but may spread hematogenously to other organs if it is not controlled by normal cell-mediated host defenses. Progressive disseminated histoplasmosis is one of the defining opportunistic infections for AIDS.
Symptoms and Signs of Histoplasmosis
Most histoplasmosis infections are asymptomatic or mildly symptomatic, and patients may not seek medical attention.
Histoplasmosis has 3 main forms.
Acute primary histoplasmosis is a syndrome with fever, cough, myalgias, chest pain, and malaise of varying severity. Acute pneumonia (evident on physical examination and chest x-ray) sometimes develops.
Chronic cavitary histoplasmosis is characterized by pulmonary lesions that are often apical and resemble cavitary tuberculosis. Manifestations are worsening cough and dyspnea, progressing eventually to disabling respiratory dysfunction. Dissemination does not occur.
Progressive disseminated histoplasmosis characteristically includes generalized involvement of the reticuloendothelial system, with hepatosplenomegaly, lymphadenopathy, bone marrow involvement, and sometimes oral or gastrointestinal ulcerations. The course is usually subacute or chronic, with only nonspecific, often subtle symptoms (eg, fever, fatigue, weight loss, weakness, malaise); the condition of patients who are HIV-positive may inexplicably worsen. The central nervous system may become involved, causing meningitis or focal brain lesions. Adrenal infection is rare but may result in Addison disease. Severe pneumonia is rare, but patients with AIDS may develop severe acute pneumonia with hypoxia, as well as hypotension, mental status changes, coagulopathy, or rhabdomyolysis.
Fibrosing mediastinitis, a chronic but rare form, ultimately causes circulatory compromise. Fibrosing mediastinitis is thought to be due to an excessive immune reaction to the persistent presence of nonviable fungal antigen, leading to scarring and obstruction of mediastinal blood vessels or airways.
Patients with histoplasmosis may lose vision, but organisms are not present in ocular lesions, antifungal chemotherapy is not helpful, and the link to H. capsulatum infection is unclear.
Diagnosis of Histoplasmosis
Histopathology and cultures
Serologic testing
Antigen testing
The index of suspicion for histoplasmosis must be high because symptoms are nonspecific.
Chest x-rays should be done and may show the following:
In acute infection: Normal or a diffuse nodular or miliary pattern
In chronic pulmonary histoplasmosis: Cavitary lesions in most patients
In progressive disease: Hilar adenopathy with diffuse nodular infiltrates in approximately 50% of patients
Bronchoalveolar lavage or tissue biopsy may be necessary to obtain histology specimens; serologic testing and culture of urine, blood, and sputum specimens are also done. Because culturing Histoplasma can pose a severe biohazard to laboratory personnel, the laboratory should be notified of the suspected diagnosis.
Microscopic histopathology can strongly suggest the diagnosis, particularly in patients with AIDS and extensive infections; in such patients, intracellular yeasts may be seen in Wright- or Giemsa-stained peripheral blood or buffy coat specimens. Fungal culture confirms the diagnosis of histoplasmosis. Lysis-centrifugation or culture of buffy coat improves the yield from blood specimens. DNA probes can rapidly identify the fungus once growth occurs in the laboratory.
A test for H. capsulatum antigen is sensitive and specific, particularly when simultaneous serum and urine specimens are tested; Histoplasma antigen is present in the serum in 80% of patients with disseminated histoplasmosis and is present in the urine in > 90% of these patients. However, cross-reactivity with other fungi (Coccidioides immitis, Blastomyces dermatitidis, Paracoccidioides brasiliensis, and Penicillium marneffei) has been noted.
Treatment of Histoplasmosis
No treatment needed for acute, self-limited infection
(See also Antifungal Medications.)
Serum concentration of itraconazole and urine or blood levels of Histoplasma antigen should be monitored during therapy.
1H. capsulatum and may be effective in the treatment of patients with histoplasmosis. Further data and experience are required to determine which medication is the best in each clinical situation.
Acute primary histoplasmosis
Fluconazole is less effective, and other azoles are not well-studied but have been used successfully.
Chronic cavitary histoplasmosis
itraconazole.
Severe disseminated histoplasmosis
Treatment reference
1. Wheat LJ, Freifeld AG, Kleiman MB, et al: Clinical practice guidelines for the management of patients with histoplasmosis: 2007 Update by the Infectious Diseases Society of America. Clin Infect Dis 45(7):807-825, 2007. doi: 10.1086/521259
Prognosis for Histoplasmosis
The acute primary form of histoplasmosis is almost always self-limited; however, very rarely, death occurs after massive infection.
Chronic cavitary histoplasmosis can cause death due to severe respiratory insufficiency.
Untreated progressive disseminated histoplasmosis has a mortality rate of > 90%.
Key Points
Histoplasmosis is a common fungal infection acquired by inhaling spores.
It is endemic to the Ohio–Mississippi River valleys, extending into parts of northern Maryland, southern Pennsylvania, central New York, and Texas.
It may cause an acute primary pulmonary infection, a chronic cavitary pulmonary infection, or progressive disseminated infection.
Diagnose using histopathology, cultures, and/or antigen testing.
Acute primary infection is almost always self-limited.
Untreated progressive disseminated histoplasmosis has a mortality rate of > 90%.
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Infectious Diseases Society of America: Practice Guidelines for the Management of Patients with Histoplasmosis (2007)
