Merck Manual

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Sanjay G. Revankar

, MD, Wayne State University School of Medicine

Last review/revision Apr 2021 | Modified Sep 2022
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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.

Histoplasmosis occurs worldwide, including parts of Central and South America, Africa, Asia, and Australia.

In the US, 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 US 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 Immune system Human immunodeficiency virus (HIV) infection results from 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ lymphocytes and impair cell-mediated immunity, increasing risk of certain... read more Immune system .

Symptoms and Signs of Histoplasmosis

Most histoplasmosis infections are asymptomatic or so mild that patients do 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 HIV-positive patients 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 suggesting Pneumocystis jirovecii infection, as well as hypotension, mental status changes, coagulopathy, or rhabdomyolysis.

Fibrosing mediastinitis, a chronic but rare form, ultimately causes circulatory compromise.

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

  • 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 about 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.

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%.

Treatment of Histoplasmosis

  • Sometimes no treatment

  • For mild to moderate infection, itraconazole

  • For severe infection, amphotericin B

Acute primary histoplasmosis

Acute primary histoplasmosis requires no antifungal therapy unless there is no spontaneous improvement after 1 month; itraconazole 200 mg orally is given 3 times a day for 3 days, then once a day for 6 to 12 weeks.

Fluconazole is less effective, and other azoles are not well-studied but have been used successfully.

Severe pneumonia requires more aggressive therapy with amphotericin B.

Chronic cavitary histoplasmosis

For chronic cavitary histoplasmosis, itraconazole 200 mg orally is given 3 times a day for 3 days, then once a day or 2 times a day for 12 to 24 months.

Other azoles or amphotericin B is used if patients are seriously ill or do not respond to or tolerate itraconazole.

Severe disseminated histoplasmosis

For severe disseminated histoplasmosis, liposomal amphotericin B 3 mg/kg IV once a day (preferred) or amphotericin B 0.5 to 1.0 mg/kg IV once a day for 2 weeks or until the patient is clinically stable is the treatment of choice. Patients can then be switched to itraconazole 200 mg orally 3 times a day for 3 days, then 2 times a day continued for 12 months after they become afebrile and require no ventilatory or blood pressure support.

For mild disseminated disease, itraconazole 200 mg orally 3 times a day for 3 days, then 2 times a day for 12 months can be used.

In patients with AIDS, itraconazole is given indefinitely to prevent relapse or until CD4 cell counts are > 150/mcL.

Blood levels of itraconazole and urine or blood levels of Histoplasma antigen should be monitored during therapy.

Fluconazole may be less effective, but voriconazole and posaconazole are very active against H. capsulatum and may be effective in the treatment of patients with histoplasmosis. Further data and experience are required to determine which drug is the best in each clinical situation.

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%.

  • For mild to moderate infection, use itraconazole.

  • For severe infection, use liposomal amphotericin B, followed by itraconazole.

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Drugs Mentioned In This Article

Drug Name Select Trade
Amphocin, Fungizone
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