Merck Manual

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(Gilchrist Disease; North American Blastomycosis)


Sanjay G. Revankar

, MD, Wayne State University School of Medicine

Last full review/revision Jul 2019| Content last modified Jul 2019
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Blastomycosis is a pulmonary disease caused by inhaling spores of the dimorphic fungus Blastomyces dermatitidis; occasionally, the fungi spread hematogenously, causing extrapulmonary disease. Symptoms result from pneumonia or from dissemination to multiple organs, most commonly the skin. Diagnosis is clinical, by chest x-ray, or both and is confirmed by laboratory identification of the fungi. Treatment is with itraconazole, fluconazole, or amphotericin B.

In North America, the endemic area for blastomycosis includes

  • Ohio–Mississippi River valleys (extending into the middle Atlantic and southeastern states)

  • Northern Midwest

  • Upstate New York

  • Southern Canada

Rarely, the infection occurs in the Middle East and Africa.

Immunocompetent people can contract this infection. Although blastomycosis may be more common and more severe in immunocompromised patients, it is a less common opportunistic infection than histoplasmosis or coccidioidomycosis.

B. dermatitidis grows as a mold at ambient temperature in soil enriched with animal excreta and in moist, decaying, acidic organic material, often near rivers.

In the lungs, inhaled spores convert into large (15 to 20 micrometers) invasive yeasts, which form characteristic broad-based buds.

Once in the lungs, infection may

  • Remain localized in the lungs

  • Disseminate hematogenously

Hematogenous dissemination can cause focal infection in numerous organs, including the skin, prostate, epididymides, testes, kidneys, vertebrae, ends of long bones, subcutaneous tissues, brain, oral or nasal mucosa, thyroid, lymph nodes, and bone marrow.

Symptoms and Signs


Pulmonary blastomycosis may be asymptomatic or cause an acute, self-limited disease that often goes unrecognized. It can also begin insidiously and develop into a chronic, progressive infection. Symptoms include a productive or dry hacking cough, chest pain, dyspnea, fever, chills, and drenching sweats.

Pleural effusion occurs occasionally. Some patients have rapidly progressive infections, and acute respiratory distress syndrome may develop.


In extrapulmonary disseminated blastomycosis, symptoms depend on the organ involved.

Skin lesions are by far the most common site of dissemination; they may be single or multiple and may occur with or without clinically apparent pulmonary involvement. Papules or papulopustules usually appear on exposed surfaces and spread slowly. Painless abscesses, varying from pinpoint to 1 mm in diameter, develop on the advancing borders. Irregular, wartlike papillae may form on surfaces. Sometimes bullae develop. As lesions enlarge, the centers heal, forming atrophic scars. When fully developed, an individual lesion appears as an elevated verrucous patch, usually 2 cm wide with an abruptly sloping, purplish red, abscess-studded border. Ulceration may occur if bacterial superinfection is present.

Images of Extrapulmonary Blastomycosis

If bone lesions develop, overlying areas are sometimes swollen, warm, and tender.

Genital lesions cause painful epididymal swelling, deep perineal discomfort, or prostatic tenderness detected during rectal examination.

Central nervous system involvement can manifest as brain abscess, epidural abscess, or meningitis.


  • Fungal cultures and smear

  • Blastomyces urine antigen

If blastomycosis is suspected, a chest x-ray should be taken. Focal or diffuse infiltrates may be present, sometimes as patchy bronchopneumonia fanning out from the hilum. These findings must be distinguished from other causes of pneumonia (eg, other mycoses, tuberculosis [TB], tumors).

Skin lesions can be mistaken for sporotrichosis, TB, iodism, or basal cell carcinoma. Genital involvement may mimic TB.

Cultures of infected material are done; they are definitive when positive. Because culturing Blastomyces can pose a severe biohazard to laboratory personnel, the laboratory should be notified of the suspected diagnosis. The organism’s characteristic appearance, seen during microscopic examination of tissues or sputum, is also frequently diagnostic.

Serologic testing is not sensitive but is useful if positive.

A urine antigen test is useful, but cross-reactivity with Histoplasma is high.

Molecular diagnostic tests (eg, polymerase chain reaction [PCR]) are becoming available.


  • For mild to moderate disease, itraconazole

  • For severe, life-threatening infection, amphotericin B

(See also Antifungal Drugs and the Infectious Diseases Society of America’s Practice Guidelines for the Management of Blastomycosis.)

Untreated blastomycosis is usually slowly progressive and is rarely ultimately fatal.

Treatment of blastomycosis depends on severity of the infection.

For mild to moderate disease, itraconazole 200 mg orally 3 times a day for 3 days, followed by 200 mg orally once a day or 2 times a day for 6 to 12 months is used. Fluconazole appears less effective, but 400 to 800 mg orally once a day may be tried in itraconazole-intolerant patients with mild disease.

For severe, life-threatening infections, IV amphotericin B is usually effective. The Infectious Diseases Society of America’s guidelines recommend a lipid formulation of amphotericin B at a dosage of 3 to 5 mg/kg once a day or amphotericin B deoxycholate 0.7 to 1.0 mg/kg once a day for 1 to 2 weeks or until improvement is noted.

Therapy is changed to oral itraconazole once patients improve; dosage is 200 mg 3 times a day for 3 days, then 200 mg 2 times a day for ≥ 12 months.

Patients with central nervous system blastomycosis, pregnant patients, and immunocompromised patients should be treated with IV amphotericin B (preferably liposomal amphotericin B), using the same dose schedule as for life-threatening infection.

Voriconazole, isavuconazole, and posaconazole are active against B. dermatitidis, but clinical data are limited, and the role of these drugs has not yet been defined.

Key Points

  • Inhaling spores of the dimorphic fungus Blastomyces can cause pulmonary disease and, less commonly, disseminated infection (particularly to the skin).

  • In North America, blastomycosis is endemic in the regions around the Great Lakes and the Ohio–Mississippi River valleys (extending into the middle Atlantic and southeastern states).

  • Diagnose using cultures of infected material; serologic testing is specific but not very sensitive.

  • For mild to moderate disease, use itraconazole.

  • For severe disease, use amphotericin B.

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