Background: Blastomyces dermatitidis is a thermally dimorphic fungus that causes the systemic pyogranulomatous disease termed blastomycosis. Blastomycosis is the least common of the endemic systemic mycoses; the other more common mycoses include histoplasmosis and coccidioidomycosis. Lungs, and to a lesser extent, skin and bone, are the most common organs involved with this fungus. Hematogenous dissemination can occur.

Pathophysiology: Analysis of sporadic cases indicates that middle-aged men with outdoor occupations that exposed them to soil are at greatest risk for blastomycosis. Exposure to soil, whether at work or at play, appears to be the common link in reports of sporadic disease and outbreaks. The disease occurs more commonly in wooded areas and in hunters.

The respiratory system is considered the portal of infection. After inhalation of the conidia, neutrophils are the first cells recruited to the sites of infection, followed by lymphocytes. A reaction to the infection results in granuloma formation with central microabscesses (termed pyogranuloma) but does not result in caseation as occurs in histoplasmosis or tuberculosis. Cellular immunity is an important factor in defense against Blastomyces dermatitidis, but to a lesser extent than against other endemic mycoses. Infection is more extensive and outcome is worse in patients who are immunosuppressed or infected with HIV.

Frequency:

• In the US: Endemic areas in North America include the southeastern and south central states, especially those bordering the Mississippi and Ohio river basins; the midwestern states and Canadian provinces (Quebec, Ontario, Manitoba) that border the Great Lakes; and a small area in New York and Canada along the St Lawrence River.

  Most data are from Wisconsin, where the disease is a reportable condition. From 1986-1995, in a report by the Centers for Disease Control and Prevention, 670 cases were identified (mean annual incidence of 1.4 per 100,000).

• Internationally: Outside of North America, well-documented cases have been reported by Baily et al to occur most frequently in Africa. Occasional cases have been reported by DiSalvo to occur in Central America, South America, India, and the Middle East.

Mortality/Morbidity: Although the disease is relatively common, the exact mortality rate remains unknown. The case fatality rate reported to the Wisconsin Department of Health from 1986-1995 was 4.3% (29 of 670 patients). The disease is more aggressive in immunocompromised patients; in 1 series of these patients reported by Wheat, the mortality rate was approximately 30%.

Race: No racial predominance or seasonal predilection appears to exist for blastomycosis.

Sex: Men are affected more commonly with a male-to-female ratio of 3:2 in Wisconsin (1986-1995). In other series, it varies from approximately 5:1-15:1.

Age: Most patients are middle aged (mean age in Wisconsin data was 46 years). Although the disease is uncommon in children, a 1979 review of the literature documented 110 patients younger than 20 years.

Clinical Details: Infection commonly presents as a flulike illness associated with fever, cough, dyspnea, and pleuritic chest pain. Insidious weight loss, arthralgias, and myalgias are not uncommon, and erythema nodosum develops occasionally. The disease may affect only the pulmonary system or it can be extensively disseminated.

Pulmonary

Patients present with either 1) localized ill-defined lung opacity or a more discrete lung nodule or 2) with disseminated disease. Each manifestation occurs in approximately one half of patients. Disseminated pulmonary disease can be rapidly progressive and complicated by miliary (hematogenous) spread, leading to acute respiratory distress syndrome. A more indolent course clinically resembling tuberculosis can occur. The infection can be self-limited or chronic and progressive. Fibrosing mediastinitis and laryngeal, tracheal, and endobronchial invasion also have been reported.

Disseminated

Skin and bone lesions are the most common manifestations of disseminated disease (50% of patients). Skin lesions are as common as those found in the lung and tend to resemble neoplasms, both clinically and pathologically. Cutaneous disease appears similar to disease seen with pyoderma gangrenosum, leishmaniasis, Mycobacterium marinum infection, giant keratoacanthoma, and squamous cell carcinoma. Typical lesions are painless erythematous nodules that develop verrucous or ulcerative surfaces.

Bone and joint lesions occur in fewer than one half of patients with disseminated blastomycosis and are characterized by osteolysis. Involvement of the vertebrae can result in spinal cord compression.

The genitourinary system is involved in approximately 10% of male patients. Central nervous system impairment manifesting as meningitis, brain lesions, or epidural abscesses is relatively common (a minimum of 15% of patients).

Other rare sites of involvement include the eye, paranasal sinuses, pericardium, peritoneum, spleen, liver, adrenal gland, and thyroid.

Preferred Examination:

Mycology

Diagnosis is based on demonstration of organisms in culture or on fungal stains (10% potassium hydroxide) of sputum, bronchoscopy specimens, or secretions obtained from cerebrospinal fluids or dermal, subcutaneous, or other lesions. Cultures are positive in more than 90% of patients. Culture growth may take from 1 to several weeks.

Radiologic modalities

Radiographic findings are nonspecific and variable. Chest radiography is the first imaging study performed. The most common pattern observed is acute nonspecific focal lung opacity, which is found in 25-75% of patients.

Limitations of Techniques: Radiographic patterns of pulmonary disease are indistinguishable from those of other mycotic infections.

DIFFERENTIALS

Coccidioidomycosis, Thoracic
Histoplasmosis, Thoracic
Sarcoidosis, Thoracic

Other Problems to be Considered:

Mycobacterium tuberculosis
Nonmycobacterial tuberculosis
Miliary tuberculosis
Lymphoma
Bronchoalveolar carcinoma

X-RAY

Findings: