eMedicine Specialties > Radiology > Chest

Blastomycosis, Thoracic

Author: Fahad M Alhameed, MD, AmBIM, FCCP, FRCPC, Deputy Chairman of Intensive Care Department, Consultant Critical Care and Pulmonary Medicine, Department of Intensive Care and Pulmonary Medicine, King Khalid National Guard Hospital, Jeddah, Saudi Arabia
Coauthor(s): Sat Sharma, MD, FRCPC, Professor and Head, Division of Pulmonary Medicine, Department of Internal Medicine, University of Manitoba; Site Director, Respiratory Medicine, St. Boniface General Hospital; Bruce Maycher, MD, Director of Pulmonary Radiology, St Boniface General Hospital; Associate Professor, Department of Radiology, University of Manitoba
Contributor Information and Disclosures

Updated: Oct 17, 2008

Introduction

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.^1,2,3,4,5

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Pathophysiology

Analysis of sporadic cases of thoracic blastomycosis indicates that middle-aged men who work outdoors in occupations that expose them to soil are at greatest risk. Exposure to soil, whether at work or at play, appears to be the common link in reports of sporadic cases and outbreaks. The disease occurs more commonly in wooded areas; the prevalence is higher among 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 it 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 outcomes are worse in patients who are immunosuppressed or who are infected with HIV.

Frequency

United States

Areas in North America where thoracic blastomycosis is endemic 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).⁶

According to the Wisconsin Department of Health Services, there were 428 reported cases of blastomycosis between January 1, 2005, and December 31, 2007. Of these cases, 267 individuals were hospitalized.⁷  

International

Outside of North America, well-documented cases of thoracic blastomycosis have been reported by Baily et al to occur most frequently in Africa.⁸ Occasional cases have been reported by DiSalvo to occur in Central America, South America, India, and the Middle East.⁹

Mortality/Morbidity

Although thoracic blastomycosis 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).⁶ The disease is more aggressive in immunocompromised patients; in 1 series of these patients reported by Wheat, the mortality rate was approximately 30%.¹⁰

Race

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

Sex

Thoracic blastomycosis occurs more commonly in men than in women. In Wisconsin from 1986-1995, the male-to-female ratio was 3:2; in other series, the ratio varies from approximately 5:1-15:1.⁶

Age

Most patients with thoracic blastomycosis are middle aged (in Wisconsin, the mean age was reported to be 46 years).⁶ Although the disease is uncommon in children, a 1979 review of the literature documented 110 patients younger than 20 years.

Presentation

Thoracic blastomycosis 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 may be extensively disseminated.^11,12,13,14

Pulmonary

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

Disseminated

Skin and bone lesions are the most common manifestations of disseminated disease (occurring in 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 may 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

The diagnosis of thoracic blastomycosis is made on the basis of a 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 cases. 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.^{15,16,17,18,19}

Limitations of Techniques

Radiographic patterns of thoracic blastomycosis are indistinguishable from those of other mycotic infections.

Differential Diagnoses

Coccidioidomycosis, Thoracic
Histoplasmosis, Thoracic
Sarcoidosis, Thoracic

Other Problems to Be Considered

Mycobacterium tuberculosis
Nonmycobacterial tuberculosis
Miliary tuberculosis
Lymphoma
Bronchoalveolar carcinoma

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