Blastomycosis

Abstract Blastomycosis, one of the endemic mycoses in the United States, is caused by the dimorphic fungus, Blastomyces dermatitidis. The organism exists in nature in the mould or mycelial phase and converts to the parasitic or yeast phase at body temperature. The fungus may either produce epidemics of infection after a point-source exposure or cause disease in a sporadic manner in the endemic areas. Blastomyces dermatitidis can cause an infection with a subclinical illness and subsequent protection against progressive infection afforded by cellular immune mechanisms. By contrast, a patient may present with pneumonia or with extrapulmonary disease or both. Lung involvement in blastomycosis may mimic acute bacterial pneumonia, while chronic pulmonary presentations mimic lung cancer or tuberculosis. Skin disease, usually presenting as either verrucous or ulcerative lesions, is the most common extrapulmonary site followed by bone, prostate, and CNS disease. Diagnosis is often easily made by visualization of the yeast in smears, or in tissue specimens or by culture; recognition of B. dermatitidis by smear or culture provides a definitive diagnosis. Itraconazole has been shown to be the drug of choice for either pulmonary or extrapulmonary infection, except in cases of life-threatening infection when amphotericin B should be used. Gilchrist first described blastomycosis in Baltimore in the 1890s as a skin infection caused by a protozoan organism (Gilchrist, 1894) and the illness was known for a time as Gilchrist’s disease. There were some errors in the initial description. For example, blastomycosis is not common in the areas surrounding Balti-more, and infection of the skin occurs secondarily rather than primarily. Moreover, the organism is not a protozoan but a fungus. Gilchrist was the first to refute portions of his own descriptions when he isolated and named the fungus B. dermatitidis (Gilchrist et al, 1898). Because skin manifestations of blastomycosis are often very striking, the initial cases were perceived as mainly dermatologic in nature (Sarosi and Davies, 1979). The concept of primary pulmonary blastomy- cosis was not recognized until pathologic descriptions allowed the pathophysiologic mechanisms to be delineated (Witorsch and Utz, 1968). There are rare cases of cutaneous inoculation of B. dermatitidis in laboratory workers and veterinarians, but almost all cases of blastomycosis are considered to originate from a pulmonary portal of entry (Sarosi and Davis, 1979).