Blastomycosis

David Wayne Dockweiler

Blastomycosis is a relatively uncommon disease caused by the dimorphic fungus Blastomyces dermatitidis. After inhalation of fungal spores by the host, the organism may give rise to a spectrum of clinical syndromes ranging from an acute, self-limited, flulike illness to rapidly progressive, widely disseminated, fatal disease. The primary reservoir for human infection is soil that is moist and rich in organic debris.

The vast majority of cases have been described in North America in endemic areas that include the southeastern and south-central United States and the region around the Great Lakes; thus, the disease also has been known as North American blastomycosis. With the demonstration of cases in Europe, Africa, South America, and the Middle East, this term is clearly no longer appropriate.

Blastomycosis most often is a disease of young to middle-aged men, although cases have been described in both sexes and in ages ranging from newborn to the elderly. A significant number of patients have a history of outdoor activities, especially hunting. Interestingly, canine blastomycosis, a condition similar to human disease, is more commonly seen in animals that are used in hunting, pointing to exposure of both humans and animals from a common outdoor source. Blastomycosis is not considered an opportunistic infection, per se, and has not been identified in immunocompromised patients to the same degree as other fungal pathogens, such as histoplasmosis or cryptococcosis; however, cases have occurred in patients with renal transplants, chronic steroid use, and AIDS. Not surprisingly, disseminated disease and an increased incidence of central nervous system (CNS) involvement are seen in such individuals.

Primary infection with B. dermatitidis almost always results from inhalation of fungal spores. Far less commonly, the disease is sexually transmitted through prostatic secretions or spread by maternal–fetal transmission. A laboratory accident associated with percutaneous inoculation has resulted in disease. Person-to-person transmission by aerosol has never been documented.

After deposition of the spores in the alveoli, an inflammatory response is initiated that consists of polymorphonuclear leukocytes, monocytes, and alveolar macrophages. Granuloma formation follows in most cases. Several clinical courses may ensue, depending on factors such as host resistance and the inhaled dose. First, there may be spontaneous resolution of mild or clinically silent pulmonary involvement that, in a presumably small number of cases, may disseminate. Second, there may be the development of acute pneumonitis with nonspecific symptoms, such as fever, cough, purulent sputum, chills, myalgias, pleuritic chest pain, and occasionally, erythema nodosum. Third, patients may present with a severe progressive pulmonary process leading to hypoxemia, respiratory failure, and prostration. Fourth, the infection may present as a chronic pulmonary infiltrate, easily confused with other fungal diseases or tuberculosis. Patients with this chronic form of blastomycosis often present with a long history of constitutional complaints, such as malaise, weight loss, chronic cough, fever, and blood-tinged sputum. Significant hemoptysis is rare. Dissemination is said to take place in up to 70% of patients with chronic pulmonary blastomycosis, and multiple-organ involvement is the rule. Lastly, there may be disseminated disease, which may occur with the previously mentioned patterns of pulmonary involvement or in isolation, representing reactivation of the disease long after the primary focus of infection has resolved.

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