Blastomycosis is a cause of serious, potentially life-threatening pulmonary infection in this geographic region.

Blastomycosis is a granulomatous infection caused by the thermally dimorphic fungus, Blastomyces dermatitidis, for which seasonal variation has been proposed. We conducted a retrospective review of medical records of 324 patients with blastomycosis in Manitoba and northwestern Ontario. The average age of patients at the time of diagnosis was 39+/-20 (range, 0-85) years. Symptoms referable to blastomycosis were first noted in the autumn and winter (September to February) by 63% of the patients. The seasonal distribution of cases was different for localized pulmonary infection than the disseminated disease (P<0.0001). For localized lung disease, the peak incidence of symptom onset occurred in the autumn, and lowest incidence in the spring; one half (50%) of the patients with diffuse lung disease had onset of symptoms in the spring months and a few (11%) cases occurred during the summer. We noted a distinct seasonal variation in the clinical presentation of blastomycosis. The observed pattern suggests that summer environmental exposure and acquisition of the infection results in an early (1-6 months) localized pneumonia in the majority of cases, followed by later (4-9 months) reactivation or slow progression of asymptomatic infection resulting in isolated extrapulmonary or disseminated hematogenous disease in the minority.