Seasonal variations in the clinical presentation of pulmonary and extrapulmonary blastomycosis

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 3920 (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 (P0.0001). 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.