BACKGROUND: Blastomycosis is an environmentally acquired fungal infection that can result in severe pulmonary illness and high hospitalization rates. In 2023, a blastomycosis outbreak was detected among workers at a paper mill in Delta County, Michigan. METHODS: We included patients with clinical and laboratory evidence of blastomycosis who had spent ≥40 hours in Delta County since 1 September 2022 and had illness onset 1 December 2022-1 July 2023. We assessed epidemiological and clinical features of patients and evaluated factors associated with hospitalization. We performed whole-genome sequencing to characterize genetic relatedness of clinical isolates from 8 patients. RESULTS: In total, 131 patients were identified
all had worked at or visited the mill. Sixteen patients (12%) were hospitalized
1 died. Compared with nonhospitalized patients, more hospitalized patients had diabetes (P = .03) and urine antigen titers above the lower limit of quantification (P <
.001). Hospitalized patients were also more likely to have had ≥1 healthcare visits before receiving a blastomycosis diagnostic test (P = .02) and to have been treated with antibiotics prior to antifungal prescription (P = .001). All sequenced isolates were identified as Blastomyces gilchristii and clustered into a distinct outbreak cluster. CONCLUSIONS: This was the largest documented blastomycosis outbreak in the United States. Epidemiologic evidence indicated exposures occurred at or near the mill, and genomic findings suggested a common exposure source. Patients with diabetes may have increased risk of hospitalization, and elevated urine antigen titers could indicate greater disease severity. Early suspicion of blastomycosis may prompt earlier diagnosis and treatment, potentially reducing unnecessary antibiotic prescriptions and improving patient outcomes.