The Billirud Mill Blastomycosis Outbreak: Comparison to Historical Controls.

A large outbreak of blastomycosis among paper mill workers in Escanaba, Michigan began with the first cases reported on February 28, 2023 and expanded to 120 cases. Analysis of baseline regional data and the data collected during this unprecedented outbreak provide insight on the outbreak's unique characteristics. The Michigan Disease Surveillance System provided descriptive and outcome data on blastomycosis cases in the Upper Peninsula from 2007 through 2022 and the 2023 outbreak. Baseline incidence for the region was estimated and outbreak cases were compared to historic controls using quantitative methods such as t-tests, chi-square, and Poisson and logistic regression. The Billirud blastomycosis outbreak has resulted in 28 confirmed and 92 probable cases. Blastomycosis incidence for the Upper Peninsula was 2.13 (95%CI: 1.75, 2.59) per 100,000 person-years with counties bordering Wisconsin having a higher incidence more than other counties (4.56, 95%CI: 3.48, 5.97; RR=3.46, 95%CI: 2.35, 5.11). Outbreak cases were significantly younger, more likely to be male, reported more respiratory symptoms, and had a shorter time from onset of illness to diagnosis. The Billirud paper mill blastomycosis outbreak is the largest reported to date, the first in an industrial facility, and the first to specifically involve Blastomyces gilchristii. Blastomyces species are endemic in regional forests, so other facilities handling forestry products may be at risk.