Assessment of environmental risk factors for blastomycosis during a large outbreak at a Michigan paper mill

Abstract

Background: Blastomycosis is a rare, potentially fatal fungal infection caused by inhalation of Blastomyces spores, typically acquired outdoors in the midwestern and eastern United States. In 2023, the largest recorded U.S. blastomycosis outbreak occurred among workers at a paper mill in Michigan's Upper Peninsula. Few data exist on occupational risk factors or indoor exposure to Blastomyces, limiting prevention efforts.

Objectives: We assessed workplace environments and conditions associated with blastomycosis risk through a cross-sectional medical survey and environmental sampling.

Methods: During April 22-28, 2023, we conducted a voluntary medical survey, including a work and health questionnaire and urine antigen testing, for 603 workers out of approximately 1,000 at the mill. We compared worker characteristics, work locations, and environmental exposures by blastomycosis case status and modeled disease risk using Poisson regression. We tested 533 environmental samples of outdoor soil, indoor surface dust, and raw materials for Blastomyces using polymerase chain reaction and culture-based methods.

Results: Twenty percent of workers were classified as blastomycosis cases based on positive urine antigen testing during the survey, self-reported provider diagnoses, or confirmed or probable case status reported by state or local health departments. Prevalence was highest among workers in paper machine line #1 (27%) and maintenance areas (25%). Adjusted analyses indicated a 40% [Prevalence Ratio (PR): 1.40; 95% confidence interval (CI): 1.00, 1.95] and 53% (PR: 1.53; 95% CI: 1.04, 2.25) higher risk for workers in these locations, respectively, compared to workers working elsewhere. Working in both locations doubled blastomycosis risk. Daily exposure to indoor pooling water was associated with a nearly two-fold higher prevalence of blastomycosis (PR: 1.79; 95% CI: 1.25, 2.57). All indoor and outdoor environmental samples were negative for Blastomyces.

Discussion: Blastomycosis was associated with specific indoor work locations and environmental conditions, suggesting the potential for occupational exposure to Blastomyces in indoor industrial settings. These findings may guide future outbreak investigations and occupational prevention strategies.

Fig 1. Map of blastomycosis prevalence by mill area.

The prevalence of blastomycosis at the mill was evaluated for each process area based on workers’ reported primary and secondary work locations, office spaces, and breakrooms/lunchrooms/shacks from October 2022–April 2023.

Fig 2. Blastomycosis risk by work location.

Adjusted prevalence ratios (PR) and 95% confidence intervals (CI) estimated risk of blastomycosis for working versus not working in each location, including for workers’ primary and secondary work locations, office spaces, and breakrooms/lunchrooms/shacks, from October 2022–April 2023. PRs were adjusted for mill tenure and sex.

Fig 3. Blastomycosis risk associated with Paper Machine Line #1 and the Maintenance areas.

Adjusted prevalence ratio (PR) and 95% confidence intervals (CI) estimated risk of blastomycosis for working in paper machine line #1 and/or the maintenance areas compared to not working in either of these mill locations from October 2022–April 2023. PRs were adjusted for mill tenure and sex; n indicates number of workers per location.

Fig 4. Blastomycosis risk associated with daily exposure to environmental conditions at the mill.

Adjusted prevalence ratios (PR) and 95% confidence intervals (CI) estimated risk of blastomycosis associated with daily versus less than daily exposure to environmental conditions at the mill from October 2022–April 2023. PRs were adjusted for mill tenure, sex, and primary work location.

Fig 5. Blastomycosis risk associated with daily exposure to multiple environmental conditions.

Adjusted prevalence ratio (PR) and 95% confidence intervals (CI) estimated risk of blastomycosis for daily versus less than daily exposure from October 2022–April 2023 at the mill to: (A) pooling water indoors and pooling water outdoors, (B) pooling water indoors and visible mold indoors, and (C) pooling water indoors and wind indoors. PRs were adjusted for mill tenure, sex, and primary work location; n indicates number of workers reporting daily exposure to the conditions.