Outbreak of Blastomycosis Among Paper Mill Workers - Michigan, November 2022-May 2023

Blastomycosis is a fungal disease caused by inhalation of Blastomyces spores from the environment that can result in severe pulmonary illness and high hospitalization rates. In early March 2023, Public Health Delta and Menominee Counties (Michigan) reported a cluster of blastomycosis cases among paper mill workers to the Michigan Department of Health and Human Services (MDHHS). MDHHS subsequently notified CDC. On March 17, paper mill management requested a health hazard evaluation (HHE) from CDC's National Institute for Occupational Safety and Health (NIOSH) to investigate potential workplace exposures to Blastomyces and recommend prevention and control measures at the mill. The workplace epidemiologic investigation combined a NIOSH HHE medical survey consisting of a questionnaire on work and health with Blastomyces urine antigen testing of specimens obtained from workers to assist in case finding, with additional case information from MDHHS blastomycosis surveillance data. Assessment of 645 mill workers identified 162 cases of blastomycosis with illness onset during November 1, 2022-May 15, 2023, with the weekly case count peaking at 21 cases in early March 2023. HHE environmental sampling in and around the mill did not identify the source of workers' Blastomyces exposure in the mill. This outbreak was the largest documented blastomycosis outbreak in the United States, and the first associated with a paper mill or an industrial setting. A coordinated public health response facilitated swift prevention measures with recommendations focused on reducing workers' exposure to Blastomyces, including hazard communication, respiratory protection, mill cleaning, and ventilation system improvements.