On Dec. 31, 2024, the U.S. Centers for Disease Control and Prevention released a study of the largest outbreak of recorded blastomycosis in the U.S. at a paper mill in Michigan during 2022–2023. Although environmental sampling did not identify the source of Blastomyces exposure in the mill, this was the first recognized blastomycosis outbreak in an industrial, largely indoor setting.

Blastomycosis is a rare fungal disease often initially misdiagnosed, which can contribute to severe pulmonary illness. Community outbreaks have been reported after soil disruption and outdoor recreational activities in blastomycosis-endemic areas.

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, 2023. 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.

Additional environmental samples were collected during August 1–2, 2023, while excavation activities took place for a bridge being constructed over the river near the mill. Among 533 indoor and outdoor environmental samples analyzed, no Blastomyces was detected by PCR or culture.

As of April 2024, no additional cases were identified among mill workers. During October 1, 2022–July 1, 2023, one blastomycosis case without an association to the mill was identified in Delta County. Based on 120 blastomycosis cases identified among 603 NIOSH medical survey participants, estimated blastomycosis case prevalence among mill workers was 20%. Rapid and coordinated public health actions by local, state, and federal public health authorities and information sharing among partners enabled a swift response to protect mill workers.