By Ethan Covey
The largest documented outbreak of blastomycosis in the United States took place at a Michigan paper mill from 2022 to 2023, and is the first such outbreak to occur in a largely indoor industrial setting. A total of 162 cases of blastomycosis were identified (MMWR Morb Mortal Wkly Rep 2025;73[5152]:1157-1162).
“Although occupational blastomycosis has been reported in outdoor workers in industries such as farming, construction and landscaping, this outbreak was unusual in that it occurred among a workforce that works primarily indoors,” R. Reid Harvey, DVM, a research epidemiologist with the Respiratory Health Division of the National Institute for Occupational Safety and Health (NIOSH), told Infectious Disease Special Edition. “Blastomyces typically does not grow and spread indoors; however, spores potentially infiltrated mill buildings through unfiltered ventilation systems or open bay doors, leading to indoor fungal exposure among mill workers, likely over several months.”
In early March 2022, the Michigan Department of Health and Human Services was notified of a cluster of blastomycosis cases among paper mill workers. An investigation identified 162 cases of blastomycosis among 645 mill workers, with illness onset from Nov. 1, 2022, through May 15, 2023. In early March 2023, the weekly case count peaked at 21 cases.
The final case was identified in May 2023, and the outbreak was declared over on July 1, 2023.
While the paper mill was located near a river in a wooded area—conditions consistent with the habitat of Blastomyces—investigators were unable to determine a specific cause of the outbreak.
“Industries in the Midwestern and Southeastern U.S. where blastomycosis is endemic should consider providing worker training and education to enhance awareness of Blastomyces, particularly industries with workers performing high-risk activities like disturbing soil, including digging or excavation,” Dr. Harvey said.
Dr. Harvey added that healthcare providers and public health authorities in areas endemic for blastomycosis should consider potential work-related exposure to Blastomyces to detect future outbreaks and implement public health interventions quickly, especially since blastomycosis often presents as community-acquired pneumonia in primary and urgent care settings.
“Healthcare providers should consider testing patients with community-acquired pneumonia who live in or recently traveled to an endemic area and have not improved with at least one course of empiric antibiotics,” he said.
Dr. Harvey said several aspects included among field activities of the NIOSH Health Hazard Evaluation (HHE) program are not incorporated routinely into blastomycosis outbreak investigations and might benefit from future studies.
“First, the NIOSH HHE medical survey supported case finding efforts through urine antigen testing that proved useful to identify workers with potentially undiagnosed blastomycosis, who could be further evaluated by their healthcare providers and treated, if necessary,” he said. “Second, analysis of environmental samples collected in and around the paper mill did not identify the exact source of workers’ Blastomyces exposure in the mill.
“We anticipated challenges identifying Blastomyces from environmental samples for several reasons, including limitations to the sampling and analytical methods and the timing of environmental sampling, given the potentially monthslong delay between exposure and illness onset,” he added. “Both the application of urine antigen testing to screen for Blastomyces infection among potentially exposed persons and the use environmental sampling for Blastomyces in air, indoor dust and soil could benefit from future studies to support outbreak investigations.”
