Source of ‘unusual’ paper mill blastomycosis outbreak undetermined
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Key takeaways:
- Blastomycosis is a rare fungal disease caused by breathing in a fungus called Blastomyces.
- The estimated blastomycosis case prevalence among the mill workers was 20%.
The largest documented outbreak of blastomycosis in the United States was reported at a paper mill in Michigan in 2022-2023. However, researchers were unable to identify the source of Blastomyces exposure, according to a MMWR report.
“Blastomycosis is a rare fungal disease caused by breathing in Blastomyces — a fungus often found in the midwestern and southeastern U.S. in areas with moist soil or decaying wood,” Reid Harvey, DVM, MPH, epidemiologist with the National Institute for Occupational Therapy and Health (NIOSH), told Healio.
“This outbreak was unusual in that it occurred among a workforce that works primarily indoors. Blastomyces typically does not propagate 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.”
The outbreak investigation — which Harvey noted was the largest documented blastomycosis outbreak in the U.S. and the first associated with a paper mill — began after a cluster of atypical pneumonia cases among workers at a local paper mill in Delta County, Michigan, were identified on Feb. 28, 2023.
According to the study, urine antigen testing yielded positive results for Blastomyces. A NIOSH health hazard evaluation began on March 17, 2023, and consisted of an initial site visit, an environmental survey and ventilation assessment at the mill, a medical survey and a follow-up environmental survey months later.
Among approximately 1,000 workers, 603 participated in the medical survey, with data from an additional 42 mill workers with blastomycosis diagnoses who did not participate in the evaluation also being included in the analysis.
Among the 645 workers, 162 (25%) with blastomycosis were identified with illness onset between Nov. 1, 2022, and May 15, 2023. The investigation revealed that new cases peaked at 21 the week of Feb. 27-March 5, 2023.
According to the study, the last case was identified in May 2023 and the outbreak was declared over on July 1, 2023, with no new cases being identified as of April 2024.
Based on 120 blastomycosis cases identified among 603 NIOSH medical survey participants, the investigators estimated that the blastomycosis case prevalence among mill workers was 20%.
Harvey said that an analysis of environmental samples collected from the paper mill did not identify the exact source of worker exposures. He added, however, that identifying Blastomyces from environmental samples is “very difficult.”
“Health care 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,” Harvey said. “Health care 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 added that industries with workers who routinely work outdoors in the midwestern and southeastern U.S. should consider providing worker training and education to enhance awareness of Blastomyces, “particularly workers performing high-risk activities like disturbing soil, including digging or excavation.”
“It is important to train workers on minimizing their risk of exposure to Blastomyces, to recognize symptoms of blastomycosis, and to promptly seek medical care if they develop symptoms,” he concluded.
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