NIOSH Investigates Largest Documented Blastomycosis Outbreak in the United States
NIOSH’s investigation of the largest documented outbreak of blastomycosis in the United States, which was also the first associated with an industrial work site, is the subject of CDC’s Morbidity and Mortality Weekly Report (MMWR) for Jan. 2, 2025. Blastomycosis is a rare fungal disease caused by inhalation of Blastomyces spores. The outbreak involved 162 cases among workers at a paper mill in Delta Count, Michigan, between Nov. 1, 2022, and May 15, 2023. Eighteen workers with Blastomycosis were hospitalized, and one worker died.
Local public health officials were notified of a cluster of unusual pneumonia cases among paper mill workers on Feb. 28, 2023. All patients experienced respiratory symptoms beginning in January and February 2023 and had positive urine antigen tests for Blastomyces. In April, the management of the paper mill voluntarily paused production for three weeks to clean ventilation ducts and upgrade air filters, while NIOSH and other public health agencies conducted training on Blastomyces for mill workers. Management also requested NIOSH to conduct a health hazard evaluation (HHE) to investigate sources of Blastomyces exposure and recommend prevention and control measures. NIOSH conducted environmental and medical surveys and a ventilation assessment between March and August 2023.
Of the mills’ roughly 1,000 workers, 645 participated in NIOSH’s investigation. Twenty-five percent of participants were identified as having been ill with blastomycosis since Nov. 1. Cough, shortness of breath, fatigue, and fever or chills were the most reported symptoms. The last case was identified in May 2023, and on July 1, the outbreak was declared over. NIOSH estimated that 20 percent of all mill workers had blastomycosis.
Workers with blastomycosis tended to be younger and to have worked at the mill for a shorter tenure than workers without blastomycosis, NIOSH found. Workers in all areas of the mill were affected, including workers in administrative offices, but blastomycosis cases were highest among those who worked mainly around one of the paper machines and in the maintenance area.
Blastomyces was not detected in environmental samples taken from the mill’s heating, ventilation, and air conditioning system or the surrounding area. However, “the lack of positive samples from the mill does not rule out the presence of Blastomyces,” MMWR states, due to the difficulty of identifying the fungal spores in the environment.
Blastomyces is found in moist soil or decaying wood and leaves in the midwestern and southern United States, MMWR explains. The mill where the outbreak occurred was located on a river in a wooded area consistent with Blastomyces’ habitat. The fungus does not typically propagate indoors. Previous cases of occupational blastomycosis have been reported in outdoor industries, such as farming, construction, and landscaping.
NIOSH Research Industrial Hygienist Ryan LeBouf, PhD, CIH, discussed the blastomycosis outbreak and HHE at AIHA Connect 2024. LeBouf predicted that the source of Blastomyces exposures was likely outside of the plant. An outbreak in an occupational setting like the mill is "unheard of," he said.
According to MMWR, “the specific environmental factors in or around the mill that led to this outbreak remain unknown.”
More information about the blastomycosis outbreak may be found in CDC’s MMWR.
