Blastomycosis at a Michigan Paper Mill, 2022–2023: The Largest U.S. Outbreak on Record

Summary

• Blastomycosis is a rare fungal disease, often misdiagnosed at first, that can cause severe lung illness. Earlier outbreaks followed soil disturbance or outdoor recreation where the fungus is common.
• In 2022–2023, 162 workers at a paper mill in Michigan fell ill — the largest documented blastomycosis outbreak in the United States, and the first in an industrial, mostly indoor workplace.
• Environmental sampling did not find the source inside the mill.
• Quick work by local, state and federal health authorities with the mill's managers and workers found cases fast and put protections in place.

The fungus

People catch blastomycosis by breathing in spores of Blastomyces from the environment. It usually affects the lungs, from mild illness that clears on its own to severe, potentially fatal pneumonia; about 25% of cases spread beyond the lungs, often to the skin. Up to half of infected people have no symptoms. Because it looks like other respiratory infections, diagnosis and treatment are often delayed, though antifungal medicine matters for preventing progression or relapse. The fungus lives in moist soil and decaying wood and leaves in the Midwest and Southeast, often near rivers and lakes. Work-related cases had been reported in farming, construction and landscaping — never before at a paper mill.

How it was found

• February 28, 2023: the county health department, Public Health Delta and Menominee Counties, learned of a cluster of atypical pneumonia among workers at a paper mill in Delta County. All had fallen ill in January–February, and urine antigen tests were positive for Blastomyces. It reported the cases to the Michigan Department of Health and Human Services (MDHHS).
• March 6: area health care providers were alerted to watch for more cases. Within the week, MDHHS told CDC's Mycotic Diseases Branch and the National Institute for Occupational Safety and Health (NIOSH) of eight cases, with 14 more under investigation — all in people who had worked at or visited the mill.
• March 17: mill management asked NIOSH for a health hazard evaluation, which ran in four parts: a site visit (March 27–28), an environmental and ventilation survey (April 24–28), a medical survey with a work-and-health questionnaire and urine antigen testing (April 22–28), and a follow-up environmental survey (August 1–2).

The workers who fell ill

Every employee, contractor and visitor was invited to the medical survey; of about 1,000 workers, 603 took part. Another 42 diagnosed workers were added from state data with their consent, for 645 in all.

• 162 (25%) had blastomycosis, with illness beginning between November 1, 2022 and May 15, 2023. New cases peaked at 21 in the week of February 27–March 5, 2023. The last case was found in May, and the outbreak was declared over on July 1, 2023; no more mill cases had appeared by April 2024. Among survey participants alone, 20% were estimated to have had the disease.
• The workforce was mostly men (83%), White (94%) and non-Hispanic (98%), with a median age of 46. Those who fell ill were younger and had worked at the mill for less time.
• The sick reported cough (90%), shortness of breath (76%), fatigue (76%) and fever or chills (73%); 63% had abnormal lung scans. But 61% of workers without blastomycosis also had a cough, in a winter season of other respiratory illness.
• 18 were hospitalized, and one person died. Earlier reports found that about 65% of patients with blastomycosis are hospitalized. Here, early detection let officials look actively for cases and urge workers with symptoms to seek care promptly.
• Urine testing found hidden cases. Of 573 workers tested, 52 (9%) were positive; half of them had never been diagnosed, and three had no symptoms at all.
• Every part of the mill was affected, including the offices; the most cases were among workers based on paper machine line #1 (35) and in maintenance (22), with 17 in administration.

Cases by week of illness onset. Figure from the CDC report.

Where the workers who fell ill mainly worked. Figure from the CDC report.

Looking for the source

The mill sits beside a river on 2,200 acres of buildings, outdoor woodyards, water treatment lagoons and a landfill, with about 400 heating, ventilation and air conditioning systems. Of 67 air-handling units checked, 59 (88%) drew unfiltered outdoor air into the mill, and many had dirt and organic debris in their coils.

Soil, wood chips, indoor dust, water, and dust, duct lining and filters from the ventilation systems were tested — along with samples taken while a bridge was being built over the nearby river. None of 533 samples held Blastomyces, by PCR or culture.

That does not rule the fungus out: it is notoriously hard to recover from the environment. Blastomyces does not usually grow indoors, but spores may have come in through the unfiltered ventilation or open bay doors, exposing workers indoors over several months — in a 2019 Wisconsin outbreak, it was detected in indoor air. Given an incubation period of 2 weeks to 3 months, exposure probably began as early as mid-October 2022 and lasted at least through February 2023, possibly into April. A riverside, wooded site suits the fungus, but exactly what caused this outbreak is unknown.

Background: blastomycosis is reportable in Michigan, with 186 cases in 2007–2017. Delta County usually saw fewer than one a year, and only one unrelated case turned up there during the outbreak — though the disease may be underreported: some similar northern Wisconsin counties record more than 20 cases per 100,000 people a year.

What was done

• From April 17, 2023, the mill voluntarily halted production for 3 weeks to clean its ventilation ductwork and upgrade filters in every air-handling unit.
• On April 20–21, federal, state and local staff held 10 sessions for workers on blastomycosis and the investigation; managers trained workers on the hazard and on protective practices, and urged them to report symptoms to their doctors.
• NIOSH advised fit-tested N95 respirators, especially for workers at risk of severe disease (such as those with weakened immune systems) or doing high-risk tasks such as changing ventilation filters or disturbing soil.

Complications included how hard the fungus is to find, the timing of the urine screening given the variable incubation period, and antifungal treatment already under way in many workers, which may have affected their test results. Paper dust, dampness and other fungi may also have caused some of the workers' respiratory symptoms.

Lessons for workplaces

• Where blastomycosis is common, employers whose workers disturb soil outdoors can train them to recognize the hazard and reduce exposure.
• Routine ventilation maintenance — changing filters, fan belts and parts, cleaning the system as needed — and good housekeeping may make indoor exposure less likely.
• Doctors and health departments in these areas should consider work-related exposure, so the next outbreak is caught early.

Sources

Based on R. Reid Harvey, Allyson W. O'Connor, Marcia L. Stanton and others, "Outbreak of Blastomycosis Among Paper Mill Workers — Michigan, November 2022–May 2023," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain.