MARSHFIELD, Wis. — Industries with workers who routinely work outdoors in the midwestern and southeastern United States should consider providing worker training and education to enhance awareness of Blastomyces, the fungal spores that cause blastomycosis lung infections, according to Marshfield Clinic Research Institute.

“Blastomycosis is difficult to diagnose because its symptoms often mimic other respiratory illnesses, like pneumonia, making it hard to distinguish clinically,” said Jennifer Meece, Ph.D., executive director of Marshfield Clinic Research Institute.

“Diagnosis requires seeing the fungus in tissue samples through a culture or microscope. It’s important for patients to begin treatment with antifungal medication as soon as possible, and to attempt to find the source of the exposure to prevent others from inhaling the spores.”

In a news release Monday, Marshfield researchers reviewed a 2022-23 outbreak at an Escanaba paper mill that sickened workers and resulted in one death. Nearly 500 environmental samples were tested by Marshfield Clinic Research Institute during a multi-state, multi-agency investigation into the outbreak, which was largest known blastomycosis outbreak in the United States.

In March 2023, a cluster of atypical pneumonia cases, many of which were attributed to mill workers at the Billerud paper mill in Escanaba, was reported to the local health department. Investigators from the Centers for Disease Control and Prevention’s National Institute for Occupational Safety and Health, working with public health partners, facilitated blastomycosis case-finding through a medical survey and conducted an environmental assessment.

A survey of 645 mill workers showed that 162 people were identified with blastomycosis between Nov. 1, 2022, and May 15, 2023, according to a study published online Monday in the CDC’s weekly Morbidity and Mortality Weekly Report. Overall, an estimated 20% of workers at the paper mill tested positive for blastomycosis.

Blastomycosis is a rare infectious disease caused by inhalation of Blastomyces fungal spores from the environment. Blastomyces is found in moist soil or decaying wood and leaves, often near rivers and lakes. Blastomycosis incubates over a two-week to three-month period and pulmonary illness is most common, ranging from mild, respiratory symptoms to severe, potentially fatal pneumonia.

Blastomycosis is an endemic and reportable disease in Michigan with 186 cases reported between 2007-2017. Blastomycosis does not spread from person to person.

Meece has researched blastomycosis for more than 20 years, investigating outbreaks throughout the Midwest. She said it’s critical to launch a comprehensive investigation as soon as possible when clusters of potential blastomycosis cases are reported.

In addition to being the largest blastomycosis outbreak ever reported in the U.S., this was the first associated with a paper mill or other industrial worksite. The mill’s location along a riverway in a wooded environment is consistent with Blastomyces habitat. Blastomyces typically does not propagate indoors; the spores potentially entered the building through unfiltered ventilation systems, open doors, and caused the exposure over several months.

Meece and her team of researchers were asked to join the Escanaba outbreak investigation based on their past experience researching public health outbreaks. Their previous blastomycosis work included looking at people who have certain racial and ethnic backgrounds and may need more aggressive treatment and closer monitoring.

Marshfield Clinic Research Institute and the Wisconsin State Laboratory of Hygiene analyzed environmental samples for Blastomyces by polymerase chain reaction and culture for identification. Environmental samples collected included soil, wood chips, indoor surface dust, and water, dust, duct lining and filters from HVAC systems.

Among 533 indoor and outdoor samples analyzed, no Blastomyces was detected and the specific environmental factors in or around the mill that led to this outbreak remain unknown.

“This was not an unexpected outcome, as Blastomyces is an elusive organism,” Meece said. “It is rare, if not unheard of, that the exact source of a blastomycosis outbreak has ever been detected in the environment.”

While many people who recreate in areas where blastomycosis is endemic are aware of the disease, this first-time outbreak at an industrial business opens new potential risks for exposure. In conjunction with worker training and education, health care providers and public health authorities should consider work-related exposure to Blastomyces to detect future outbreaks and implement public health interventions quickly, Meece said.

For more information on fungal diseases, go to https://www.cdc.gov/fungal/index.html.