CDC Issues Final Report in Blastomycosis Outbreak

Following the largest documented outbreak in the country, clinicians should consider a diagnosis of blastomycosis in work-related respiratory disease outbreaks in areas where the fungal disease is endemic, a CDC group advised.

Investigators detailed an outbreak of blastomycosis — characterized by cough, shortness of breath, fatigue, and fever or chills — that occurred among workers at a paper mill in Delta County, Michigan, from November 2022 to May 2023. Among the 645 workers studied, 162 received a blastomycosis diagnosis, of whom 12% were hospitalized, and one died.

A search for the source of the Blastomyces exposure was conducted with 533 indoor and outdoor environmental samples taken from in and around the paper mill, located in a wooded area next to a river.

The source was never found, according to researchers led by R. Reid Harvey, DVM, of the CDC’s National Institute for Occupational Safety and Health (NIOSH), in Morbidity & Mortality Weekly Report.

However, “because of the challenges in identifying Blastomyces from the environment, the lack of positive samples from the mill does not rule out the presence of Blastomyces. Given the 2-week to 3-month incubation period for blastomycosis, Blastomyces exposure at the mill likely began as early as mid-October 2022 and extended through at least February 2023, and as late as April 2023,” Harvey’s group wrote.

Blastomycosis is caused by Blastomyces fungal spores found in the environment. In the U.S., the spores are found in moist soil or decaying wood and leaves in the Midwest and Southeast, often near rivers and lakes, the researchers noted. The rare disease is often initially misdiagnosed, which can contribute to severe pulmonary illness.

The authors suggested that the Michigan paper mill outbreak has implications for public health.

For one, “work-related exposure to Blastomyces might be considered by health care providers and public health authorities in these areas to detect future outbreaks early and implement public health interventions quickly,” Harvey and colleagues wrote.

“Industries and occupations that routinely perform outdoor work activities such as disturbing soil in areas with endemic blastomycosis might consider providing worker training and education to enhance awareness of Blastomyces and reduce the likelihood of work-related blastomycosis among workers,” they added.

Moreover, they stressed the routine maintenance of heating, ventilation, and air conditioning (HVAC) components, which “might help reduce the likelihood of Blastomyces propagating indoors in areas where the disease is endemic.”

The CDC group reported that at the Michigan paper mill with about 1,000 workers, 603 had agreed to participate in a medical survey conducted by NIOSH. An additional 42 workers received a blastomycosis diagnosis but did not agree to participate in the survey, giving investigators a total of 645 workers for analysis.

Most workers were male (83%), white (94%), and non-Hispanic (98%), with a median age of 46 years.

Of the 483 who did not receive a blastomycosis diagnosis, 61% reported having a cough. Workers with blastomycosis tended to be younger and have a shorter tenure at the mill compared with those without the disease.

The NIOSH medical survey included 573 workers who had received a Blastomyces urine antigen test. Of those, 9% received a positive result, half of whom hadn’t been diagnosed with the disease before the medical survey; that latter group included three workers who reported no signs or symptoms, the researchers wrote.

“Despite a relatively healthy workforce, respiratory symptoms, particularly cough, were common in workers without blastomycosis and were potentially attributed to other respiratory illnesses, during the 6-month exposure window that spanned winter,” the investigators said. “Conditions associated with paper milling, including generation of and exposure to paper dust, indoor dampness, and fungi other than Blastomyces might also have contributed to respiratory symptoms among mill workers.”

Public health authorities and the plant’s management responded to the outbreak in several ways.

Beginning April 17, 2023, paper mill management voluntarily idled production for 3 weeks to clean ventilation ductwork and upgrade filters in air handling units, the researchers reported. During April 20-21, public health workers conducted sessions for workers to provide information about blastomycosis. Also, during the outbreak, NIOSH recommended that workers wear fit-tested, NIOSH-approved N95 respirators, especially those workers at risk for severe disease — such as immunocompromised employees — or those who were performing potentially high-risk activities like changing HVAC filters.

The Marshfield Clinic Research Institute and the University of Wisconsin-Madison analyzed environmental samples for Blastomyces by both polymerase chain reaction and culture for identification. Environmental samples from the mill collected during the evaluation included soil, wood chips, indoor surface dust, and water, dust, duct lining, and filters from the HVAC systems.

Additional environmental samples were collected during August 1-2, 2023, while excavation activities took place for a bridge being constructed over the river near the mill.

In addition to the challenge of finding the spores in the environment, a major limitation of the investigation was the timing of the NIOSH medical survey — particularly the urine antigen screening, given the variable incubation period for blastomycosis, Harvey and colleagues cautioned.

Also, most workers who self-reported blastomycosis had been prescribed antifungal medication from their healthcare provider, which could have affected urine antigen test results from the NIOSH survey.