'Exceedingly rare' fungal infection sickens dozens at Michigan paper mill

illustration of Blastomyces fungal spores growing against blue background
An outbreak of fungal disease is affecting employees of a paper mill in Escanaba, Michigan. (Image credit: KATERYNA KON/SCIENCE PHOTO LIBRARY via Getty Images)

An "exceedingly rare" fungal infection has sickened at least 19 employees of a paper mill in Escanaba, Michigan, and more than 70 additional employees have "probable" cases of the disease, health officials reported. The exact source of the fungus has not yet been found.

Public Health Delta & Menominee Counties (PHDM) was first notified of the fungal outbreak in late February 2023, when the department was informed of 15 "atypical" pneumonia cases among employees of the Escanaba Billerud Paper Mill. These individuals' symptoms began in January and February and early tests suggested that their illnesses might be blastomycosis, an infection caused by a fungus in the genus Blastomyces

In the U.S., Blastomyces mainly grows in midwestern, south-central and southeastern states, where the fungus can typically be found in moist soil and decomposing organic matter, such as wood and leaves, according to the Centers for Disease Control and Prevention (CDC). In states where blastomycosis is reportable, each year there are about 1 to 2 cases reported per 100,000 people.

"These infections are exceedingly rare. On average over the past five years, only 26 cases have been reported for the entire State of Michigan annually," PHDM wrote in a statement published March 9. "However, the Upper Peninsula (UP) of Michigan is a known risk area for blastomycosis infection."

Related: A man's voice grew hoarse for no obvious reason. It turns out, he had fungus in his throat.

After PHDM learned of the initial cases of blastomycosis among paper mill employees, the department launched an investigation. And in late March, outbreak investigators from the CDC, National Institute for Occupational Safety and Health (NIOSH) and Michigan Department of Health and Human Services (MDHHS) performed an on-site inspection of the mill.

As of April 7, the investigators had identified 19 confirmed cases of blastomycosis. These individuals showed symptoms of the disease and their clinical specimens grew Blastomyces fungus when cultured in a lab. In addition to these confirmed cases, the team has identified 74 probable cases, meaning people who showed blastomycosis symptoms and tested positive for antigen — identifiable bits of fungus in their blood — or antibodies — immune molecules that latch onto the fungus. 

"Although the source of the infection has not been established, we continue to take this matter very seriously and are following recommendations from health and government officials and implementing numerous, proactive steps to protect the health and safety of our employees, contractors and visitors," Brian Peterson, Operations Vice President of the Escanaba Billerud Paper Mill, said in an April 7 update from PHDM. The statement did not note how or whether the mill's operations have been adjusted in light of the ongoing investigation. 

People can catch blastomycosis by breathing in Blastomyces spores from the air, but most people who breathe in the spores don't actually get sick, according to the CDC. Symptoms of the disease include fever, cough and muscle aches, and in rare cases, the infection can become severe and spread from the lungs to other organs, like the skin, bones and brain. Severe cases are most likely in people with weakened immune systems. 

Blastomycosis is treated with antifungal medications. "Depending on the severity of the infection and the person's immune status, the course of treatment can range from six months to one year," the CDC states

Nicoletta Lanese
Channel Editor, Health

Nicoletta Lanese is the health channel editor at Live Science and was previously a news editor and staff writer at the site. She holds a graduate certificate in science communication from UC Santa Cruz and degrees in neuroscience and dance from the University of Florida. Her work has appeared in The Scientist, Science News, the Mercury News, Mongabay and Stanford Medicine Magazine, among other outlets. Based in NYC, she also remains heavily involved in dance and performs in local choreographers' work.