Outbreak of Blastomycosis Among Paper Mill Workers — Michigan, November 2022–May 2023

Abstract

Blastomycosis is a fungal disease caused by inhalation of Blastomyces spores from the environment that can result in severe pulmonary illness and high hospitalization rates. In early March 2023, Public Health Delta and Menominee Counties (Michigan) reported a cluster of blastomycosis cases among paper mill workers to the Michigan Department of Health and Human Services (MDHHS). MDHHS subsequently notified CDC. On March 17, paper mill management requested a health hazard evaluation (HHE) from CDC’s National Institute for Occupational Safety and Health (NIOSH) to investigate potential workplace exposures to Blastomyces and recommend prevention and control measures at the mill. The workplace epidemiologic investigation combined a NIOSH HHE medical survey consisting of a questionnaire on work and health with Blastomyces urine antigen testing of specimens obtained from workers to assist in case finding, with additional case information from MDHHS blastomycosis surveillance data. Assessment of 645 mill workers identified 162 cases of blastomycosis with illness onset during November 1, 2022–May 15, 2023, with the weekly case count peaking at 21 cases in early March 2023. HHE environmental sampling in and around the mill did not identify the source of workers’ Blastomyces exposure in the mill. This outbreak was the largest documented blastomycosis outbreak in the United States, and the first associated with a paper mill or an industrial setting. A coordinated public health response facilitated swift prevention measures with recommendations focused on reducing workers’ exposure to Blastomyces, including hazard communication, respiratory protection, mill cleaning, and ventilation system improvements.

Investigation and Results

Public Health Notification and Response

On February 28, 2023, Public Health Delta and Menominee Counties (PHDM) in Michigan was notified of a cluster of atypical pneumonia cases among workers at a local paper mill in Delta County, Michigan; all patients experienced onset of respiratory symptoms during January–February 2023, and urine antigen testing of patient specimens was positive for Blastomyces. PHDM reported the blastomycosis cases to the Michigan Department of Health and Human Services (MDHHS). On March 6, 2023, PHDM notified area health care providers to be alert for additional cases. Within the week, MDHHS notified CDC’s Mycotic Diseases Branch (MDB) and National Institute for Occupational Safety and Health (NIOSH) of eight blastomycosis cases with an additional 14 under investigation; all cases occurred among persons who worked at or visited the paper mill (workers) before illness onset. PHDM, MDHHS, MDB, and NIOSH, in conjunction with mill management and workers, initiated outbreak response and prevention measures.

On March 17, 2023, paper mill management requested a NIOSH health hazard evaluation (HHE) to investigate potential Blastomyces exposures and recommend prevention and control measures. NIOSH coordinated four HHE field activities: 1) an initial site visit during March 27–28; 2) an environmental survey and ventilation assessment at the mill during April 24–28; 3) a medical survey, which included a work and health questionnaire and Blastomyces urine antigen testing to identify potential cases during April 22–28; and 4) a follow-up environmental survey during August 1–2. This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.^†

Epidemiologic Investigation

All paper mill employees, contractors, and visitors were invited to participate in the NIOSH medical survey. Among approximately 1,000 workers, 603 participated in the medical survey. Data from an additional 42 mill workers who received a diagnosis of blastomycosis but did not participate in the NIOSH HHE were included in the analysis, coordinated through a data use agreement between NIOSH and MDHHS and voluntary worker consent to share HHE information with public health authorities, resulting in a total of 645 participants. An outbreak case was defined as confirmatory or presumptive^§ laboratory evidence of blastomycosis or self-reported^¶ health care provider–diagnosed blastomycosis in a person who worked at or visited the mill during October 1, 2022–July 1, 2023.** Among the 645 workers included in the epidemiologic investigation, 162 (25%) persons with blastomycosis were identified with illness onset during November 1, 2022–May 15, 2023; during the week of February 27–March 5, 2023, the number of new onset cases peaked at 21 (Figure 1). In May 2023, the last case was identified, and on July 1, 2023, the outbreak was declared over. As of April 2024, no additional cases were identified among mill workers. During October 1, 2022–July 1, 2023, one blastomycosis case without an association to the mill was identified in Delta County. Based on 120 blastomycosis cases identified among 603 NIOSH medical survey participants, estimated blastomycosis case prevalence among mill workers was 20%.

Worker Characteristics

A majority of workers were men (83%), White (94%), and non-Hispanic (98%), with a median age of 46 years (range = 19–73 years) (Table). Workers with blastomycosis were younger with a shorter tenure at the mill compared with workers without blastomycosis. Among the 645 workers included in the epidemiologic investigation, 162 workers received a diagnosis of blastomycosis and frequently reported signs and symptoms, including cough (90%), shortness of breath (76%), fatigue (76%), and fever or chills (73%); 63% of patients had abnormal lung findings on chest imaging. Among 483 workers without blastomycosis, 294 (61%) reported cough. Eighteen (12%) workers with blastomycosis were hospitalized, and one patient died. Among 573 workers from the NIOSH medical survey with Blastomyces urine antigen results, 52 (9%) received a positive test result, 26 (50%) of whom did not report receiving a diagnosis of blastomycosis before the medical survey, including three workers (6%) who reported no signs or symptoms. Based on workers’ primary work locations, paper mill areas with the highest blastomycosis case counts were paper machine line #1 (35) and maintenance (22), although workers in all areas of the mill were affected, including the administrative offices (17) (Figure 2).

Environmental Assessment

The paper mill is located adjacent to a river and occupies 2,200 acres, including the indoor mill buildings, outdoor woodyards, water treatment lagoons, and landfill. Mill buildings contained approximately 400 heating, ventilation, and air conditioning (HVAC) systems. The ventilation assessment involved visual assessment of the air handling units and makeup air units (these units supply fresh air from outside the building into the occupied space). Among the 67 units assessed, 59 (88%) were makeup air units providing unfiltered outdoor air into the mill; many had dirt and organic debris observed in the coils.

The Marshfield Clinic Research Institute (https://marshfieldresearch.org/) and the University of Wisconsin-Madison analyzed environmental samples for Blastomyces by both polymerase chain reaction (PCR) and culture for identification. Environmental samples from the mill collected during the HHE 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. Among 533 indoor and outdoor environmental samples analyzed, no Blastomyces was detected by PCR or culture.

Public Health Response

Beginning April 17, 2023, paper mill management voluntarily idled production for 3 weeks, for ventilation ductwork cleaning and upgrading filters in all air handling units. During April 20–21, NIOSH, MDB, PHDM, and MDHHS staff members conducted 10 sessions for workers to provide information about blastomycosis and the NIOSH HHE. Mill managers facilitated training for workers regarding potential hazards and associated safe practices, procedures, and protective measures to reduce Blastomyces exposure and encouraged workers to report potential blastomycosis symptoms to health care providers. During the outbreak, NIOSH recommended that workers wear fit-tested NIOSH-approved N95 respirators,^†† especially those workers at risk for severe disease (e.g., persons who were immunosuppressed) or performing potentially high-risk activities (e.g., changing HVAC filters or disturbing soil). NIOSH communicated HHE findings and recommendations to paper mill managers, workers, and public health partners through routine conference calls and interim letters.

Discussion

Blastomycosis is a rare infectious disease caused by inhalation of Blastomyces fungal spores from the environment. Pulmonary illness is most common, ranging from mild, self-limited respiratory symptoms to severe, potentially fatal pneumonia; approximately 25% of cases include extrapulmonary disease, often involving skin lesions (1). As many as one half of people infected with Blastomyces remain asymptomatic (2). Diagnostic and treatment delays are common because of clinical similarities with other respiratory infections, although antifungal medication is important to prevent progression or recurrence of disease in symptomatic patients (1). Blastomyces is found in moist soil or decaying wood and leaves in the midwestern and southeastern United States, often near rivers and lakes. This outbreak was the largest documented blastomycosis outbreak ever reported in the United States. Although work-related blastomycosis has been reported in industries such as farming, construction, and landscaping (3), this report investigates the first recognized blastomycosis outbreak associated with a paper mill or other industrial worksite. The mill’s location along a riverway in a wooded area is consistent with Blastomyces’ habitat^§§; however, the specific environmental factors in or around the mill that led to this outbreak remain unknown.

Blastomyces typically does not propagate indoors (4); however, spores potentially infiltrated mill buildings through unfiltered ventilation systems or open bay doors, leading to indoor fungal exposure, likely over several months. During a 2019 outbreak in Wisconsin, Blastomyces was detected by PCR from indoor air samples (5). Because of the challenges in identifying Blastomyces from the environment (1), 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 (6), 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.

Blastomycosis is an endemic and reportable disease in Michigan, with 186 cases reported during 2007–2017 (mean annual incidence = 0.2 cases per 100,000 population) (7). In Delta County, fewer than one blastomycosis case was reported annually; the single non-mill–related case identified during the workplace outbreak was consistent with previous surveillance. Blastomycosis might be underreported in Delta County; some northern counties in neighboring Wisconsin with similar environments have an annual reported incidence exceeding 20 cases per 100,000 population (7). Urine antigen testing in the NIOSH medical survey was useful for identifying workers with potentially undiagnosed blastomycosis. Case finding through HHE using urine antigen testing identified three asymptomatic patients. In addition, approximately one half of workers with positive urine test results did not report blastomycosis diagnoses, indicating that urine antigen testing helped to identify cases not included in MDHHS surveillance data.

Twelve percent of workers with blastomycosis were hospitalized in this outbreak; previous reports indicate that approximately 65% of reported patients with blastomycosis require hospitalization (8). Early outbreak detection enabled active case finding and directed public health messages, urging symptomatic workers to seek medical care promptly. 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. Conditions associated with paper milling, including generation of and exposure to paper dust (9), indoor dampness, and fungi other than Blastomyces (10) might also have contributed to respiratory symptoms among mill workers.

Challenges of the investigation included the difficulties identifying Blastomyces from environmental samples (1) and the timing of the NIOSH HHE medical survey, particularly the urine antigen screening, given the variable incubation period for blastomycosis (6). In addition, most workers who self-reported blastomycosis had been prescribed antifungal medication from their health care provider, which could have affected urine antigen test results from the NIOSH medical survey. Paper mill managers acted quickly after outbreak recognition to engage public health authorities and implement prevention measures. Rapid and coordinated public health actions by local, state, and federal public health authorities and information sharing among partners enabled a swift response to protect mill workers.

Implications for Public Health Practice

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. Although not specific to Blastomyces, routine HVAC maintenance, including changing filters, fan belts, and other parts, along with cleaning the system when necessary and employing housekeeping best practices might help reduce the likelihood of Blastomyces propagating indoors in areas where the disease is endemic. 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.