Blastomyces

Blastomycosis is a fungal disease caused by inhalation of Blastomyces spores from the environment. Most blastomycosis infections in the US are caused by Blastomyces dermatiditis or Blastomyces gilchristii. They are dimorphic fungi that are endemic throughout the Midwestern, South-central, and Southeastern United States. It prefers moist soil and decomposing wood and leaves especially near lakes and rivers. Blastomyces produces fungal spores that are released into the environment and can be inhaled by animals and humans. Activities that disturb soil or plant matter increase the risk of inhalation. Examples include excavations, construction, and some outdoor sports. 

Between November 1, 2022 and May 15, 2023, workers at a paper mill in Michigan experienced the largest blastomycosis outbreak in US history. A total of 162 of the 645 mill workers developed respiratory symptoms including cough, shortness of breath, fever, and fatigue. Eighteen workers (12%) were hospitalized and one patient died. Urine samples tested positive for Blastomyces antigen. The mill’s location along a river in a wooded area was consistent with Blastomyces’ habitat, but the specific source of the infection was not determined.

At body temperature, spores mature into yeast. Symptoms usually occur within 3 weeks to 3 months following inhalation. As many as 50% of people infected with Blastomyces remain asymptomatic. The other 50% develop symptoms including fever, cough, shortness of breath, night sweats, muscle and joint aches, fatigue, and weight loss. People who are immunocompromised  can develop severe, potentially fatal pneumonia. In some cases, the yeast enters the blood and is disseminated to skin, bone, joints, and central nervous system. In these organs, Blastomcyes produces ulcers and granulomas. An estimated 8 to 10% of patients hospitalized with blastomycosis in the US die each year. 

Diagnosis of blastomycosis can be made by culture, antigen detection, and serologic evaluation. Culture is the gold standard for diagnosis but takes several weeks to grow. Urine enzyme immunoassays to detect Blastomyces antigens have the highest sensitivity and the quickest turnaround time. Antibody testing by immunodiffusion has low sensitivity and specificity. 

References

Schwartz IS, Kauffman CA. Blastomycosis. Semin Respir Crit Care Med 2020;41:31–41

Linder KA, Kauffman CA, Miceli MH. Blastomycosis: a review of mycological and clinical aspects. J Fungi (Basel) 2023;9:117

Harvey RR, O’Connor AW, Stanton ML, et al. Outbreak of Blastomycosis Among Paper Mill Workers — Michigan, November 2022–May 2023. MMWR Morb Mortal Wkly Rep 2025;73:1157–1162.