A 2021-2022 blastomycosis outbreak that sickened five people—one fatally—and six dogs in a Wisconsin neighborhood may have stemmed from sources such as a riverbank, riverside trails or yards, or dust from extensive construction and excavation, a Centers for Disease Control and Prevention (CDC)-led research team reported yesterday in Emerging Infectious Diseases.
Blastomycosis, which can lead to life-threatening disease in people and animals, is caused by the infectious Blastomyces fungus. Blastomyces thrives in moist, rich soil that, when disrupted, can lead to aerosolization of spores that can then be inhaled. Most US cases occur sporadically in the midwestern, south-central, and southeastern states, but clusters related to waterway-related occupational and recreational activities occasionally occur.
Half of people tested had antibodies
In January 2022, a veterinarian notified the CDC, St. Croix County Public Health, and the Wisconsin Department of Health Services of four dogs with blastomycosis living within a radius of a few miles near the Willow River in St. Croix County. Two infections in humans in the same area were also reported.
The county is in western Wisconsin, along the Minnesota border.
Investigators administered questionnaires to households located within a 1.5-mile diameter of the cluster and gathered information on exposures, symptoms, and risk factors occurring from September 2021 to March 2022. Of 60 households, 46 opted to participate in serologic testing for themselves and their dogs. The median distance from the households to the river was 342 meters (0.2 miles).
During the investigation, three more human cases and two additional canine blastomycosis cases were identified after public health officials notified veterinarians, clinicians, and residents. The median age of the human patients was 54 years, and all reported cough, fever, and fatigue. Two patients were hospitalized, and one died.
Infected dogs were lethargic and very thin, with five having difficulty breathing and four having cough and fever. The CDC identified the causative species as Blastomyces gilchristii.
Blastomyces source undetermined
Serologic testing was conducted in 89 of 147 human participants (61%) and 51 of 65 dogs (79%) with enzyme immunoassay (EIA) and immunodiffusion in the former and EIA in the latter. Of the 87 people who consented to further testing with a new EIA, one had Blastomyces antibodies, and 44 (51%) had immunoglobulin G antibodies.
Extensive neighborhood construction and excavation during the exposure period might have increased risk for exposure, because most recent home construction occurred close to the river.
"Although this result suggests widespread exposure, the length of time that Blastomyces antibodies remain detectable in the blood and the baseline antibody prevalence in the community are unknown," the investigators wrote.
Male participants had 1.5 times the antibodies than females. People who had lived in the neighborhood for more years had a lower prevalence of antibodies (prevalence ratio [PR], 0.32).
Eight of the 51 dogs (16%) tested positive for antibodies, 6 (75%) of whom had a blastomycosis diagnosis.
The origin of the Blastomyces spores wasn't determined, but the authors said the riverbank, riverside trails or yards, or construction dust could have been sources. "Extensive neighborhood construction and excavation during the exposure period might have increased risk for exposure, because most recent home construction occurred close to the river," they wrote. "Construction and excavation have been implicated in previous clusters of blastomycosis."
The researchers recommend that residents of blastomycosis-endemic areas—particularly those with weakened immune systems—avoid activities that disrupt dirt and leaf litter and, if they do, wear a well-fitted, high-quality face mask during these activities.
"Risk for blastomycosis cannot be eliminated, and symptoms of blastomycosis can be indistinguishable from other illnesses," they concluded. "Increasing disease awareness is critical to improve early identification and treatment of patients."
