Blastomycosis, caused by the dimorphic fungus Blastomyces, is a potentially severe disease of humans and animals. Blastomyces favors moist and organically rich soil, and soil disruption can lead to aerosolization and inhalation of infectious Blastomyces conidia (1,2). Most blastomycosis cases in the United States are sporadic and occur in the midwestern, south-central, and southeastern states. Infrequently, clusters of blastomycosis are reported because of common occupational and recreational activities near waterways (3).

In January 2022, St. Croix County Public Health, the Wisconsin Department of Health Services, and the Centers for Disease Control and Prevention (CDC) were notified of a cluster of human and canine blastomycosis cases near the Willow River in St. Croix County, Wisconsin (4). A clinical veterinarian reported 4 canine cases living within a radius of a few miles; 2 cases among persons living in the same area were reported to public health. The multiagency team investigated this cluster to identify risk factors for blastomycosis and to encourage persons in blastomycosis-endemic areas with consistent symptoms to seek healthcare.

The population at risk was defined as persons living within a 1.5-mile diameter of where the human and canine cases occurred. Members of the investigation team administered questionnaires to consenting households (Appendix 1), and collected information on exposures, symptoms, and risk factors for blastomycosis occurring during September 2021–March 2022.

Respondents were eligible to participate in serologic testing; pet owners were offered serologic testing for their dogs. We collected 5–10 mL of venous blood; serum samples were sent to MiraVista Diagnostics (Indianapolis, IN, USA) for Blastomyces spp. IgG testing by enzyme immunoassays (EIA) for human and canine serum samples and by immunodiffusion assay for human serum samples (5). This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.

We defined human cases of blastomycosis among neighborhood residents who had illness onset during September 2021–March 2022. Included residents met clinical case criteria according to the Council of State and Territorial Epidemiologists case definition and met either confirmatory laboratory criteria or presumptive laboratory criteria, including a positive Blastomyces antigen test or a detection of serum antibody through immunodiffusion or EIA (6). Canine cases were dogs that exhibited clinical signs and either had received a diagnosis of blastomycosis from a veterinarian during September 2021–March 2022 or had detectable Blastomyces spp. antibodies.

We described the demographics, recreational exposure history, symptoms, and any symptom-related healthcare visits of persons by clinical case status. We compared recreational exposures between those with antibodies detected (positive or intermediate results) with exposures of those without antibodies detected. Prevalence ratios (PRs) and 95% CIs were calculated using robust Poisson models. All analyses were completed using R version 4.1.3 (The R Project for Statistical Computing, https://www.r-project.org).

Figure

Figure. Flowchart of enrollment of households into investigation of human and canine blastomycosis cases associated with riverside neighborhood, Wisconsin, USA, December 2021–March 2022. Enrolled refers to a household in which ...

We interviewed members of 60 households, 46 (77%) of whom participated in serologic testing (Figure). The median length of time lived in the neighborhood at time of interview was 4.9 years (range 1 month–56.5 years) (Table 1). The median distance to the in-neighborhood river was 342 (range 1.7–791.0) meters.

During our investigation, 3 previously unreported human cases and 2 additional canine cases of blastomycosis were identified clinically after public health notification to local veterinarians, clinicians, and residents. A total of 5 human and 6 canine cases were included in this investigation (Appendix 2 Tables 1, 2).

The median age among human case-patients was 54 years, and most (60%, n = 3) had >1 comorbidity. All persons reported cough, fever, and fatigue associated with their illness. The initial 2 reported patients were hospitalized, and 1 died (4) (Appendix 2 Table 1). Isolates from the 2 initial cases were sent to the CDC Mycotic Diseases Branch laboratory for species identification, which determined the species to be Blastomyces gilchristii. In the canine cases, 5 of 6 (83%) were in sporting breeds; less than half of participating dogs were sporting breeds (n = 26, 42%). All dogs with a blastomycosis diagnosis were lethargic and anorexic; most had difficulty breathing (83%, n = 5) and cough and fever (67%, n = 4) (Appendix 2 Table 2). Human and canine case exposures varied.

We performed serologic testing on 89 (61%) of 147 human participants and 51 (79%) of 65 dogs (by EIA for dogs and immunodiffusion and EIA for humans). A total of 87 persons consented to further testing; their serum samples were later tested by a newly approved EIA. Among persons tested for antibodies, 1 person had detectable Blastomyces antibodies detected by immunodiffusion (also detected by EIA), and 44 (51%) persons had IgG detected by EIA (Table 2). Demographics and recreational exposures did not vary by antibody status, but male participants had 1.5 (95% CI 1.0–2.3) times the prevalence of antibodies than did female participants. Persons who reported a recreational activity involving the in-neighborhood river had 1.5 (95% CI 0.9–2.4) times greater prevalence of detectable antibodies, but was not statistically significant.

Among the 51 dogs tested, 8 (16%) had detectable antibodies, of which 6 (75%) had a clinical diagnosis. Higher number of years lived in the neighborhood was associated with decreased antibody prevalence (PR 0.32; 95% CI 0.1–0.9). Dogs that walked on the trails (PR 10.0; 95% CI 1.3–75.4), specifically those that walked off-leash (PR 6.8; 95% CI 2.0–23.9) had a higher prevalence of antibodies compared with dogs who did not (Table 3). Dogs who lived closer to the river had higher prevalence of antibodies (PR 0.98; 95% CI 0.97–0.99).

We investigated a Wisconsin cluster of blastomycosis in which 5 human cases, resulting in 2 hospitalizations and 1 death, and 6 canine cases were identified. Using a multidisciplinary One Health approach to trigger notification and investigation of this cluster expanded both the investigative team and the outreach and effects among residents, pet owners, veterinarians, construction workers, and healthcare providers. The elevated awareness likely led to additional residents and dogs identified with blastomycosis illness and exposures.

The serological survey revealed nearly half of residents had Blastomyces antibodies detected by EIA. 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 (5,7). Although low sensitivity of immunodiffusion limits its use for detecting antibodies (8), this study used an EIA targeting the BAD-1 antigen with increased sensitivity.

Although we were unable to identify a definitive exposure as the likely cause of this cluster, several sites such as the riverbank, riverside trails or yards, or construction dust, could have plausibly been sources of Blastomyces spores. Although not statistically significant, antibody prevalence was increased in persons who reported participating in activities along the river, as did dogs who walked on in-neighborhood trails. 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. Construction and excavation have been implicated in previous clusters of blastomycosis (9–11).

Top

Suggested citation for this article: Segaloff HE, Wu K, Williams SL, Shaw S, Miko S, Parnell LA, et al. Human and canine blastomycosis cases associated with riverside neighborhood, Wisconsin, USA, December 2021–March 2022. Emerg Infect Dis. 2024 Dec [date cited]. https://doi.org/10.3201/eid3012.240390