BACKGROUND: Blastomycosis, an uncommon fungal disease caused by Blastomyces dermatitidis, is an endemic and reportable disease in Wisconsin. Infection develops after inhalation of airborne fungal spores from the environment. On 7/31/2015, the Wisconsin Division of Public Health (DPH) was informed of a suspected cluster of blastomycosis cases among individuals who participated in recreational tubing on the Little Wolf River (LWR) on 6/17/2015. During initial interviews, many participants reported stopping at wooded river islands during their trip. An investigation was initiated to determine outbreak magnitude, features, risk factors, and control measures.
METHODS: Case-patients and members of their tubing cohort(s) were interviewed using a standardized investigation questionnaire to obtain clinical signs/symptoms, relevant medical history, tubing trip details, soil disruption behavior during trips, and other exposures and risk factors unrelated to the LWR. Outbreak cases were classified using a modified version of the DPH blastomycosis case definition. A retrospective case-control study was conducted using interview data. Soil samples were collected from exposure locations and tested for Blastomyces by PCR. Available clinical isolates of Blastomyces from patient specimens were characterized using Sanger sequencing of the ITS2_19 gene.
RESULTS: Ninety cases of blastomycosis (51 confirmed, 39 probable) were identified among 54 tubing cohorts. Exposures occurred during 5/23/2015–8/1/2015; illness onsets occurred during 6/22/2015–9/13/2015. Median case-patient age was 23 (range 5–48) years. Incubation intervals ranged from 14–74 (median 33) days. All 10 available Blastomyces clinical isolates were characterized as B. gilchristii. Among participants in one tubing trip, disturbing soil or brush was associated with developing blastomycosis (OR 2.63, 95% CI 1.23–5.61). Among 13 case-patients who neither stopped during their trip nor disturbed soil, 12 tubed during one busy June weekend. On 8/6/2015, health advisory signage discouraging soil disruption was posted along the tubing route. No additional cases were identified with exposure occurring after signage was posted. B. gilchristii was detected in one soil sample collected from an island.
CONCLUSIONS: This outbreak of blastomycosis, the largest reported, was caused by B. gilchristii, a cryptic species within B. dermatitidis. Behaviors resulting in soil disruption remain associated with increased risk of blastomycosis in endemic regions. This outbreak further supports that exposure to aerosolized spores while in the proximity of soil disturbing activities causes disease. Posting health advisory signage should be considered in areas of demonstrated risk to reduce exposure. Incubation periods as short as 14 days should be considered consistent with blastomycosis, based on known exposure and onset dates.