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BACKGROUND: After >5,000 persons ran a weekend obstacle course mud race, hundreds of racers experienced a severe pruritic rash. Along with contacting the race organizers, media, and local health department, affected runners created a Facebook page. We investigated to determine the rash etiology.
METHODS: On Facebook, we reviewed rash photographs and race videos and posted messages asking affected racers to call the Illinois health department. We created an electronic survey of exposures and illness outcomes based on interviews with 50 of the >100 racers who called. We sent the survey to all registered racers and conducted 3 site visits to assess environmental exposures.
RESULTS: Of the 5,184 registered racers, 1,848 (36%) completed the survey. Of these, 1,341 (73%) reported rash (attack rate: 26% among registered racers). Respondents had a mean age of 32.9 years and lived in 41 states and 3 foreign countries. Among those with rash, 65% had unresolved lesions ~ 3 weeks after racing. Rash commonly affected the abdomen (97%), legs (95%), arms (92%), and chest (85%). Only 4% experienced facial rash. Respondents described their rashes as raised (98%) and linear (75%), with blisters (54%) and crusting (47%). Fewer than 5% reported fever, vomiting, or diarrhea. Rash symptoms caused 23% of affected racers to miss work or school and 57% to visit a physician. Of the 649 who reported a physician diagnosis, the majority received diagnoses of poison ivy/sumac/oak rash (57%) or contact dermatitis (16%).Video posted on Facebook revealed racers in mud-soaked clothing crawling on their abdomens through recently constructed mud pits, sparing only their faces. Approximately all respondents (97%) showered at race completion, but only 44% reported using soap. Site visits confirmed substantial poison ivy in areas where vegetation had been uprooted recently to create mud pits. No other unusual exposures were identified. Eight days after our initial Facebook request, we posted survey results and recommendations.
CONCLUSIONS: Survey findings (descriptions of rash locations and patterns, lack of systemic symptoms, and physician diagnoses), posted photographs and videos, and environmental findings indicated poison ivy contact dermatitis. The unusual severity likely resulted from massive exposure to uroshiol, the oil released when poison ivy plants are crushed, after it soaked into racers’ clothing and was not washed off for hours. Social media offered direct and rapid communication to those affected, which was crucial for this investigation because racers were highly dispersed geographically and seeking care from numerous providers.