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A large outbreak of gastrointestinal illness at an open-water swimming event in the River Thames, London

  • V. HALL (a1) (a2) (a3), A. TAYE (a4), B. WALSH (a4), H. MAGUIRE (a2) (a3) (a5), J. DAVE (a6), A. WRIGHT (a3), C. ANDERSON (a3) and P. CROOK (a3)...

Summary

Open-water swimming is increasingly popular, often in water not considered safe for bathing. Limited evidence exists on the associated health risks. We investigated gastrointestinal illness in 1100 swimmers in a River Thames event in London, UK, to describe the outbreak and identify risk factors. We conducted a retrospective cohort study. Our case definition was swimmers with any: diarrhoea, vomiting, abdominal cramps lasting ⩾48 h, nausea lasting ⩾48 h, with onset within 9 days after the event. We used an online survey to collect information on symptoms, demographics, pre- and post-swim behaviours and open-water experience. We tested associations using robust Poisson regression. We followed up case microbiological results. Survey response was 61%, and attack rate 53% (338 cases). Median incubation period was 34 h and median symptom duration 4 days. Five cases had confirmed microbiological diagnoses (four Giardia, one Cryptosporidium). Wearing a wetsuit [adjusted relative risk (aRR) 6·96, 95% confidence interval (CI) 1·04–46·72] and swallowing water (aRR 1·42, 95% CI 1·03–1·97) were risk factors. Recent river-swimming (aRR 0·78, 95% CI 0·67–0·92) and age >40 years (aRR 0·83, 95% CI 0·70–0·98) were protective. Action to reduce risk of illness in future events is recommended, including clarification of oversight arrangements for future swims to ensure appropriate risk assessment and advice is provided.

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Copyright

Corresponding author

*Author for correspondence: Ms. V. Hall, c/o Paul Crook, Field Epidemiology Service – London, Public Health England, Skipton House, 80 London Road, London SE1 6LH, UK. (Email: victoria.hall4@nhs.net)

References

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A large outbreak of gastrointestinal illness at an open-water swimming event in the River Thames, London

  • V. HALL (a1) (a2) (a3), A. TAYE (a4), B. WALSH (a4), H. MAGUIRE (a2) (a3) (a5), J. DAVE (a6), A. WRIGHT (a3), C. ANDERSON (a3) and P. CROOK (a3)...

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