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Epidemiol Infect. 2017 Apr;145(6):1246-1255. doi: 10.1017/S0950268816003393. Epub 2017 Feb 6.

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

Author information

UK Field Epidemiology Training Programme,Public Health England,London,UK.
South West London Health Protection Team,Public Health England,London,UK.
European Programme for Intervention Epidemiology and Training (EPIET),European Centre for Disease Control,Stockholm,Sweden.
London Public Health Laboratory,Public Health England,London,UK.
Field Epidemiology Service - London,Public Health England,London,UK.


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.


Community epidemics; gastrointestinal infections; water (quality); waterborne infections

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