Background Mass gatherings at major sport events such as Olympic Games pose unique health risks because having the large number of people in a small space can aid the spread of infectious diseases. That can also pose risks for athletes’ health and can endanger their performance. Water quality testing of the fields of play (FOP) in Tokyo are showing potential environmental problems (temperature and water pollution) that could endanger the health and safety of the athletes. It is necessary to develop adequate preventive and mitigation measures that would be used in Tokyo Olympics. This research aims to provide international and local authorities with evidence-base for such an intervention.

Hypothesis Athletes of open water sports are more prone to gastrointestinal infections (TD) due to the exposure of polluted water on the FOP. This hypothesis has been tested by investigating data obtained from the survey and identifying the influence of contact with the water of the FOP on athletes’ health.

Design Cross sectional study.

Setting Pre-Olympic test event Ready Steady Tokyo 2019 (17–22 August)

Patients (or Participants) athletes (sailing and triathlon) and their accompanying teams on shore.

Main Outcome Measurements Surveys among athletes in contact with the water and their accompanying teams on shore. The whole population defined in investigation was approached. Anonymous TD questionnaire was distributed in hard copy and collected on the last day of the event. The online version was available for one month after the event for participants that omitted to fulfill it on site. Data were processed using MedCalc statistical software. The incidence of diarrhea, its duration, the number of stools, impact on performance was recorded.

Results The study presents the epidemiology and the impact of TD to athletes participating at Pre-Olympic test event Ready Steady Tokyo 2019.

Conclusions Conclusions on risk are made and compared with the data from Rio 2016 Olympics.