Norovirus is the "most likely" explanation for 88 athletes becoming unwell after the UK leg of the World Triathlon Championship series in Sunderland, the UK Health Security Agency North East said.
Data published by the Environment Agency had shown a reduced water quality in the Roker Beach area at the time of the event while elevated levels of E.coli were also reported outside the swim area.
However, tests conducted in the immediate swim area for British Triathlon rated the quality of bathing water as excellent.
Preliminary investigations from the UK Health Security Agency North East (UKHSA North East) are yet to detect any E.coli associated with severe illness from 31 samples received, the agency said.
Norovirus was detected in 60% of the samples making it the "most likely" explanation but it may not be possible to determine the exact result of the infections as investigations continue.
Around 2,000 people took part in the triathlon which took place in Sunderland for the first time over the weekend of 29 and 30 July.
After the event, 88 people contacted the North East Health Protection Team to report experiencing diarrhoea and/or vomiting symptoms after the event. 31 faecal samples were then sent to be tested for a range of viral and bacterial pathogens, the UKHSA said.
The agency said 19 showed evidence of Norovirus infection while the remaining samples either tested negative or were positive for other infections including sapovirus, astrovirus and rotavirus.
E.coli was present in four samples but not the kind associated with severe illness. It can be carried naturally in the gut of healthy individuals, so it is not possible to say whether the presence is a result of participation in the event.
An in-depth investigation is still on-going and anyone who has not yet contacted British Triathlon to report symptoms after the event is still encouraged to do so.
Participants are being updated on their lab results as and when they are available, and a fuller outbreak report will be made available once all investigations have been undertaken.
Dr Kirsty Foster, consultant in health protection said: “We thank everyone who took the time to contact British Triathlon to undertake sampling and respond to our epidemiological investigations and I hope that today’s preliminary findings will offer some reassurance.
“Norovirus is a very unpleasant stomach bug but tends to pass after a few days with most people usually making a complete recovery without any specific treatment. Rest and drinking lots of fluids are important to avoid dehydration. Norovirus can easily spread from person to person, particularly in large groups. The risk to the wider public remains very low.
“Many people enjoy open swimming and the considerable social and wellbeing benefits it brings. However, it is important to remember that anyone can become unwell from swimming in open water as there will always be micro-organisms present.
"There are a number of ways people can reduce their risk of illness though and these are detailed in the Swim Healthy guidelines.”
A statement from Sunderland City Council read: "UKHSA's North East Health Protection team have identified Norovirus as the most likely cause in their preliminary findings into a number of participants reporting feeling unwell after the World Triathlon Championship Series event.
"We are sorry to hear that a number of people have been unwell but we hope these findings reassure those who live in and visit our area that our award-winning blue flag beaches are safe to swim from and are there to be enjoyed by all, during the summer months and beyond.
"British Triathlon carried out water quality testing in the swim area in the two years leading up to the World Triathlon Series event, as well as in the build-up to and over the race weekend, to ensure that water in the area where participants swam was safe to do so. Test results passed the required guidelines for hosting a World Triathlon Championship Series event."
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