Triathletes were most likely struck down by a severe case of norovirus after swimming in the North Sea coast off Sunderland, experts said.
But the exact source of the infection that led to the horror outbreak of sickness and diarrhoea among scores of swimmers may never be known.
Norovirus was detected in samples that were given by 33 of the 88 competitors in the British Triathlon-organised event last month.
But it remains unclear whether the illness was spread by swimming in the sea, or by another means.
The UK Health Security Agency (UKHSA) said tests were carried out for a range of viral and bacterial pathogens that can cause sickness and diarrhoea, with 19 of the 33 – 60% – showing evidence of norovirus infection.
The UKHSA said the remainder were positive for other infections including sapovirus, astrovirus and rotavirus.
E.coli levels were found to be high off the coast at the time, leading to complaints that the swimming event went ahead.
Australian triathlete Jake Birthwistle, who competed in the event, posted the Environment Agency’s results on Instagram and said he had felt ill after the event.
He said: “Have been feeling pretty rubbish since the race, but I guess that’s what you get when you swim in s***. The swim should have been cancelled.”
But a UKHSA spokesperson warned “we may never know the exact sourceW of infection.
They said: “The predominance of norovirus makes it the most likely explanation of illness in participants.
“Norovirus is very easily transmitted through contact with people with the infection and any food, water, surfaces or objects that have been contaminated with the virus.
“It is possible for norovirus infection to occur following exposure to contaminated water, but again it has not been possible to identify the likely cause of infection from investigations to date and there are other scenarios in which infection could also have been spread.”
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.”
She added: “Many people enjoy open swimming and the considerable social and wellbeing benefits it brings.
The British Standard Distance Championships kicked off day two of AJ Bell 2023 World Triathlon Championship Series Sunderland yesterday 🏊🚴🏃
— British Triathlon (@BritTri) July 31, 2023
Well done to everyone who took part and congratulations to the medal winners 🏅
Read more ➡️ https://t.co/VpkQCF1trv#Triathlon
“However it is important to remember that anyone can become unwell from swimming in open water as there will always be micro-organisms present.”
About 2,000 people took part in the UK leg of the World Triathlon Championship series in Sunderland, which included a swim off the city’s Roker Beach.
Environment Agency sampling at the beach on July 26 detected 39 times the amount of E.coli found in the water during typical readings.
E.coli is a bacterial infection that can cause stomach pain and bloody diarrhoea.
