Seagulls could be the reason swimmers leave the beach with diarrhea or nausea. Here is why

A seagull sits at the edge of the beach in Brigantine, N.J., on Wednesday, Sept. 18, 2024. (AP Photo/Wayne Parry)

A day spent at the beach is likely filled with the squawks of seagulls and the threat they may steal unsupervised food. But Canadian beachgoers may have more to blame them for than missing French fries: they could be the ones making them sick after going for a swim.

A new study, currently under peer review and led by professor Ian Young at Toronto Metropolitan University, found beachgoers who swallowed water that contained bacteria linked to seagull feces were more likely to develop acute gastrointestinal illness. Though, the study stresses the overall risk of getting sick from swimming at the beach remains low.

What launched this study was that, in Young’s view, there wasn’t a lot of good data available for Canadians specifically on what the actual risk is of getting sick from going swimming at a beach.

“So, we kind of looked into it a bit more and realized there hadn’t been any studies done on that in Canada for over 30 or 40 years,” Young said in an interview with CTV News Toronto.

“It was necessary to kind of look at this and see what are the real risks here, and do the current sort of guidelines we have about safe swimming, are they still valid or should we kind of revisit those.”

The three-year study surveyed 4,085 beachgoers at 10 beaches across Canada between 2023 and 2025. Four of those beaches were in Ontario — two in Toronto and two in the Niagara Region, but the study did not disclose which ones.

Researchers surveyed participants of all ages on the day of their visit, and would follow up with them a week later to determine whether they experienced any gastrointestinal symptoms, like diarrhea, vomiting, nausea or stomach cramps.

Roughly two-thirds (67.6 per cent) of participants responded to the follow-up survey, and of them only 73 people (2.6 per cent) reported developing an acute gastrointestinal illness after their day at the beach.

The risk of contracting an illness was higher among those who actually went for a swim, particularly among those who swallowed some water. The study found higher levels of E. coli and DNA markers from seagull feces were associated with greater illness risk among swimmers.

“In our study, we actually found that more adolescents had the highest rates of illness, around the 10 years to 19 years old group,” Young said, instead of younger children.

“We’re not quite sure why the reason for that is. It could be that maybe they’re engaging in more intense water play that maybe leads to them swallowing water.”

Young tells CTV News Toronto that most of the studies done in the past were conducted in the U.S. and examined how human sewage impacted beaches. This study, he says, may be the first that has shown a direct link between seagull contamination and illness.

“I think it’s a good news story overall, in terms of that the rate of illness is quite low and the level of human sewage markers that we found were also low, but it’s still interesting that the seagulls contamination can be linked to human illness,” Young said.

“We know that seagull feces can contain different microbes that could get you sick, so that’s sort of the hypothesis, is that if you have too much of that in the water, it’s probably not great for your risk of getting sick.”

The study reinforces that Canadian beaches are generally clean and safe to swim in, as researchers found low levels of fecal contamination, which could be a signal of the efficacy of a city or province’s water quality monitoring and beach management programs.

Young says Canadians can feel confident in the quality of the beach water at their local beach, but advises to monitor any health advisories that could be in place before they go for a dip.

How Toronto supervises its beaches

Toronto Public Health tells CTV News Toronto that it measures the E.coli levels at 10 supervised public beaches from the first weekend of June until Labour Day. During that time, TPH determines the water quality and the safety conditions for public swimming.

The health unit publishes these levels daily, on its Beach Water Quality website.

“When E. coli levels exceed the City beach water quality standard, this is indicated by a red icon (alongside the actual level) and a red flag is posted at the beach on the back of the lifeguard station,” TPH said in a statement. Toronto’s beach water quality standard is 100 E.coli per 100 millilitres of water.

While swimmers aren’t banned from swimming when there are high levels of E. coli in the water, TPH says they increase their risk of infection.