A large randomized controlled trial has found that swimming in lakes and rivers raises the risk of gastrointestinal illness and skin rashes compared with simply standing on the shore, according to a study published this week in the International Journal of Hygiene and Environmental Health. [1]

Researchers led by the University of East Anglia randomly assigned 2,368 volunteers at four freshwater sites in Hungary either to swim for 10 minutes, including three full head immersions, or to remain dry on the bank. [1] Swimmers were more than twice as likely to report skin ailments such as rashes or itching, a risk that held steady regardless of how contaminated the water tested — evidence, researchers say, that the irritation may come from naturally occurring organisms, including the parasites behind "swimmer's itch," rather than only fecal pollution. [1] For gastrointestinal illness, the risk tracked the water's bacterial load directly: each tenfold rise in E. coli concentration was tied to a 73% increase in GI-illness risk, and a comparable rise in indicator viruses called somatic coliphages was tied to a 45% increase. [2]

Overall illness rates were low — just over 2% of participants developed GI symptoms or skin problems, and most cases were mild. [1] But lead author Paul Hunter said the pattern held consistently across the four Hungarian sites and matched an earlier German study, suggesting the findings likely extend to UK and US waters as open-water swimming continues to grow in popularity. [2]

-- NORA WHITFIELD, Chicago