A quiet pond, ducks paddling near the shore, children throwing them bread and then wading into the water themselves. A few hours later, their legs are covered in itchy red spots that look like midge bites. It's not mosquitoes or an allergy to the water: the culprits are tiny parasite larvae whose main hosts are ducks.
Every summer, the same photos appear on social media: legs and thighs covered in little red spots, with the caption "went for a swim in the lake". In June 2026, a woman from the Sverdlovsk Region came out in blisters after swimming in Lake Turgoyak, one of the most popular lakes in the Chelyabinsk Region.
And in July, Rospotrebnadzor, Russia's consumer health watchdog, again warned of the risk of what's known as cercarial dermatitis, which is recorded every year in various regions of Russia. It's popularly called swimmer's itch. We look at where it comes from and why ducks on a beach are a reason to be wary.
What Swimmer's Itch Is
Swimmer's itch, or cercarial dermatitis, is a skin reaction to the larvae of flatworms that parasitise waterfowl. The larvae are called cercariae: tiny creatures with a forked tail that can't be seen with the naked eye.
Cercariae search the water for their host: a duck, gull or swan. But sometimes they mistakenly burrow into human skin.
"In this case, a human is a dead-end host: it can't keep the cercariae alive," explains Dmitry Molodoy, an emergency doctor and paediatrician.
The larva dies in the skin, and the body responds with inflammation and itching.
What Ducks and Snails Have to Do With It
The parasite has a complex life cycle, and ducks are its key link. The adult worms live inside waterfowl: ducks, gulls and swans. Their eggs end up in the water along with the birds' droppings.
Larvae hatch from the eggs and penetrate freshwater snails, which become an intermediate host. After four to six weeks, an infected snail starts releasing cercariae into the water. From there, the larvae look for a bird, or a swimmer who happens to be nearby.
That's why the danger lies specifically in water that has both birds and snails. The more ducks there are, the more parasites in the water.
Where the Risk Is Highest
"The riskiest thing is trying to cool off in shallow ponds with muddy bottoms, stagnant water and lots of ducks," says Dmitry Molodoy.
Parasitologist Eduard Bartuli explains why still water is more dangerous than flowing water: in rivers the water is constantly renewed and parasites don't build up, while in ponds and lakes without a current, a persistent source of infection develops.
Cercariae stay mostly in the upper layer of water, to a depth of 30–40 cm, and on aquatic plants. There are most of them in overgrown shallows, where the snails live. That's exactly where children usually splash about and where adults who don't dare go deeper stand. Rospotrebnadzor advises staying in areas deeper than one metre.
When the Danger Is Greatest
Infections peak in July and August, when the water warms up well, to around 20–25°C. Cercariae are most active in warm water, and according to sanitary services, they live for up to three days.
A peculiarity of swimmer's itch is that it can get worse. On first contact, the reaction may be mild or not noticeable at all, but on repeated swims in the same water the body responds with a full-blown allergic reaction. So if your legs have already itched after swimming in a particular pond, it's best not to go in there a second time.
How to Tell It's Cercarial Dermatitis
The rash appears a few hours after swimming and looks like midge bites: red spots or blisters that itch badly. The legs, thighs and shins, the parts that were in the water longest, are most often affected.
The rash usually clears up by itself within one to two weeks, although for some people the symptoms last up to three weeks.
"The rash goes away by itself, without any special treatment and without any consequences for the body," says Dmitry Molodoy.
Don't scratch it: the scratches can become infected. If the rash is very heavy, comes with a fever or doesn't go away, see a doctor.
How to Avoid Getting It
- Swim at equipped beaches
The water there is tested, and it's best not to go into untested ponds with ducks. Back in 2022, Rospotrebnadzor's Moscow office advised against going into water where ducks swim, as these birds are often infected with parasites.
- Don't feed ducks near the beach
Bread thrown from the shore draws birds together in one place, and the parasites with them. So don't swim where people are feeding waterfowl.
- Avoid overgrown shallows
Reeds, waterweed near the shore and a muddy bottom are snails' favourite places. If you want a swim, get through the shallows quickly and swim where it's deeper.
- Don't stay in the water for long
During swimmer's itch season, sanitary doctors advise not staying in the water for more than 5–10 minutes at a time.
- Rinse off and dry yourself straight away
After swimming, take a shower or rinse off with clean water, then rub your skin briskly with a rough towel, especially your shins and thighs. Don't dry off in the sun in wet swimwear.
- Keep an eye on children
They spend the longest time sitting in the shallows by the shore, and are more likely than anyone to come home with a rash.
