Cercarial dermatitis, also known as swimmer’s itch, is an itchy rash caused by a tiny parasitic worm. It’s contracted by swimming or wading in infested fresh water lakes or ponds. The parasite’s usual hosts are waterfowl and rodents. After the parasite is excreted from the waterfowl or rodent, it then enters a snail. After further development, it leaves the snail and comes into contact with human skin. It can’t enter your bloodstream or deeper tissues, but it can cause an uncomfortable, itchy rash when it burrows into your skin.
The rash begins to itch and appears while one is still in the water. After a few hours, the itching and rash disappears. However, about 10–15 hours after the initial rash the papules and itch return. The rash appears as small, itchy red bumps that can turn into blisters. It usually clears up within a week.
If you swim or wade in water that is infested with the parasite, you may experience:
- tingling, burning, or itching on your exposed skin
- small, red pimples
- blisters, on rare occasions
The itching can last up to several days. The rash is only located on skin that was exposed to the water. It’s important to try not to scratch, as scratching can lead to skin infection.
The itch is an allergic reaction to the parasite, and the more frequently you swim in contaminated waters, the worse your symptoms may be each time. Also, some people may be more sensitive to the parasite.
Cercarial dermatitis is caused by a parasitic worm that burrows into your skin. The parasite can’t live in humans. It accidently attaches to you and other humans while it’s looking for a water bird or another animal host. Its normal life cycle is from waterfowl or water animal to snail and back again. You just happen to be there swimming or wading when the parasite, in its infective stage, is searching for its natural host.
In its life cycle, the parasitic worm infects the blood of waterfowl and some animals that live near the water. The birds or animals then pass the eggs of the parasite into the water through their feces. When the parasite’s eggs hatch in the water, the larvae swim around trying to find their second host, a species of snail. After the parasite infects the snail and develops in it, the snail excretes a second form of parasitic larvae into the water. These tiny larvae, known as cercariae, swim around looking for waterfowl or water animals to begin the cycle again.
These larvae live only for about 24 hours and hunt for a proper host to continue their life cycle. If you swim or wade in infested water, some of these parasitic worms may mistakenly land on your skin.
Cercarial dermatitis may be hard to distinguish from other skin reactions, such as insect bites, poison ivy, jellyfish stings, or bacterial infections. There is no specific test for it. Your doctor may ask you questions to help make a diagnosis. Their questions may include:
- When did the rash start?
- Have you been swimming or wading recently in fresh water?
- Did other people who were in the water with you get a rash?
The doctor may also ask about your medical history, any allergies you may have, and medications and supplements you take. If your itching is severe, the doctor may prescribe something stronger than over-the-counter remedies.
Most of the time, you can use home treatments to calm the itch from swimmer’s rash. The Centers for Disease Control and Prevention list several remedies for itch relief, including:
- anti-itch lotion or corticosteroid cream
- cool compresses
- baths with colloidal oatmeal, baking soda, or Epsom salts
- baking soda paste
You’re at risk of getting swimmer’s itch if you swim in a body of fresh water that’s infested. Swimming in shallow water near the shoreline also puts you at a higher risk because this is where the larvae are most likely to be. Children may be particularly at risk because they wade and play near the shore. If you’ve had swimmer’s itch before, you’re likely to have a worse reaction the next time you’re exposed.
It’s important to note that well maintained, chlorinated pools are safe to swim in and won’t pose a risk of catching swimmer’s itch.
Swimmer's itch also isn’t contagious.
Swimmer’s itch is a common summer ailment around the world. There are few statistics kept on its occurrence. Based on reports in the last decade, a 2015 review article calls swimmer’s itch an emerging disease, responsible for the majority of dermatitis outbreaks worldwide.
Most of the time, the rash clears up on its own without complications. After the rash is gone, you may have a pigmented spot where the pimple was for a few weeks.
How long the water remains infective depends on many factors:
- There are almost 100 species that can carry the parasite.
- There are also different species of the parasite itself, each with specific attributes.
- Waterfowl are migratory and may pick up the parasite around the world.
- The parasite’s life cycle depends on having both infected birds or animals and snails present.
- Temperature and sunlight also play a role in the parasite’s life cycle.
The 2015 review article in Clinical Microbial Reviews suggests that more research is needed to help in control measures. Knowledge of the particular species involved, for example, will help target control efforts. The authors also suggest that a warming climate may tend to increase the incidence of swimmer’s rash.
Control efforts are not easy. It’s possible to treat waterfowl with an anti-worm drug to reduce the parasite population. This requires capturing, treating, and releasing the birds individually. Eliminating or reducing the waterbird population from recreational areas is difficult. It’s also possible to use poison to reduce snail populations, but this was reported to have a damaging effect on other animals.
While it may be difficult to control the prevalence of the parasites in certain bodies of water, swimmer’s itch is not a serious medical condition.
When you’re planning a beach outing to fresh water, you should avoid swimming or wading in places where you know people have gotten swimmer’s itch.