What is Swimmer's Itch?
Swimmer's itch is a temporary skin rash that is caused by an allergic reaction to microscopic parasites that are carried by waterfowl, semi-aquatic mammals, and snails. As a part of their life cycle, these parasites are released by infected snails into the water, where they may come in contact with people and burrow into their skin.
The good news is the organism that causes swimmer's itch cannot complete its life history in the human body. Your body's immune system detects it as a foreign protein, then attacks and kills it shortly after it penetrates your skin. The itching and welts are not caused by the organism living under your skin, but by an allergic reaction.
Not everyone is sensitive to swimmer's itch. Some people show no symptoms of swimmer's itch even though others swimming at the same time and place break out severely. Much like poison ivy, however, your sensitivity to swimmer's itch will increase with each exposure. There are several over the counter remedies your pharmacist can recommend to help relieve the discomfort, but see your physician for a definitive diagnosis.
Read more about Swimmer's Itch from the Minnesota Department of Health
Where does it come from?
The organism that causes swimmer's itch has a complicated life history.
- It starts out as a worm (parasite) in the intestinal lining of aquatic birds and mammals (host).
- The worms lay eggs inside the host animal.
- The eggs are eliminated by the host and drop to the bottom of the lake.
- Then the eggs hatch into miracidia that swim around in the water until they find a snail.
- Once they find a snail, they live inside them and develop into cercariae.
- The snails then release the cercariae back into the water where they look for another host - aquatic birds, mammals or humans.
- When the cercariae find a potential host, they burrow into the host's skin.
- If the host is suitable, the life cycle starts all over again.
- Humans are not a suitable host and the cercariae die after penetrating the skin.
Swimmer’s itch is not spread from person to person. It is found throughout the world, and in many ways, it is similar to other naturally occurring nuisances in the outdoors that cause allergic reactions - things like poison ivy.
Reducing the odds of getting swimmer's itch
The parasite that causes swimmer’s itch only lives for a day or so and typically swim around in the upper few inches of lake water. This increases their chances of coming into contact with a duck. Once the parasite is in the duck, it easily moves around the lake, and ultimately along the shoreline.
There are actions you can take to help reduce your odds of getting swimmer's itch.
- Keep waterfowl away from your dock and shoreline. If you are feeding waterfowl (ducks and geese) from your dock, stop. If ducks like to rest on your dock, do what you can to discourage them. You can try putting an owl windsock or statue on your dock and move it around occasionally, so the ducks don't become accustomed to it.
- Stay out of the water by the shore. The swimmer's itch organism may originate somewhere else in the lake and is being brought to your shoreline by wave action or currents. You may want to consider swimming from a raft or boat farther out from shore where you are less likely to come into contact with the parasite that causes swimmer’s itch. Of course, this strategy may not be practical if you don't swim or have young children who want to play in the water near shore.
- Apply a water repellant substance such as petroleum jelly, waterproof sunscreen or other skin oils to reduce the ability of the parasite that causes swimmer’s itch from penetrating the skin.
- Rash guard clothing: Wearing protective swimwear has been shown to create a physical barrier against parasite entry.
- Dry off with a towel as soon as you get out of the water. When you get out of the lake, don't let the water evaporate off your skin. The organism in the droplets of water on your skin will look for somewhere to go as the droplet of water evaporates.
- Timing of swimming: Concentrations of the parasite that can cause swimmer’s itch are typically higher in the morning and in warm, shallow water. Swimming in deeper water and later in the day reduces risk.
- Consult with your doctor on potential use of oral antihistamines to reduce or eliminate allergic response to exposure.
Waterfowl and their role in swimmer’s itch
In Minnesota, several species of waterfowl are hosts that sustain the parasite lifecycle. These include common mergansers, mallards, wood ducks and Canada geese.
All waterfowl species contribute to ecosystem biodiversity. For example, common mergansers serve as top-level aquatic predators, helping control populations of small fish and maintaining balanced lake ecosystems.
Removal of these birds may unintentionally disrupt food webs, potentially resulting in overabundant forage fish or increased pressure from invasive species. Widespread adoption of waterfowl removal could threaten regional populations, undermining broader conservation goals.
The science is not conclusive that removing waterfowl is an effective way to minimize swimmer’s itch. Research findings suggest removal does not eliminate swimmer’s itch unless all potential hosts are removed. Even brief presence of non-resident (migratory) birds can introduce or maintain parasite levels sufficient to cause swimmer’s itch, reducing the long-term effectiveness of local brood removal.
However, there is scientific consensus that personal preventative methods (such as those listed above) are effective for people who are concerned about exposure to the swimmer’s itch.