What is Swimmer's Itch?
Swimmer's Itch is a transient rash on the uncovered skin of fresh water bathers and swimmers. It is caused when cercariae, a larval form of animal flat worms
(schistosomes), penetrates the epidermal skin layer. After penetration, these parasites remain and die, thus causing an allergic
reaction (in some people). The embedded parasite does not mature, reproduce or create permanent infection. These parasites ordinarily infect snails, semi-aquatic
mammals and birds.
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Will Swimmer's Itch spread to other parts of the body or to other people?
No! It can not spread. A papule only forms where a cercaria has entered the skin. Additional papules can form from continual exposure to more cercariae.
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When does the first outbreak of Swimmer's Itch occur? Will the water be unsafe? What are some of the contibuting factors?
The first outbreak typically occurs in late May or early June during the first warm period in the spring. The water is not unsafe. Many variables
must be present before it becomes a problem. Since these factors change from year to year, it is difficult to gauge the severity of an outbreak.
Larvae are usually infective for 24 hours once they are released from snails into the water. An infected snail will continue to produce cercariae throughout
its lifespan. For future snails to become infected, migratory birds or mammals must also be infected. At this time, there is no way to determine how long the
outbreak will last. It can occur once or even persist for the entire summer. Contributing factors include, but are not limited to:
- Water Currents
- Wind Direction
- Time of Day
- Number and Distribution of snail populations (that can serve as hosts)
- Number and Distribution of migratory birds (that can serve as hosts)
- An Individual's sensitivity to the larvae
- Number of Hours that swimmers remain in the water
Please note: These factors are subject to change on an annual basis.
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What is the lifecycle of Swimmer's Itch?
The causative agent for swimmer's itch is the larval, free-living and aquatic stage of a group of flatworms that are called schistosomes. Most of species that
cause swimmer's itch use bird hosts for the adult parasite and aquatic snails as intermediate hosts for the larval stages. The life history of the dermatitis-producing
worms is cyclic. Eggs released from the adult worms that reside in the blood vessels (usually veins around the intestine) of the bird host, make their way into the
digestive tract of the bird and then pass out of the host with the feces. If the eggs are deposited in water, they will hatch within an hour if conditions are right.
The miracidium, an aquatic stage, is free-swimming, but non-feeding. It has enough energy to keep it moving for about a day. Once the miracidium comes in contact with
the proper snail it will either penetrate into the snail via the integument or it may enter through its mouth. Within the snail, the miracidium will elongate to form
a reproductive sac called the sporocyst. This germinating structure will produce a second generation of sporocysts. In approximately a month, the sporocysts produce
another stage, called the cercaria. This stage burrows out of the snail, becomes a second type of non-feeding, swimming aquatic stage that must enter the bird host.
It does this either by penetrating the skin of the bird or by being ingested and then entering the blood vessels in the walls of the pharynx or esophagus. In the bird
host, the parasite migrates through various organs of the bird and finally matures in the blood vessels. The adult worms then begin producing large numbers of eggs
which again are voided with the feces. Avian schistosomes usually complete their life cycle in two months, however, the specific time varies with each species.
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How many hosts are there in this lifecycle of parasites? What are some of the common hosts to these organisms?
Two - always two. A snail intermediate and the vertebrate host. The parasites can only be transmitted from bird-to-snail or snail-to-bird.
It can NEVER occur in bird-to-bird or snail-to-snail patterns. Birds that are common hosts include: Ducks, Canadien Geese, Swans, Mallards, Mergansers, Grackles
and Red-Winged Blackbirds. Some semi-aquatic mammals that harbor adult parasites include Muskrats, Moles and Mice.
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Can I observe cercariae in the water?
No. It is impossible to see the larvae with the naked eye. They are transparent in color and about 1/80 of an inch in length. A microscope is necessary to study these larvae.
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How soon do the symptoms start? What are the symptoms of Swimmer's Itch? How long will symptoms last?
Symptoms may appear anywhere from 5 minutes to 2 hours. Some people experience a tingling or burning sensation. At each point of penetration, a raised, red spot
(called a papule) will develop on the skin. The papule may increase in size during the first 24-30 hours.
Itching may last as long as a week. In some people, itching may subside for 10 - 15 hours and then re-occur with a greater intensity.
Some individuals may develop blisters which may later become pustules. Symptoms typically disappear within a week.
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What groups of people are more likely to get Swimmer's Itch?
Young children have the greatest tendency to spend prolonged periods of time in
the water near the shore. Researchers contend that cercariae are concentrated
towards the shoreline. About 1/3 of the people who come in contact with the
parasites develop Swimmer's Itch. All people who swim or wade in infested water
may experience this itchy rash. Continual exposure increases the likelihood of
getting Swimmer's Itch; repeated contact with the parasites can increase a person's
allergic reaction. Elderly people and those with weakened immune systems may also be
considered vulnerable.
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Do I need to see my health care provider for treatments? What are the available treatments for Swimmer's Itch?
No - most cases do not require medical attention. When a rash develops, you can try applying the following items for relief:
- (Phenolized) Calamine Lotion
- Anti-itch Lotion, such as benedryl
- Colloidal oatmeal baths, such as Aveeno
- Topical creams, such as Ken-Tox, to reduce swelling
- Baking Soda Paste
- 0.5% Hydrocortisone or Corticosteroid Cream
- Cool Compresses
- Baking Soda baths
- Antihistaminic Cream
Avoid scratching. Scratching may cause the skin reaction to worsen and become infected. When itch is severe, your doctor may presribe lotions and creams
to lessen your symptoms. NOTE: When symptoms persist for more than 3 days, make an appointment with a dermatologist.
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If Swimmer's Itch occurs on a lake, does that mean that the lake is polluted?
No; the opposite is probably true. Natural lake conditions promote the diversity of species, including the birds and snails that are potential hosts for the
causative agents of swimmer's itch.
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How is the diagnosis confirmed?
Specific diagnosis is difficult. Skin biopsies are not always helpful. There is no widely available blood test that gives specific indication that cercariae
have caused the itching. The best confirmation of the cause is based on knowing from other people that Swimmer's Itch occurs in the place where one swam or
entered the water. Other diseases have been confused with Swimmers Itch include: impetigo (bacterial infection of the skin), chickenpox, poison ivy, herpes
or rare skin diseases.
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What does Swimmer's Itch look like? What do the cercariae look like?
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Can Swimmer's Itch be prevented in the future? What can be done to lower the risk?
No, it can not be prevented. However, it can be reduced and you can follow similar steps to lower your risk of exposure.
- Avoid swimming in areas where Swimmer's Itch is a known problem.
- Avoid swimming in areas where posted warning signs of unsafe water.
- Avoid swimming in areas where snails are commonly found.
- Avoid swimming for long periods in shallow areas.
- Avoid swimming in marshy areas.
- Applying waterproof suntan lotion or baby oil to your body prior to swimming.
- Do not feed birds to areas where people are swimming. Reasons for not feeding them include:
- It may propagate the occurrence of Swimmer's Itch.
- It may make the birds dependent on humans for survival.
- It may stimulate additional fecal deposits at the feeding location.
- Swim in water away from the shoreline.
- Avoid water areas where ducks and other birds tend to aggregate.
- Keep your distance from drainpipes leading to the water, especially after a storm or heavy rain.
- Wait 24 hours after a storm before swimming at the beach.
- Avoid cloudy or murky waters, as they be risky for bacteria.
- Encourage health officals and Pond Association board members to post signs when known outbreaks have occurred.
- (Briskly) Towel dry or shower immediately after leaving the water. Cercariae of some species only enter the skin upon water evaporation.
- Avoid placing rip-rap on your shoreline. This provides an excellent surface for some snail species to attach their eggs.
- Applying chemicals to kill off migrating snails.
- Reduce pond vegetation so that snails do not have an ideal environment to grow.
- Educating yourself and others.
- Having treatment remedies available (as needed).
- Regular monitoring and assessments of the lake and its water conditions.
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What other sources of information are available?
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Michigan Swimmer's Itch Literature