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Swimmer's Itch

Swimmer's itch (sometimes called schistosome dermatitis or cercarial dermatitis or sedge pool itch) is a skin irritation caused by immatures (cercariae) of certain species of nonhuman blood flukes (parasitic flatworms of the Phylum Platyhelminthes, Class Trematoda) belonging to the famly Schistosomatidae.

The cercariae can penetrate the outer layer of skin of people who are wading, swimming, or floating on inner tubes in infested waters. During skin penetration, an itching or stinging sensation may develop and persist for up to one hour. A skin eruption may occur at the site of penetration and a more diffuse reddish rash may occur during this phase. After penetration, the cercariae die, causing an allergic reaction characterized by intense itching and the formation of papule irritations or small blisters in sensitized individuals. The condition usually subsides after several days to one week. However, problems can persist for more than a week if secondary infection occurs.

The small (less than 0.04 inches in length) cercariae which penetrate human skin die because people are not suitable hosts. However, various birds and small mammals are suitable hosts for these flukes. If cercariae successfully penetrate a suitable host (i.e. duck), they enter the circulatory system, where they grow and become adult schistosomes. These adults are wormlike and usually less than 0.5 inches in length. The adult female may mate and produce eggs which are passed with the duck's feces. If the droppings fall into the water, then the egg hatches into a larval form called a miracidium. The free-swimming miracidium can only survive if it penetrates an aquatic snail (the intermediate host). Miracidia give rise to sporocysts (in the snail tissue) which can produce cercariae. The cercariae burrow out of the snail and swim away. If the cercariae contact a warm-blooded animal, they penetrate the skin. If the animal is a suitable host, the life cycle is continued.

If medical personnel have not identified cercariae in the skin of individuals who have developed swimmer's itch symptoms after being in lake or pond water, other possible causes of skin irritation might be considered. Some species of algae reportedly can irritate the skin but this has not been identified as a problem in Lake County. In other areas of the United States (i.e. Louisiana), roundworms (nematodes) in the water can penetrate human skin and cause severe inflammation and itching (sometimes called "nutria itch"). These roundworms probably do not occur in Clear Lake. Backswimmers (Notonectidae), toe-biters (Belostomatidae), and some other aquatic insects which can inflict painful bites on swimmers occur in some ponds and lakes in Lake County. These insect bites can cause acute temporary pain, but swimmer's itch cercariae can cause more prolonged allergic reactions.


  • Do not feed ducks or geese near swimming areas. Increased numbers of waterfowl in an area can result in increased swimmer's itch problems.
  • Do not swim in areas frequented by large numbers of ducks or geese.
  • Avoid swimming in areas where recent swimmer's itch problems have been reported.
  • Swim in relatively deep water. The cercariae are usually found in shallow waters and most easily attach to inactive bodies. Babies sitting along the shoreline are especially vulnerable to swimmer's itch.
  • Swim in open water areas and do not approach areas infested with aquatic weeds and snails.
  • Upon exiting the water, rubbing the skin with a rough dry towel may dislodge cercariae.
  • Swim for short periods of time (i.e. 10 minutes or less). If the skin becomes soft due to excessive time in the water, it may be easier for cercariae to penetrate.
  • If skin develops symptoms of swimmer's itch, antiseptic anesthetic lotions or ointments may provide some relief.
  • Keep affected skin clean and do not scrach to avoid the possibility of secondary infection.
  • If swimmer's itch symptoms persist, consult your family physician.
  • Be aware that schistosome dermatitis cannot be transferred to other parts of the body or to other people by skin contact.