Also called "clamdigger's itch," swimmer's itch is caused by skin contact with cercariae, which are the immature larval forms of parasitic schistosomes (flatworms) found throughout the world in both fresh and salt waters. Snails and birds are the intermediate hosts for the flatworms. They release hundreds of fork-tailed microscopic cercariae into the water.
The affliction is contracted when a film of cercariae-infested water dries on exposed (uncovered by clothing) skin. The cercariae penetrate the outer layer of the skin, where itching is noted within minutes. Shortly afterwards, the skin becomes reddened and swollen, with an intense rash and, occasionally, hives. Blisters may develop over the next 24 to 48 hours.
Untreated, the affliction is limited to 1 to 2 weeks. Persons who have suffered swimmer's itch previously may be more severely affected on repeated exposures, which suggests that an allergic response may be a factor.
Swimmer's itch can be prevented by briskly rubbing the skin with a towel immediately after leaving the water, to prevent the cercariae from having time to penetrate the skin. Once the reaction has occurred, the skin should be lightly rinsed with isopropyl (rubbing) alcohol and then coated with calamine lotion. If the reaction is severe, the injured person may be treated with oral prednisone.
Because the cercariae are present in greatest concentration in shallow, warmer water (where the snails are), swimmers should try to avoid these areas.
For more information on marine life injuries, see the complete article by Paul S. Auerbach, M.D., M.S. on Marine Life Trauma from the Jan/Feb 1998 issue of Alert Diver.