"Swimmer's Itch" and Gravel Pits in Fairbanks

If you itch, as I do, and have been swimming recently in waterfilled gravel pits in Fairbanks, "Cheer-Up", you have probably been parasitized! The itch-producing organism is an Alaskan, non-human schistosome, classed as a Trematode and a member of the Platyhelminthes, or Flat Worms, Phylum of the Animal Kingdom.

This schistosome, or blood fluke, is normally parasitic in wild ducks, where it is to be found in the blood vessels near the small intestine. Eggs laid by the female schistosome drop out with the bird's excrement. In a watery environment, ciliated embryos, termed miracidia, hatch from these eggs and attack and penetrate freshwater snails. The snails, such as Lymnaea stagnalis and palustris, serve as intermediate hosts. In the snail, the miracidia undergo morphological and reproductive changes which result in the production of forked-tailed larvae, or cercariae. These cercariae leave the snails in July and August and swim as free-living organisms in the water of the gravel pits. They are seeking out another aquatic bird to start the cycle over again.

Schistosome dermatitis, or "swimmer's itch", is caused by the penetration into human skin of the 0.5 mm long cercariae. This penetration produces a prickling sensation, and small red spots of pinpoint to pinhead size may develop. These may disappear in a few hours, but they may persist until replaced by papules, or pimples, in some 10-15 hours. These papules vary in size from 1 to 5 mm in diameter. Intense itching accompanies development of the papules. These may be surrounded by an area of erythema, or reddened skin. The rash may persist for a week. The reaction apparently depends upon individual sensitivity. But, don't worry, once the cercariae finds out that it has not penetrated a bird, it will soon expire.

The major danger of "swimmer's itch" is probably secondary infection caused by breaking through the skin while scratching with fingernails. Go see your friendly doctor for palliative treatment of the itch. If you persist in swimming in the gravel pits, towel-dry vigorously after coming out of the water, especially under your bathing suit; you may be able to reduce the number of forked-tailed "pasties" on your skin that way!