Swimmer's itch, also known as "cercarial dermatitis," is a rash caused by the swimming larvae of certain types of flatworms, called schistosomes, burrowing into a person's skin. The larvae are actually looking for birds, and end up in humans by mistake.

A schistosome's normal life cycle starts with eggs hatching out in the water into a small, free-swimming stage called the miracidium. A miracidium can swim for about a day, and in that time it has to penetrate into a particular type of snail either through its mouth or its skin. Inside the snail, the miracidium transforms into a reproductive sac called a sporocyst, which in turn produces other sporocysts, which release another swimming stage called a cercaria. The cercaria burrows out of the snail and goes looking for a bird, which it enters either by being eaten or by penetrating the bird's skin, eventually making its way into the bird's blood vessels, and usually ending up in the blood vessels that surround the intestine. There it matures into an adult and mates with another worm. The females lay eggs that enter the bird's intestines and pass into the water with the feces, to start the life cycle over.

One species of schistosome that causes swimmer's itch, Austrobilharzia variglandis, has been found in San Francisco Bay. Its cercaria form in theAtlantic mudsnail Ilyanassa obsoleta, an Atlantic Coast native that was first collected on the Pacific Coast in San Francisco Bay in 1907. The snail probably arrived with shipments of Atlantic oysters (Crassostrea virginica) that were planted in large numbers in San Francisco Bay to grow to market size.

Austrobilharzia variglandis occurs in the North Atlantic on both the North American and European coasts, and could have been introduced into San Francisco Bay along with the mudsnail. It was first collected in San Francisco Bay in 1955, after many cases of dermatitis were reported in bathers on the Alameda shore in 1954. Austrobilharzia variglandis was identified in 3% of the Ilyanassa obsoleta collected that were greater than 25 mm in height, in 1% of the 20-24 mm snails, and in none of the smaller snails.

We know of no other cases of swimmer's itch in San Francisco Bay until a recent outbreak at Crown Beach in Alameda. In June 2005, over 90 people, primarily schoolchildren, reported rashes after wading in the water. The culprit this time seems to be a schistosome in the genus Gigantobilharzia, carried by a recently arrived Japanese bubble snail, Haminoea japonica

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