THAT TIME FORGOT
The weird life cycle of swimmer's itch
By CARY SHIMEK
Pure and pristine though they seem, western Montana's lakes and rivers sometimes offer an itchy surprise.
Bill Granath Jr., a UM microbiologist, says from mid-July to the end of August is the peak time to contract swimmer's itch: an irritating rash caused by an aquatic parasite. The itch, also called schistosome dermatitis, afflicts people in waterways around the world and is especially prevalent in the Upper Midwest. In Montana, Flathead Lake is a prime location to catch swimmer's itch.
"It's horribly itchy and takes about seven to 10 days to clear up," Granath says. "It affects everybody differently, but some people think it's worse than poison ivy. And there is no cure."
Granath did a study on Flathead Lake itch in the 1990s. He and his fellow researchers learned that the parasitic flatworm — Trichobilharzia ocellata — has a convoluted life cycle before it ends up as a series of red bumps on some unfortunate swimmer.
In Flathead Lake, he says, the natural hosts of the parasite are common merganser ducks and pea-sized snails called Stagnicola elrodi. Seagulls and Canada geese sometimes are hosts as well.
The adult flatworms are about a quarter inch long and customarily start their life cycle in the intestinal veins of mergansers. Granath says these parasites are an unusual species of flatworm because they have separate sexes. (Most flatworm species have both male and female sex organs.) The male is leaf-shaped, and the female cigar-shaped. Both have two suckers to affix to their host. These worms normally are found with the male wrapped in a canoe shape around the female, and they copulate for life. Because they reside in veins and feed on blood, they are often called blood flukes.
After they mate, the female lays spiny eggs, each containing a larval stage. These eggs then are passed out of the merganser duck in feces. When the eggs hit water they hatch, and microscopic critters called miracidia swim forth using hairlike cilia to propel themselves. A miracidium has one goal: to find a certain species of snail before it dies in a few hours.
If the parasite finds the right snail, it penetrates into the flesh and begins reproducing asexually. After six or seven weeks, the parasite becomes a cercaria — a sperm-shaped creature with a forked tail. The cercaria then exits the snail, departing directly from the flesh, and begins swimming around looking for a nice cozy merganser to call home. Granath says cercariae are tiny, right at the edge of human visibility.
the proper duck is found, the cercaria bores into the bird's
flesh, losing its no-longer-needed tail in the process. The
creature then migrates to the merganser's intestinal veins
where it can reach adulthood and start the cycle anew. Granath
says the ducks and snails are seldom harmed by this process
and probably don't realize they are hosts.
There is no cure for swimmer's itch, but Granath did offer some tips to help avoid the parasite:
Granath says cortisone creams work for those who contract the itch. Extreme reactions may require a prescription cream. People who catch schistosome dermatitis start noticing itchy red bumps about 12 hours after they have been in the water.
"People can enjoy the water and still stay itch free," he says, "but they should watch where they go swimming."