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2003
RIVER
RESCUERS
UM'S NEW RIVERINE SCIENCE CENTER
POISONED
HEARTS
ARSENIC EFFECTS ON CARDIOVASCULAR HEALTH
WATER
WIZARDRY
BIO STATION'S NEW FLOOD PLAIN MODEL
RIVERS
THAT TIME FORGOT
AN UNDISTURBED RUSSIAN WILDERNESS
SODIUM
SOLUTIONS
COALBED METHANE IN EASTERN MONTANA
HEAVY
METAL
SCIENTISTS STUDY IMPACT OF METALS ON MICROBES
COTTONWOOD
CONUNDRUM
MONTANA'S DISAPPEARING RIVER TREES
WATER
WARDENS
UM'S WATERSHED HEALTH CLINIC
TALLYING
TADPOLES
STUDYING MONTANA AMPHIBIANS
FISH
FINDERS
DNA IN WATER REVEALS LOCATION OF FISH
WET
AND WILD
A PRIMER ON MONTANA AQUATIC LIFE
WATER
THAT WAS
THE SECRETS OF GLACIAL LAKE MISSOULA
NEWS
TO USE
THE WEIRD LIFE CYCLE OF SWIMMER'S ITCH
BACKTALK
GIARDIA: A WATER DRINKER'S GUT-WRENCHING SURPRISE
The weird life cycle of swimmer's itch
By CARY SHIMEK
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| The aquatic creature responsible for swimmer's itch. |
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.
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| Microbiologist Bill Granath Jr. |
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.
If
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.
However, cercariae can cause problems for humans. If the parasite
bores into wet human skin in cercaria form, it soon recognizes
it's not in a duck and dies. Penetration enzymes and all sorts
of waste products then are released, causing intense inflammation
and itching to the accidental host after about 12 hours. Each red
bump contains one dead cercaria.
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| Host snails carry the parasite. |
There is no cure for swimmer's itch, but Granath did offer some tips to help avoid the parasite:
- Don't swim in July and August. This is a hard sell, but Granath says these two months are when the parasite swims around in its cercaria stage and can impact humans.
- Stay in deep, cold water. Snails, one of the parasite's primary hosts, prefer warm, shallow areas with vegetation.
- Wear a coating of baby oil or waterproof sunscreen. Any barrier on the skin may prevent cercariae from burrowing in ... at least until it washes off.
- Towel
off promptly and thoroughly after being in the water. Often
the parasites
can be rubbed off. Slowly evaporating water on skin prompts cercariae to burrow in. Especially watch areas that trap moisture such as the belt line on swimming trunks.
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."