Sometimes a sudden change in temperature can trigger a greater release of cercariae from the snails. For that reason the windward shore can be hit the hardest during an outbreak.Ĭertain days can be worse than others. Some species of these itch-causing parasites rise in the water column and become trapped in the surface tension where they can travel for miles due to wind and current. The weak-swimming larvae can live and penetrate the skin for up to 24 hours. The greatest release of cercariae for most species of parasites comes in the morning, soon after sunrise. Unfortunately, those who are will become more and more sensitive each time they are exposed. It appears only 30 to 40 percent of all swimmers are sensitive to the larvae. To date there is no easy way to know if the parasites are cycling in your lake, short of sending the neighbor’s kids out in the water as guinea pigs. The swimmer’s itch-causing cercariae are transparent and only about 0.7 mm (1/32 inch) long, so they are essentially undetectable with the naked eye. The free-swimming larvae, called cercariae (pronounced sir-care-e-e), penetrate the human skin, die and cause intense itching that can make life miserable for several days and ruin a family vacation.
It’s this second larval stage that causes all the trouble.Īlthough the new larvae’s intended target is another duck, where it can mature and begin the cycle all over again, human swimmers sometimes get in the way. That sporocyst becomes a veritable “itch factory,” producing upwards of 4,000 new larvae every day for the rest of the summer. Upon penetrating the snail the larva begins a month-long metamorphosis into what’s called a sporocyst. Given warm water and sunlight, a very small larva will hatch from each egg within an hour and swim in search of an intermediate host, a certain species of snail. Heavily infected birds can pass hundreds of eggs with each defecation. The microscopic eggs find their way into the digestive tract and exit the bird via the feces. (Biologists have identified at least a dozen such worm species that cause swimmer’s itch in the United States.)Īt maturity, the parasites mate and lay eggs. Parasites live on or in “hosts,” although they are not welcome “guests.” The adult worms ultimately responsible for swimmer’s itch enjoy life safely tucked away in the small blood vessels surrounding the intestines of an unsuspecting host – most likely a duck, goose or swan. But biologists have learned an enormous amount about these parasites in the last century and that knowledge is leading to new approaches for control. Even science has a tough time with that one. Parasites are another matter, especially those that cause swimmer’s itch. Even mosquitos, those bloodsucking little pests! Hard to believe, but biologists tell us mosquitos have a niche in the great web of life – as a food source for dragonflies, trout, bats and birds. There’s a purpose for most things in nature.
Similar results were obtained in the last few years on a large recreational lake in Maine. On beautiful Glen Lake in the heart of the Sleeping Bear Dunes National Lakeshore in northern Michigan, the infection rate of the parasite in the snail intermediate host was lowered twentyfold in the late 1980s, reducing the swimmer’s itch problem to a minor inconvenience at worst. The reason this works is that the waterfowl population on most lakes actually is very stable with resident birds. Of course, this approach required that all the ducks on a lake be trapped and administered an adequate dose of the drug. When a new parasite-killing drug came on the market in the early 1980s, Blankespoor reasoned that it might be better to inoculate all the ducks on the lake, instead of killing the snails that carried the parasite. Harvey Blankespoor, a world-renowned authority on itch-causing parasitic worms, developed a novel approach for swimmer’s itch control and has found success on recreational lakes in Michigan and Maine. Increasing environmental concerns about this heavy metal have caused many states to ban its use.ĭr. On large recreational lakes this amounted to tons of copper applied each year, often with disappointing results. Historically, the only recourse for a lake association to combat swimmer’s itch was the widespread application of copper sulfate, a compound toxic to most aquatic organisms, including snails.
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