Ginanne Brownell Mitic June 2017
It is most assuredly the biggest topic of conversation at the Crystal View, at Crystal beach, on the ball field, in the Assembly building and most everywhere else across the Congregational Summer Assembly at the start of each season: How bad is the swimmer’s itch this summer? How many cases have been reported? How many duck broods are on the lake? And what is being done about it to make this problem go away?
I have spent a good portion of time since last summer doing investigative work into swimmer’s itch and my story on the dreaded itch (also called cercarial dermatitis), came out a few weeks ago in Scientific American. For me, swimmer’s itch (let’s call it SI for short) is personal as I get a bad case at least once a summer (there are between 20% to 25% of people who luckily do not have an allergic reaction to SI). Both my own anecdotal research as well as research done by experts have found that the more times you are exposed the worse your reaction gets: the itchiness, the red spots and I now also get a low-grade fever and lightheadedness. It is, as Patrick Hanington from the University of Alberta’s School of Public Health told me, similar to an allergic response that people get from bee stings or pollen.
So let’s quickly go over the biology: in general terms, SI is caused by something called schistosomes, which are also known as flatworms or blood flukes. On the planet, there are about 100 species of schistosomes—70 of which are avian schistosomes (so that means they infect birds) and about 30 of which infect mammals (but don’t worry, only three species can infect humans causing human schistosomiasis —a neglected tropical disease— and are found only in Africa, Asia and South America). While cercarial dermatitis has been found in 30 states in the US (and there have been cases reported from France to New Zealand, China, Chile and Russia), 95% of the reported cases have been in the northern states that border Canada. So that’s from Maine to Washington.
Here’s where it gets rather confusing—each schistosome species is particular not only to a specific kind of bird but also a specific kind of snail. So in northern Michigan—north of Cadillac—our SI schistosome is called the trichobilharzia stagnicolae and it has found the perfect host in both the common merganser duck (we had five broods last summer on Crystal) and the stagnicola emarginata snail. Eric Samuel Loker, a snail expert who teaches in the biology department of the University of New Mexico, told me that while the trichobilharzia stagnicolae is very common in places like Michigan and the upper Midwest, you do not find them elsewhere. “There are different species in different locations and these different parasites rely on different birds for their transmission,” he told me. That means, for example, that the Canadian geese and seagulls that we see on Crystal, aren’t infected by the trichobilharzia stagnicolae. It’s just the mergansers. (But those geese and seagulls could carry other schistosome species that, with the right snail species, could be infecting lakes somewhere else that they may migrate to or from).
However the general premise of getting the itch is pretty much the same: the adult schistosomes pass their eggs through the GI track of birds—and in the case of the schistosoma douthitti, it’s through muskrats—so when the animal defecates in the water the eggs take about an hour to hatch into the larvae stage called miracidia, which then looks for its host, the snail. Once it penetrates the snail, it develops into another intermediate stage where over a period of four weeks they can asexually produce thousands of cercariae that then leave the body of the snail, heading up to the sunlight. Dr. Loker told me that what makes them “most happy” is to encounter the foot of a duckling (their feet and skin are easier to penetrate than adult ducks and also they have not build up strong immunities yet). But if they don’t find—in our case on Crystal, mergansers—then they will try to burrow into our skin instead. Though the cercariae die trying to burrow into our skin—they can’t get through the epidermis layer—75% of us have a reaction to them. And that’s where the itching, the red welts and the general drama come from.
There was an old legend that toweling off would stop SI, but that is simply not true. Many people, including myself, can actually feel them (or at least begin to feel the allergic reaction) while still in the water. Toweling off only works in the case of schistosoma douthitti and another schistosome species that target muskrats—those cercariae try burrowing in as the water dries off on people’s skin. There are a few researchers this summer that are going to be experimenting—not on humans—with what kinds of skin creams seem to work best. Some, like me, swear by creams like Safe Sea that are marketed to protect from jellyfish stings, while liposome-based deet products are also said to help prevent getting SI. Since cercarial dermatitis is an allergy, a number of people (including doctors) suggest taking an antihistamine like Claritin or Zyrtex before going for a swim. These are only anecdotal of course so if you have a formula you swear by, by all means keep it up.
So how do we get rid of the itch? There is a debate in the field over whether it makes sense to get rid of the snails (that’s why the copper sulfate is rumored to have been used back in the 1960s and 1970s) or get rid of the mergansers. Hunting in the autumn has minimal effect—first of all they are said to taste horrid and those adult ducks are probably commuting down from Canada to Florida for the winter and so aren’t the actual summer culprits (plus for every one cercariae that gets in an adult duck there are 100 that are in ducklings). On Lake Cadillac and Lake Mitchell a number of years ago they too had a problem with SI. But turns out, it was the mallard ducks that were the hosts (for a different species of schistosome and snail) and researchers found that the heartworm medicine given to dogs and cats could also kill the SI. So scientists spread the medicine on corn and left it out for mallards, who, as anyone who has ever fed them bread will know, mallards love people food. The medicine killed the schistosomes and the mallards, bellies full, were none the wiser. Not more itch on those lakes.
But mergansers are fish eating ducks and are very shy as well, so the corn option won’t work on them. But hopefully a solution to SI was started in earnest in June. Thanks in large part to the lobbying efforts of the Michigan Swimmer’s Itch Partnership (MISIP), co-founded by our very own Joel Buzzell, the state of Michigan this year has given $250,000 to the Department of Natural Resources (to be distributed by Tip of the Mitt), for SI control and research. Biologist and swimmer’s itch expert Curtis Blankespoor (who will be speaking after the Women’s Association annual meeting on July 18) has been working this spring and summer to identify the nests of mergansers. They will be captured, along with their less than eight week old ducklings and transported to the Lake Huron side of the state where there are few Stagnicola emarginata snails. If you capture a merganser duckling before eight weeks, places like Crystal will not imprint on their memories so they will instead nest next year where they were released this summer.
Meanwhile while the stagnicola emarginata snails in Crystal are infected from last summer (they live from about June until the following September), the snails that hatch this summer won’t be infected because the ducklings are gone. So next year, we could be fairly SI free. Researchers are also keen to find out how air temperature, wind velocity and wind temperature can be factors when there are particularly bad cases of the itch reported. And the MISIP, the first of its kind in the US that focuses on lobbying and sharing research on cercarial dermatitis, hope to help lake associations in Wisconsin and Minnesota set up similar bodies. “We can show people what we have done, that we have a model they can use to do the same kinds of things that we are doing,” says Jim Vondale, the co-founder of the MSIP. “And we believe as the idea expands our influence and our ability to move forward will increase even further.” Let's hope the knowledge increases while the number of SI cases decreases. Happy swimming!