‘Tis the season for vacations, and travel to a freshwater lake or an ocean comes to mind. But a condition known as swimmer’s itch can ruin your vacation. Swimmer’s itch is far more complex and fascinating than the name suggests and occurs worldwide. This intensely itchy rash can be contracted by swimming in fresh or salt water, is more common in the northern half of the U.S., and most common in the states bordering the Great Lakes.
Let’s begin with the bad news. Swimmer’s itch starts as an extremely itchy raised rash that occurs on exposed skin within an hour of swimming in infested fresh (lakes and rivers) or salt (ocean) water. The condition can be severe with nausea, fever, and several sleepless nights. Risk is increased with increased exposure (multiple exposures, length of time in water), swimming or wading in weedy, shallow areas near the shore, and visiting areas frequently home to ducks and geese.
Creepy science — the medical term for swimmer’s itch is cercarial dermatitis, and the organism responsible is a flatworm parasite from the schistosome genus Trichobilharzia. Schistosome parasites exist in different forms and pass from host to host during their life cycle (egg-larva-adult). The adult flatworm lives in the blood of the host (e.g. duck) and produces eggs that are released in the feces of the infected water fowl or even a few mammals such as muskrats and raccoons. In water, the eggs hatch into miracidia, swimming larvae that search for their host, a certain species of snail. These miracidia larvae multiply in the snail and develop into the next larval stage, the cercariae (note the medical name of the rash-cercarial dermatitis). These cercariae then search for an appropriate host (usually a duck or goose) that is required to complete the life cycle back to the adult flatworm. (An excellent animated depiction of the life cycle is available at www.swimmersitch.org/flash/itch3.swf, presented by Hope College in Holland, Michigan.) However, the cercariae causing swimmer’s itch are not very discriminating, and when they encounter human skin, they burrow right in. Since the parasites are “foreign” to humans, an allergic reaction sets in, resulting in the characteristic intensely itchy rash. (For some really creepy reading, you might google schistosome mansoni, a similar parasite that does infect humans and grows to adult worms in human circulation.)
There is good news — fortunately, the cercariae that cause swimmer’s itch don’t prefer humans and cannot develop further in this host. The organism gets into the skin and then dies. It’s an allergic reaction in the skin that causes the itchy rash. Although the rash will go away without treatment, cool compresses, corticosteroid creams, and oatmeal or Aveeno baths can help relieve itching. More good news is that swimmer’s itch cannot be transmitted from human to human. Recommendations to avoid swimmer’s itch include: avoid areas posted as hazards; avoid shallow edges of lakes; avoid waterfowl infested areas and don’t encourage by feeding geese and ducks; towel off briskly after swimming or use a brush to remove (invisible) cercariae, or avoid freshwater or ocean swimming altogether — well maintained and properly chlorinated swimming pools are recommended.
How to tell if you have swimmer’s itch? Could it be something else? Itching is intense and has been compared with scabies. Children may be more likely to have it. Each red pimple-looking bump represents the site of entry of one parasite, so the legs are the most likely site of infection in those who have been wading in shallow waters. The rash can be confused with insect bites, chiggers, and poison ivy. You already know my recommendation — a visit to your primary care provider. You are probably overdue for a checkup anyway.
Dr. Fleming is the Medical Director of The Jane Pauley Community Health Center (closely affiliated with Community Health Network). Ideas for this column can be e-mailed to Dr. Fleming at AskDrFleming@gmail.com. Or you can write your medical questions to Dr, Fleming at AskDrFleming, 8931 E. 30th Street, Indianapolis, IN 46218. On written correspondence, please include your name and city (names will not be published).