Enjoy the water again!

Swimmer’s Itch Prevention

By Harvey D. Blankespoor, Ph.D.

50 years of Research

My first association with swimmer’s itch came in 1965 at the Iowa Lakeside Laboratory, located in northwest Iowa on Lake Okoboji. As a graduate student, I became interested in the host-parasite relationships of the avian schistosomes that cause a dermatitis in many people who enter the water, especially the pristine lakes found in the northern states of our country. From 1972-76, I taught Parasitology and conducted research at the University of Michigan in the Mollusk Division. In 1973, I supervised four student projects that were associated with the host-parasite relationships of avian schistosomes that cause swimmer’s itch at their field station that is located just below the Mackinaw Bridge in the Lower Peninsula. I have continued teaching and doing research on swimmer’s itch there during even-numbered years until 2010. Also, I taught and conducted research at the Michigan State University field station (Kellogg Biological Station) during the odd-numbered years from 1975-83. During the early 1980’s, I worked on a related human disease, Schistosomiasis in the African country of Sudan. Finally, for the past 35 years, I have also been teaching biology and doing research on swimmer’s itch at Hope College, Holland, MI.(swimmersitch.org) In all, I have conducted swimmer’s itch research for nearly a half century.

What is Swimmer’s Itch?

Swimmer’s itch is a skin reaction by sensitized people, to a larval stage (cercaria) of a group of flatworms whose adults usually live in the veins associated with the intestine of its vertebrate hosts. The microscopic-sized worms lay eggs that work their way into the lumen of the intestine of the vertebrate host so that they can be passed out with the feces. Upon contact with water and light, a ciliated form (miracidium) hatches and swims in a zigzag pattern hoping to contact a specific species of snail. When it gets closer, there appears to be a chemical attraction emitted from the snail which leads to the miracidium penetrating the snail. The miracidium does not enter just any snail, but members of a single species.

Once in the snail, the miracidium migrates to an internal organ, changes to form a network of tubes (sporocysts) that begin reproducing at an incredible rate. About a month later, the end products (cercariae) are released daily, usually early in the morning, stimulated by light. The number of cercariae released each day can be up to several thousand. These forms, living for nearly 24 hours, are dispersed by wind and wave action. This process is repeated daily for each snail from the late spring to early fall. Size of snail, age of the infection and water temperature are important factors in determining how many cercariae are released each day. If the cercariae come in contact with a person, they will penetrate the epidermis; however, in people, they do not have the proper enzymes to proceed further, so they die. In birds, they continue to enter their natural hosts and develop into adult worms.

In sensitized people, the cercaria enters the epidermis and dies. Immediately, a small, red area forms. Within 30 hours, this area of the skin becomes much larger in diameter and then becomes reddened and raised. It is now called a papule that will itch intensely for up to two weeks. There is often a reduced reaction when someone is exposed for the first time. Subsequent exposures usually cause a more severe reaction and children and adults with fair complexions, are usually affected the most.

Attempts at Prevention

During the past decades, there have been many attempts to prevent or to reduce the prevalence of swimmer’s itch. Initially, many lake associations applied toxic chemicals (e.g. copper sulfate) to kill the snails, thereby breaking the life cycle. This method of control continues to the present time, but is seldom used now because it is often not effective, it is extremely toxic, it never leaves the lake, and its cost has escalated dramatically because of the price of copper.

A second method, used primarily in Canada, involved the removal of the snail intermediate hosts, usually manually. Because snails migrate, have a high reproductive rate and wind and wave action can disperse the cercariae to areas where snails are not found, this method is too labor intensive and ineffective, so it is not widely used.

A third method of reducing swimmer’s itch was initiated in my laboratory. After nearly 40 years of field and laboratory research, we determined that the common merganser is the bird host responsible for carrying the the schistosomes that cause more than 90% of the swimmer’s itch in North America. Based on these data and nearly two decades of research, we have learned to trap every merganser brood from a given lake. Trapped female common mergansers and their broods are transplanted to lakes where the birds survive and develop but where there are no snail intermediate hosts present.These areas were usually protected bays on Lake Michigan or Lake Huron where the transplanted broods would continue to survive and develop to adulthood. Until 2010, this was the method of choice because is did reduce swimmer’s itch dramatically on those lakes. However, federal and state permits are required. The Michigan DNR is not willing to issue depredation permits to individual lake associations for swimmer’s itch control. Also, some riparians, especially those who do not go into the water, do not want mergansers removed from their lake because they feel common mergansers are, and always have been, an important component of the lake.

A final method of control has recently been introduced by a few lake associations. They organize bird hunting groups to shoot every duck on the lake during the opening weekend of the duck season. Unfortunately, hundreds of ducks are killed, including those species that are not carriers of the parasites that cause swimmer’s itch. Ducks form an important part of the ecosystem of those lakes. In addition, there is no guarantee that common mergansers will not return to that specific lake and have broods there the next summer.

Prevention Myths

During the past half century, many myths have resulted from attempts to reduce the adverse effects of swimmer’s itch. Unfortunately, these myths continue to be perpetuated on line, on signs posted at the beaches, and by lake association newsletters. The most common one is advising swimmers to “towel off” and or shower immediately after leaving the water. This advice was introduced decades ago when a few unimportant species were thought to cause swimmer’s itch. Today the great majority of cases of swimmer’s itch is caused by cercariae that penetrate the skin while a person is in the water. In fact, people have regularly told me that they feel a “pricking” sensation when the cercariae penetrate. Once that happens, it is too late.

Another suggestion for reducing swimmer’s itch is for swimmers to go to deeper water, away from infected snails. Although this may help in a few situations, many people have told me that they get swimmer’s itch even in deep water. It is likely that there are fewer cercariae in deeper water but water currents, wave action and off shore winds may distribute cercariae to other regions irrespective of where the cercariae originated. Furthermore, there is evidence that infected snails may go down to depth of at least 10 meters.

Ideal Method of Prevention

Given all the previously mentioned ineffective methods and myths of swimmer’s itch prevention, the ideal method would be a safe, 100% effective, water resistant, and inexpensive cream that would be applied to all areas of skin exposed to water. Applied properly and before entering the water, the water resistant cream would prevent the penetration of the cercariae while the person is in the water. It is surprising that such a cream had not been developed during all my years of teaching and studying swimmer’s itch until 2010. The cream is called Swimmer’s Itch Guard. During the summer of 2010, at the University of Michigan Biological Station, my students and I tested Swimmer’s Itch Guard by applying it to one of each of our legs. We used the other leg (with no Guard) as a control. Based on six trials, nobody got swimmer’s itch (not even one papule) on the leg that had the Guard. Several students got papules on the legs that had no Guard. For more information on this promising product, see swimmersitchguard.com.


For lake property owners with children and grandchildren, and for people using public beaches, swimmer’s itch is a serious problem. It not only ruins family fun time in the water but it causes one of the most miserable of all “itches”. Children of all ages are especially affected since they cannot stop scratching, even while they sleep. I hope I have given the reader a better understanding of swimmer’s itch and some hope of preventing it so that all may enjoy being in the water.