Swimmer's Itch ~ Calvin College, Grand Rapids, Michigan
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Frequently Asked Questions

What is swimmer's itch?

Swimmer's itch, or schistosome cercarial dermatitis, is a skin reaction that people get when the larval stage of certain parasitic flatworms enters into the epidermal layer of the skin. After entry the larva dies, and your body's immune system surrounds the parasite. A dermatitis may result, and is more likely to occur in individuals who have been previously sensitized to the parasite. This sensitivity will rarely disappear, and usually get worse with each subsequent exposure to the flatworm.

Will swimmer's itch spread?

No. Each papule represents the location where one cercaria has entered the skin. If a person has multiple papules it means that he/she has been exposed to multiple cercariae.

What are the symptoms of swimmer's itch?

Sensitized people get swimmer's itch when the cercarial stage (originating in a specific snail species) accidentally enters their skin. Usually within 30 minutes of exposure to a cercaria, a small red spot appears at the site where the cercaria penetrated into the skin. This red spot will continue to increase in size for the next 24-30 hours, and it will itch intensely. The raised, reddened spot is now called a papule, and the itching may continue for up to a week.

Because cercariae cannot live out of the water, the papules are limited to areas of the body that get exposed to lake water.

Why do children often develop the most severe cases of swimmer's itch?

When compared to adults, young children typically spend more time in the water, have more sensitive skin, and have a greater tendency to play in shallower water where cercariae most often concentrate.

When does the season's first outbreak of swimmer's itch usually occur?

The first cases of swimmer's itch usually occur just after the first extended period of warm weather in the spring (most often in late May or early June).

If an outbreak of swimmer's itch occurs in a particular lake or region of a lake, how long might it remain a problem?

Given what we know today about the science behind swimmer's itch, there is no way to determine how long an outbreak will last. In any given summer, a lake may have only one major outbreak of swimmer's itch; in other summers the same lake may have cases of swimmer's itch that persists for several weeks.

If swimmer's itch occurs on a lake, does that mean that the lake is polluted?

No. In fact, just the opposite is probably true. Natural, healthy lake conditions promote a high diversity of species, including the birds and snails that are potential hosts for the causative agents of swimmer's itch.

Why may swimmer's itch be a problem one year but not the next?

More scientific research is needed to answer that question. The following are factors that determine whether swimmer's itch may be a problem on a specific lake at a given time:

  • the distribution and number of snail species that can serve as intermediate hosts for the parasite
  • the distribution and number of bird species (migrants and summer residents) that can serve as definitive hosts for the parasite
  • wind direction
  • water currents
  • the number of hours that people swim in the water
  • time of day

All of these factors can change on an annual basis.

How common is swimmer's itch in Michigan?

Swimmer's itch occurs throughout Michigan, but is relatively uncommon in the thumb region. Major outbreaks most often occur on the larger recreational lakes in the northern part of the Lower Peninsula. Nearly every lake in Michigan has the potential to support the snails and birds that host the parasites that cause swimmer's itch.

Is swimmer's itch found only in Michigan?

No. Cases of swimmer's itch have been reported in nearly every state in the USA, as well as in all of the provinces of Canada. In North America swimmer's itch is most common in the northern tier of states, including Alaska. In addition, swimmer's itch has been reported from more than 30 countries.

What is the life cycle of swimmer's itch causing parasites?

The causative agent for swimmer's itch is the larval, free-living, aquatic stage of a group of flatworms that are called schistosomes. Most species that cause swimmer's itch use bird hosts for the adult parasite and aquatic snails as intermediate hosts for the larval stages. The life history of the dermatitis-producing worms is cyclic.

Eggs, released from the adult worms that reside in the blood vessels (usually veins around the intestine) of the bird host, make their way into the digestive tract of the bird and then pass out of the host with the feces. If an egg is deposited in water, within an hour it will hatch into a miracidium, which is free-swimming, but non-feeding. A miracidium has enough energy to keep moving for approximately 24 hours, and once it comes in contact with the proper snail species it will either penetrate into the snail via the integument or it may enter through the snail's mouth. Once inside the snail, the miracidium elongates to form a reproductive sac called a sporocyst. This germinating structure will produce a second generation of sporocysts. After living in the snail for approximately a month, the sporocysts produce another larval stage, called a cercaria. This stage burrows out of the snail, becomes a second type of nonfeeding, swimming aquatic stage that must enter the proper bird host species to continue its life cycle. It does this either by penetrating the skin of the bird or by being ingested and then entering the blood vessels in the walls of the pharynx or esophagus. In the bird host, the parasite migrates through various organs of the bird and finally matures in the blood vessels. The resulting adult worm then begins producing large numbers of eggs which again are voided with the feces. Avian schistosomes usually complete their life cycle in two months, but that time varies slightly with each species.

Can cercariae be seen in the water?

No. It is impossible to observe cercariae in the water without the aid of a microscope. They are transparent and each one is approximately 1/80th of an inch long.

How many hosts are there in the life cycle of the parasites that cause swimmer's itch?

There are always two, a snail intermediate and vertebrate final host, usually a bird. The parasite must be transmitted from snail to bird and from bird to snail. It can never go from snail to snail or from bird to bird.

How many species of avian schistosomes can cause swimmer's itch in Michigan?

There are at least 12-15 different species of avain schistosome in Michigan alone. The exact number is difficult to determine for several reasons. First, there are a large number of bird species that potentially can serve as hosts for the adult worms. Second, the adult worms are so small and so difficult to remove from the blood vessels that few people have attempted to work out the classification scheme. Third, for most species the life cycles are completely unknown. In other words, the snail intermediate host and the vertebrate host species for some avian schistosome species have not been discovered.

Do all of these species of schistosomes use the same species of snails and birds as their hosts?

No. Most species of avain schistosomes are very host-specific, meaning they can use only one species of snail and one species of bird to complete their life cycle. This is an important concept to remember in the implementation of any swimmer's itch control program.

What is the relationship of snails to swimmer's itch?

Certain stages of the parasites that cause swimmer's itch must cycle through snails. Larval stages develop and reproduce in the internal organs of the snail. Each day thousands of free-swimming cercariae emerge from each infected snail.

Do all snails carry the organisms that cause swimmer's itch?

No. There are at least nine different species of snails reported in Michigan that can serve as intermediate hosts for parasites that cause swimmer's itch.

Are birds important to the organisms that cause swimmer's itch?

Yes. Many species of birds, and even some rodent species, can harbor the adult parasites within their blood vessels. Some of the more common vertebrate hosts include common mergansers, mallards, Canada geese, swans, red-winged blackbirds, as well as muskrats and mice.

What is the role of these birds and mammals in the life cycle of the parasites?

When cercariae contact a suitable bird or mammal host, they penetrate through the skin, migrate through various organs including the liver and lungs, and then reside in the blood vessels of the host, particularly the veins surrounding the intestine. There the parasites develop into extremely small and thin adult worms. The female worms (no bigger than a single hair of a paint brush) lay eggs that work their way into the host's intestine. When the host defecates into the water, the egg hatches into the next stage, a miracidium. Like a cercaria, the miracidium is non-feeding and lives only 24-32 hours, depending on the water temperature.

Why should ducks, geese, and swans not be fed?

Three good reasons for not feeding birds are:

  1. it may propagate swimmer's itch in the area where the birds are being fed
  2. it may make the birds dependent on humans for survival
  3. it may stimulate fecal deposits at the feeding site.

How do you determine which birds are carrying the schistosomes that are causing swimmer's itch on a particular lake?

Birds can be checked for avian schistosomes by hatching the miracidia from parasite eggs in the hosts' feces. If local that can't fly are positive for avian schistosomes, then it must be concluded that they contacted the parasite on that specific lake. Not only can the bird species of dermatitis-producing parasite be isolated, but it is possible to determine the level of infection. This is done by weighing the fecal content and then counting the number of miracidia that hatch from one gram of feces. It is important not only to know what bird species serve(s) as hosts, but also the level of infection. To pinpoint the bird host even further, it is possible to take the miracidia that hatch from the feces and expose suitable lab-reared snails to determine if they get the infection. If the cycle can be reared in the laboratory, cercariae from the lab cultures can be compared to those that emerge from naturally-infected snails taken from areas on the lake where swimmer's itch was a problem. The behavior, size, and morphology of each species of avian schistosome are unique to each species.

How can one be sure that common mergansers play such an important role as bird hosts on so many lakes in Michigan?

First, on most lakes where swimmer's itch is an annual problem, nearly all of the common mergansers are infected.

Second, common mergansers usually harbor heavy infections compared to other species of bird hosts. For example, the average number of miracidia that hatch from a gram of feces from common mergansers is more than 300. Mallards, Canada geese and wood ducks usually have less than 25% infected and only a couple of miracidia per gram of feces.

Third, the cercariae from the species of schistosomes that cycle through common mergansers, are much larger than average and emerge only from lymnaeid snails, particularly Stagnicola emarginata.

Can a few common mergansers have an important swimmer's itch impact on a large recreational lake?

Absolutely! Common mergansers are usually very heavily infected, have a high prevalence of infection, and are extremely mobile, capable of covering several miles of shoreline in a single day.

Are Canada geese and mute swans important hosts for swimmer's itch?

No. They usually are not important hosts for swimmer's itch parasites because the snail intermediate hosts for the schistosomes that cycle through them are snails found typically in marshy areas where people do not swim. Remember, the stage that causes swimmer's itch comes from the snail and not directly from the bird host.

Is it possible to have swimmer's itch on a lake or pond even though waterfowl aren't very often seen on the lake during the summer?

Yes. First, snails could become infected by spring and fall migrants. Remember that it takes at least 60 days for the parasite's life cycle to be completed. That means that spring birds could transmit the infection to the snails. Second, there are species of schistosomes that cycle through passerine birds such as red-winged blackbirds, grackles, etc. and one species that cycles through rodents. It is unusual to see major problems of swimmer's itch caused by schistosomes in these hosts.

What can be done to prevent or to reduce swimmer's itch?

  • Avoid swimming for long periods of time in shallow water
  • Avoid swimming in areas where swimmer's itch is a problem and where there is an onshore wind
  • Post appropriate signs on beaches where swimmer's itch is an annual problem
  • Do not encourage birds to stay in your area by feeding them
  • Avoid placing rip-rap on your shoreline. This provides an excellent surface for certain species of snails to attach their eggs. The higher the number of snails, the greater the chance for swimmer's itch

What can our lake association do about swimmer's itch?

Lake associations can do several things to help combat outbreaks of swimmer's itch:

  • educate members about swimmer's itch
  • assess the problem of swimmer's itch on its lake
  • make recommendations for relieving the itching
  • begin a control program if swimmer's itch is a regular problem.

What can individuals do who have a bad case of swimmer's itch?

They should see a doctor and ask for a prescription to relieve the itching. Also many topical, over-the-counter creams can help reduce the swelling.

What types of control programs are available in Michigan to combat swimmer's itch?

For more than 50 years, the application of copper sulfate as a molluscicide was used on some of the larger recreational lakes to break the life cycle by killing the snail intermediate hosts. Although this method is still used, fewer lakes are requesting permits because of the uncertainty of long-term consequences to a particular lake and because the desired results may not be obtained.

A second method that has recently been introduced is to trap the birds to remove the adult worms from the bird hosts with an antihelmintic drug.

Can swimmer's itch be eliminated completely?

Because of the complexity of the problem and because of the number of species that can cause swimmer's itch, no method will eliminate 100% of the cases of swimmer's itch on a given lake. But with a comprehensive control program, swimmer's itch outbreaks can be managed and reduced to acceptable levels.

Why are control measures so costly?

Most proven control efforts are labor intensive and require highly trained professionals to implement using very specialized equipment.

Is it legal to shoot common mergansers or other species of waterfowl that harbor parasites that cause swimmer's itch?

Only during the hunting season with proper licenses, under specific regulations.

Why hasn't the problem been researched more extensively?

Field and laboratory research on swimmer's itch requires expertise in Parasitology, Ornithology, Malacology (study of mollusks), and Limnology--a rare combination of backgrounds for biologists.

Is there reason for optimism for swimmer's itch control in the future?

We think so. During the last 15 years, many advances have been made with the help and commitment from various lake associations in the Leelanau Peninsula of Michigan. Currently, we are conducting experiments to develop better methods of control.


For more information on swimmer's itch contact:

Curtis L. Blankespoor
Calvin College Biology Department
123 DeVries Hall / 1726 Knollcrest Circle, SE
Grand Rapids, Michigan 49546-4403
616.526.6498 / fax 616.526.6501

Ronald L. Reimink
Scientist, SICON, LLC
6906 48th Ave.
Hudsonville, Michigan 49426