Swimmer's Itch ~ Hope College, Holland, 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 certain people get the larval stage of certain flatworms enters into the epidermal layer of the skin.

After the parasite enters, it dies and may cause dermatitis in individuals who have been previously sensitized. This sensitivity will rarely disappear; it usually get worse to subsequent exposures.

Will swimmer's itch spread?

No, a papule forms only where a cercaria has entered the skin of a person. If the person gets exposed to more cercariae, additional papules will form.

What are the symptoms of swimmer's itch?

Sensitized people get swimmer's itch when the cercarial stage (originating in specific snail species) accidentally enters their skin.

Usually within 30 minutes, a small red spot appears at the site where the cercaria penetrated.

This red spot will continue to increase in size for the next 24-30 hours. The raised, reddened spot is now called a papule. It will continue to itch for up to a week. Papules are limited to areas of the body that get exposed to water because cercariae can not live out of the water. For a few species of schistosomes that cause swimmer's itch, toweling off may help; with most other species, it will not do any good because the cercariae penetrated the skin while the person was in the water.

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

They usually swim more regularly, their skin may be more sensitive, and young children have a tendency to stay near the water's edge where cercariae may concentrate.

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

Often during the first warm period in the spring, usually in late May or early June. In 1998, swimmer's itch cases were reported during the second week of May. These outbreaks are delayed in the more northern regions of Michigan.

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

At the present time, there is no way to determine how long the outbreak will last. On some lakes, swimmers get the itch one time; in other lakes, it persists for the entire summer.

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

No, the opposite is probably true. Natural lake conditions promote the 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?

The following are factors that may determine whether swimmer's itch may be a problem on a specific lake at a given time:

  • distribution and number of snails that can serve as intermediate hosts
  • distribution and number of bird hosts that can serve as hosts for the adult worm
  • wind direction
  • water currents
  • number of hours that people stay in the water
  • time of day
  • sensitivity of the individual to swimmer's itch.

All of these factors can change on an annual basis.

How common is swimmer's itch in Michigan?

It is widely scattered throughout Michigan, but uncommon in the thumb region. Major outbreaks 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 schistosomes that cause swimmer's itch.

Is swimmer's itch found only in Michigan?

No, cases have been reported from nearly every state as well as from all of the provinces of Canada. In North America, most of the problems of swimmer's itch occur in the northern tier of states, north to 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 and 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 the eggs are deposited in water, they will hatch within an hour if conditions are right. The miracidium, an aquatic stage, is free-swimming, but nonfeeding. It has enough energy to keep it moving for about a day. Once the miracidium comes in contact with the proper snail it will either penetrate into the snail via the integument or it may enter through its mouth. Within the snail, the miracidium will elongate to form a reproductive sac called the sporocyst. This germinating structure will produce a second generation of sporocysts. In approximately a month, the sporocysts produce another stage, called the cercaria. This stage burrows out of the snail, becomes a second type of nonfeeding, swimming aquatic stage that must enter the bird host. 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 adult worms then begin producing large numbers of eggs which again are voided with the feces. Avian schistosomes usually complete their life cycle in two months, however, the specific time varies with each species.

Can cercariae be seen in the water?

No, it is impossible to observe larvae in the water without a microscope. They are approximately 1/80 of an inch long and transparent.

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?

Nobody knows the answer to this question. Based on previous work, it appears that there may be from 12-15 species in Michigan. This question is difficult to answer for several reasons. First. there are a large number of birds that potentially can serve as hosts for the adult worm. 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. A final difficulty is that for most species, the life cycles are not known. In other words, the snail intermediate and bird hosts for a specific species of schistosome remain an enigma.

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

No, most species of schistosomes that are the causative agents for swimmer's itch use only one species of snail and one species of bird to complete their life cycles. In other words, they are quite host-specific. This is an important concept to remember when control measures are employed. Most snail intermediate hosts for avian schistosomes belong to one of two families: Lymnaeidae and Physidae. Some members of a small snail (Planorbidae) can also serve as intermediate hosts.

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

Certain stages of the parasites that cause swimmers' itch must cycle through snails. Larval stages develop and reproduce in the internal organs of the snail. Each day, thousands of these free-swimming cercariae emerge from the snail but do not feed and therefore will not live for much more than 24 hours in the water.

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

No, there are at least nine species reported in Michigan that can serve as intermediate hosts for the parasites.

Remember that most species of schistosomes have only one snail species that can serve as its host.

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

Yes, many species of birds and some rodent species can harbor the adults parasites within their blood vessels. Some common hosts include common mergansers, mallards, Canada geese, swans, red-winged blackbirds, etc., 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, they penetrate through the skin, migrate through various organs such as the liver and lungs, and then reside in the blood vessels of the host, particularly those surrounding the intestine. The parasites develop to adulthood there even though they are extremely small and thin. The female worms (no bigger than a single hair of a paint brush) then lay eggs that work their way into the host's intestine. When the host defecates into the water, the eggs of the parasites in the feces hatch into the next stage, the miracidium. Like a cercaria, the miracidium is nonfeeding and can only swim in the water for up to a day, depending on the water temperature.

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

Three good reasons for not feeding the birds include:

  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. feeding the birds 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 hatch-year (young birds) 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. Furthermore, 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 relatively few common mergansers have an important swimmer's itch impact on a large recreational lake?

Absolutely, because mergansers are heavily infected and because there is a high prevalence of infections in them. Also, common mergansers are very mobile. In addition, one must consider the dynamics of the life cycle. For example, if one miracidium infects a snail, that intermediate host (after approximately 30 days) will produce several thousand cercariae every day it lives. Fortunately, only a small percentage of the miracidia every contact a suitable snail.

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

No, they usually are not 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 we see no waterfowl during the summer?

Yes, for the following reasons: first, snails could become infected by spring and fall migrants. Remember that it takes at least 60 days for the cycle to get 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 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 shore. 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?

It can do several things including the following: educate members about swimmer's itch, assess the problem of swimmer's itch on its lake, make recommendations for relieving the itching, and 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 topical creams will reduce the swelling.

What types of control are used 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 results are unpredictable. A second method that has recently been introduced is to trap the birds to remove the adult worms from the birds hosts with an antihelmintic drug.

Because of the complexity of the problem and because of the number of species that can cause swimmer's itch, no method will eliminate every case of swimmer's itch on a given lake.

Can swimmer's itch be eliminated completely?

Under normal circumstances, it can be greatly reduced, possibly eliminated. The success and cost of a control program follows the law of diminishing returns.

Why are control measures so costly?

Control of swimmer's itch must be done by professional people who have invested a lot of time and money into their education and training. In addition, most control efforts either are time-consuming or they require the purchase of costly chemicals like copper sulfate.

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. Currently, we are conducting experiments to develop better methods of control.


For more information on swimmer's itch:

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