What is swimmer's itch?
Swimmer's itch or schistosome cercarial dermatitis
is a skin reaction that certain people get the larval
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
Sensitized people get swimmer's itch when the cercarial stage
(originating in specific snail species) accidentally enters their
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
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
- 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,
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
either penetrate into the snail via the integument or it
may enter through its mouth. Within the snail, the miracidium
elongate to form a reproductive sac called the sporocyst.
This germinating structure will produce a second generation of
In approximately a month, the sporocysts produce another
stage, called the cercaria. This stage burrows out of the snail,
a second type of nonfeeding, swimming aquatic stage that
must enter the bird host. It does this either by penetrating
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
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
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
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
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
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:
- it may propagate swimmer's itch in the area where the birds
are being fed
- it may make the birds dependent on humans for survival
- feeding the birds may stimulate fecal deposits at the feeding
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.
(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
Can a relatively few common mergansers
have an important swimmer's itch impact on a large recreational
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
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
- Avoid placing rip-rap on your shore. This provides an
excellent surface for certain species of snails
The higher the number of snails, the greater
the chance for swimmer's
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
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
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
Under normal circumstances, it can be greatly
reduced, possibly eliminated. The success and cost of a control
the law of diminishing returns.
Why are control measures so costly?
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
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
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.