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Swimmer's Itch
Children are more likely targets because of their tendency to
play in shallow water.
Beverly Louis, BSc(Pharm), Drug Information Pharmacist,
British Columbia Drug & Poison Information Centre
Swimmer's itch (or cercarial dermatitis) is a rash resulting
from an allergic reaction to a parasite
called a Schistosome (a type of flatworm), which has a complex
life cycle. Although some
schistosomes can cause serious infections in humans, the ones
found in the lakes and ponds
of North America tend to infect birds (e.g., ducks) and snails.
This parasite lives as an adult worm in ducks. The worm releases
eggs, which are then excreted in the
duck feces into the water. The eggs hatch and release larvae,
which have only 24 hours to find
a specific snail host before they die. If it finds the right
host, the larva enters the snail, where after three
to four weeks of development, it is released into the water
as a cercaria (a free-swimming
larval form of the parasite). The microscopic cercaria then
swims to the surface searching for an avian
host to complete its life cycle. It is at this stage that the
parasite may encounter a human swimmer
and penetrate the skin.
Although some cercariae can penetrate skin while
it is still wet, others only penetrate dry skin. Once
they enter the skin, the cercariae quickly die. Swimmer’s
itch results in individuals who have been
previously exposed to the cercariae and who have become sensitized
to the dead parasite.
As the cercariae penetrate the skin, the person may feel a
tingling sensation. Approximately a third of
people will develop an allergic reaction to the parasite and
experience possible itching, swelling and
a rash. Scratching should be avoided as it will increase the
risk of infection. Symptoms usually begin
within a couple of hours and may last for up to two weeks.
Although swimmer’s itch can affect anyone, children are
more likely targets because of their tendency
to play in shallow water, where the cercariae are often found.
As well, children tend to
run in and out of the water without drying off. The parasite
may then enter their skin once the infested
water on their skin evaporates. Swimmer’s itch is not
contagious.
Many people may not require treatment. However,
if the itching
is bothersome, cool compresses, baking
soda baths or pastes, calamine lotion, topical antihistamines
and colloidal oatmeal baths have been
recommended. If the rash is severe, oral antihistamines and
topical corticosteroids can be used.
To help prevent swimmer’s itch, individuals should shower
off immediately after coming out of
the water and then briskly dry themselves with a towel. This
measure, however, will not affect the
cercariae that have already penetrated the skin.
For additional information, consult the following web sites:
http://wlapwww.gov.bc.ca/wat/wq/brochures/swimitch.html
http://www.cdc.gov/ncidod/dpd/parasites/schistosomiasis
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The information in this newsletter was prepared and reviewed
by pharmacists for the Canadian Pharmaceutical Journal.
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Ask Your Pharmacist newsletter is produced through
an unconditional educational grant from APOTEX/PACE. |
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