dermatitis is caused by the cercariae of certain species of schistosomes
whose normal hosts are birds and mammals other than humans. These
cercariae seem to have a chemotrophic reaction to secretions from the
skin and are not as host-specific as other types of schistosomes. They
attempt to, and, sometimes may actually, enter human skin. The
penetration causes a dermatitis which is usually accompanied with
intense itching, but the cercariae do not mature into adults in the
human body. Cases of cercarial dermatitis can occur in both fresh and
brackish water environments. One species of schistosome often
implicated in cases of cercarial dermatitis is Austrobilharzia
variglandis, whose normal hosts are ducks. The snail, Nassarius
obsoletus, is the intermediate host for this species and can be
found at marine beaches in temperate climates. Cercarial dermatitis
should not be confused with seabather's eruption, which is caused by the
larval stage of cnidarians (e.g., jellyfish). The areas of skin
affected by seabather's eruption is generally under the garments worn by
bathers and swimmers where the organisms are trapped after the person
leaves the water. Cercarial dermatitis occurs on the exposed skin
outside of close-fitting garments.
of avian schistosomes are migratory water birds, including shorebirds,
ducks, and geese. Adult worms are found in the blood vessels and
produce eggs that are swallowed and passed in the feces
On exposure to water, the eggs hatch and liberate a ciliated miracidium
that infects a suitable molluscan intermediate host
The parasite develops in the intermediate host, usually a certain
species of snail
to produce free-swimming cercariae that are released under appropriate
conditions and penetrate the skin of the birds to complete the cycle
Humans are inadvertent and inappropriate hosts; cercariae may penetrate
the skin but do not develop further
A number of species of dermatitis-producing cercariae have been
described from both freshwater and saltwater environments, and exposure
to either type of cercariae will sensitize persons to both.
dermatitis occurs worldwide with cases reported from every continent
except Antarctica. In the United States, cases are commonly reported
from the Great Lakes region.
dermatitis (swimmer's itch) is a cutaneous inflammatory response usually
associated with penetration of the skin by cercariae of bird
schistosomes. Symptoms include reddening and itching of exposed skin in
the water or immediately after emerging. This is an indication of
initial penetration of the cercariae. After a period of approximately
12 hours, pruritic papules may become vesicular. Scratching the
affected areas may result in secondary bacterial infections. An
interesting note is that previous contact with cercariae can lead to a
more immediate and intense immune response.
that would be suitable hosts for these particular avian schistosomes
(such as Nassarius obsoletus, the intermediate host for the duck
schistosome Austrobilharzia variglandis) need to be collected
from the area where cases of cercarial dermatitis have been reported.
The snails need to be checked to verify if they are shedding cercariae
by standard methodology. Sunlight may be preferred over using
artificial light to stimulate shedding. Another method is to crush the
snails and examine the body for parasite sporocysts and/or cercariae.
The cercariae then must be identified as being a type that can cause
cercarial dermatitis by using appropriate reference material.
Most cases do not
require medical attention. Topical use of corticosteroid cream may be
used for relief. In addition, cool compresses, bathing with baking
soda, applying baking soda paste to the rash, and anti-itch lotion may
also be tried. Scratching the affected area may cause the rash to
CDC. MMWR 1992
April 10;41 (14):225-228.