Avian schistosomes (cercarial dermatitis)
- Parasite
- For completeness
Bird blood flukes whose larvae burrow into a swimmer's skin and die there, leaving an itchy rash that goes away on its own.
Also called: Trichobilharzia, swimmer's itch, cercarial dermatitis, bird schistosomes
Quick facts
- Type
- Parasite · Trematodes (flukes)
- Gram / shape
- Fork-tailed cercariae released
- Spread
- Swimming or wading in fresh · Shallow
- Causes
- Cercarial dermatitis
- Treatment
- Usually self-limited
At a glance
- Shape
- Fork-tailed cercariae released by snails into water
- Family
- Schistosomatidae
- Type
- Schistosome trematodes of birds (and some mammals); humans are dead-end hosts
- Size
- Cercariae are microscopic, under 1 mm
Morphology
- Main genus: Trichobilharzia; others include Austrobilharzia, Gigantobilharzia, Ornithobilharzia and Bilharziella.
- Cercariae have a forked tail, like those of human schistosomes.
- In human skin they die in the epidermis and never reach the blood.
Transmission
- Swimming or wading in fresh, brackish or sea water where infected snails release cercariae.
- Shallow, warm water near the shore where snails and water birds gather.
- Not contagious from person to person.
Reservoir: Ducks, geese and other water birds; some mammals such as muskrats; Aquatic snails (intermediate hosts)
Isolation precautions
What the care team adds to standard precautions.
- Standard
Life cycle
- Adult flukes live in the blood vessels of water birds and release eggs in their droppings.
- In water the eggs hatch into miracidia, which infect specific snails.
- In the snail the parasite multiplies and releases free-swimming cercariae.
- Cercariae penetrate bird skin and migrate to the blood vessels, closing the cycle.
- In a human the cercariae penetrate the skin but die there, provoking an allergic reaction.
Virulence factors
- Dying cercariae in the skin trigger an immediate and delayed hypersensitivity reaction.
- Repeated exposures sensitise the person, so later rashes are faster and more intense.
- Scratching can lead to secondary bacterial infection.
Diseases it causes
- Cercarial dermatitis (swimmer's itch): itchy papules and vesicles on skin exposed to water
Clinical picture
- Tingling, burning or itching of exposed skin minutes to days after swimming.
- Small red papules within about 12 hours, which may become vesicles.
- Itch lasts about a week and fades without treatment.
- Skin covered by a swimsuit is usually spared (unlike seabather's eruption from cnidarian larvae).
Diagnosis
- Clinical: typical rash plus recent exposure to natural water.
- No routine laboratory test; in outbreaks, snails are collected and cercariae identified, sometimes by PCR.
- Distinguish it from seabather's eruption, insect bites and contact dermatitis.
Treatment
Usually self-limited; symptomatic relief with topical corticosteroids, oral antihistamines and cool compresses. No anthelmintic is needed because the larvae die in the skin.
Drug class only; doses and regimens are set by the clinician from the current guideline.
Prevention
- Avoid shallow, marshy water where snails are common or warnings are posted.
- Towel dry or shower right after leaving the water.
- Do not feed water birds at swimming sites.
Test yourself
Think of the answer, then tap the question to see it.
Why does cercarial dermatitis not progress to systemic infection?
Because humans are dead-end hosts: bird schistosome cercariae die in the skin and do not reach the blood.
Which genus most often causes swimmer's itch?
Trichobilharzia, a bird schistosome.
How does swimmer's itch differ from seabather's eruption?
Swimmer's itch affects uncovered skin and is caused by schistosome cercariae; seabather's eruption appears under the swimsuit and is caused by cnidarian larvae.
Why are rashes more intense after repeated exposures?
Prior exposure sensitises the person, so the immune response becomes faster and stronger.
Connections
← Back to the group: Trematodes (flukes)
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: CDC - DPDx - Cercarial Dermatitis (CDC, 2019); About Swimmer's Itch (CDC, 2024).
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Representative model, for educational purposes only.