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# 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.

Updated Nov 13, 2026

Also called: Trichobilharzia, swimmer's itch, cercarial dermatitis, bird schistosomes

## Quick facts

**Type**
Parasite · [Trematodes (flukes)](https://durgical.com/en/microbiology/groups/trematodes/)

**Gram / shape**
Fork-tailed cercariae released

**Spread**
Swimming or wading in fresh · Shallow

**Causes**
Cercarial dermatitis

**Treatment**
Usually self-limited

On this page

- The basics Key points
- At a glance
- Morphology
- Transmission
- Life cycle
- Memory aid
- What the colours mean

- Clinical Virulence factors
- Diseases it causes
- Clinical picture
- Diagnosis

- Treatment Treatment
- Prevention

- Study Test yourself

- Connections Connections

## Key points

- Bird schistosome cercariae die in human skin: itchy rash only, no systemic infection.
- Human schistosomiasis is different: Schistosoma cercariae survive, reach the veins and lay eggs.
- Rash on uncovered skin; worse with repeated exposure (sensitisation).
- Self-limited; symptomatic treatment only.

## 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.

## Memory aid

Duck fluke, dead end: the cercaria gets through the door of the skin and dies in the hall, leaving only the itch.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/avian-schistosomes/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

## Connections

## Related 2

- [Schistosoma mansoni / S. haematobium / S. japonicum](https://durgical.com/en/microbiology/schistosoma-mansoni-s-haematobium-s-japonicum/)
- [Fasciola hepatica](https://durgical.com/en/microbiology/fasciola-hepatica/)

[← Back to the group: Trematodes (flukes)](https://durgical.com/en/microbiology/groups/trematodes/)

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](https://www.cdc.gov/dpdx/cercarialdermatitis/index.html) (CDC, 2019); [About Swimmer's Itch](https://www.cdc.gov/swimmers-itch/about/index.html) (CDC, 2024).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/avian-schistosomes/