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Bathing Water Quality and Human Health


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Swimmer's dermatitis or "duck itch"

published on 01/08/2008

For some ten year’s now, the number of epidemic breakouts in swimmer’s dermatitis has both surged and spread, across all European countries, and during the past decade (1990-2000), with one or two breakouts identified in France.


lake with ducks
Swimmer’s dermatitis occurs when “swimming” larvae enter bathers’ skin; they are referred to as “cercarial” and come from certain parasites. The larvae infect certain fowl and molluscs. Spring and summer weather conditions seem to foster the extension of parasitic occurrences.

Health Impacts

Swimmer’s dermatitis is a passing skin affliction that can lead to allergy. The affliction is generally benign, even though the reaction is often very strong.
The dermatitis appears immediately after bathing as a tingling sensation at the larvae penetration points. Soon after, small red patches and vesicles appear and persist five to fifteen days, without leaving any marks, but can lead to severe itching. The breakouts are found on part of the body that remain uncovered by bathing wear.
When exposure is repeated, the lesions can be heightened by a hypersensitive reaction (allergic phenomenon).

Conseils sanitaires

To relieve swimmer’s dermatitis, no individual protection measures intended to prevent larval skin penetration have officially been validated. The use of a repellent used in certain anti-mosquito products is reported to be effective against the larvae. To reduce risk, taking a shower after bathing, and briskly drying the body with a towel after a bath are also reputed to reduce risk, as it would appear that the larvae remain at the skin’s surface for some time before entering.
Should swimmer’s dermatitis symptoms appear, seek a doctor’s advice or go to a pharmacy to ask for appropriate symptom treatment: local anti-inflammatory creams (dermocorticoids) or, in the event of additional infection, fusidic acid. Applying ice to the affected areas, as soon as the first signs appear, can relieve pain immediately.
More information:


Swimmer’s dermatitis is a passing affliction that occurs when larval parasites (furcocercaria) from the trematode family, of the trichobilharzia variety comprising many species, enter the skin.
The usual parasite cycle includes:
  • A final host, aquatic mammal or aquatic avian species (ducks, swans, etc. often found to be the cause), that houses adult parasites in their viscera. Little is known about the hosting time. Trichobilharzia eggs are returned to the aquatic environment, hatch and yield the first larvae (myracidium). They survive only a few hours in the water and therefore need to find the intermediate host very quickly, in the immediate vicinity.
  • and an intermediate host, often a lymnaea mollusc (snail) in which the parasite continues its lifestyle, with an indeterminate hosting time. At one point, the snail discharges the second set of larvae (furcocercaria) in its slime which, like the first ones will survive only a few hours and therefore need to find their final host as quickly as possible (aquatic fowl, mammal) to continue their life cycle.
This cycle appears to be constant, with no preferred starting time.


Man is only an accidental host in the parasitic cycle, in which he puts an end to all development. In the current state of scientific and medical knowledge, the larvae cross the skin barrier but do not migrate any further in the body and die on site.
Two types of reactions can occur:
  • the prime infection often goes unnoticed and leads to passing itching, followed by the emergence of macules shortly after the bather leaves the water and can persist for 24 hours,
  • subsequent reinfection leads to outright burning and intermittent itching, followed by the emergence of highly pruritic papules.
The symptomatology varies greatly from one individual to another and as reinfection occurs, with symptoms sometimes gaining in intensity and time of onset. Treatment shall involve the application of antihistamine creams or local corticoids on the affected areas.


For the time being, cercarian dermatitis is considered a benign skin affliction with no health risk for the population, in the state of current knowledge, and is not a major public health issue as such.
However, considering the irritation caused to bathers in the midst of the tourist season, a number of preventive measures have been taken to reduce risk, aside from prohibiting bathing, in two specific areas:
  • measures intended to break down the parasite cycle,
  • measures intended to protect bathers.


Though the parasitic lifecycle was described above, the phenomenon’s details remain little-known, such that it is impossible to specify the type and scope of actions to be implemented. For instance, uncertainties remain on the following areas:
  • influence and specifics of the local ecosystem (presence of reed belts, etc.) and the topography of the beaches (slimy bottom, etc.),
  • little knowledge of how the phenomenon arises (aside from climatic effects),
  • little knowledge of trends from year to year.
The actions relating to molluscs consist of:
  • having divers gather snails by hand or using a net,
  • destabilisation trials by trawling the environment,
  • smashing and crushing the snails using an underwater hoeing machine.
The actions relating to fowl consist of taking samples to measure the level of parasite contamination and issuing municipal orders in certain towns to prohibit feeding wild fauna and prevent the species from settling in the area.


Trials were performed to see whether certain protective creams are effective in preventing parasites from entering the skin, but without any conclusive results.
The action targeting swimmers consisted primarily of informing the public as broadly as possible about preventive actions such as:
  • swimming preferably in deep waters,
  • not remaining too long in the water,
  • keeping to the areas set off for bathing,
  • briskly towelling off immediately upon leaving the water and before taking a shower,
  • equipping beaches with group shower areas.