[The effect of praziquantel on Trichobilharzia (Digenea, Schistosomatidae), a cause of swimmer's dermatitis in humans].

Cases of swimmer's itch caused by cercariae of the genus Trichobilharzia have spread during the last years in Southwestern Germany and increased in intensity. The only consequence was to forbid swimming because no effective strategies are known which would be ecologically or financially acceptable. In this examination we therefore tried, by means of medication of ducks, the possible final hosts, to kill the different stages of Trichobilharzia in patency and prepatency in order to interrupt the parasitic cycle. For this the effectiveness of praziquantel on the parasitic stages in the final host (preadults, adults, eggs or fully developed miracidia) was tested in experimentally infected dwarf mallards (Anas platyrhynchos, hemerot.) and mallards (Anas platyrhynchos platyrhynchos). During therapy of ducks with patent Trichobilharzia infection, onefold or twofold application of this drug (1 x 500 mg/duck, 1 x 300 mg/duck, 2 x 200 mg/duck) only prevented the releasing of eggs with hatchable miracidia for up to 24 h. Only a threefold application of 200 mg/duck in 24 h intervals led to a permanent reduction of detectable miracidia. Application of praziquantel in low doses (30 or 40 mg per duck and day) did not reduce the number of released miracidia. Medication with praziquantel led to a strong shift of adult worms located in the enteric veins of the ducks to the liver veins in a little as 3 h; examinations by scanning electron microscopy revealed changes in the worms' surface and damage of the tegument. During prepatency doses of 22.5 mg praziquantel per duck and day, given continuously for one week, were sufficient to completely stop the release of miracidia. In this low dosage, a special preparation of the bitter drug was voluntarily swallowed by the ducks, thus providing in principal a simple, cheap and ecologically sound method of controlling cercarial dermatitis.