Lifecycle of Trichobilharzia szidati
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Lifecycle of Trichobilharzia szidati |
Cercarial dermatitis
of humans ("swimmer's itch", "Badedermatitis", "Weiherhippel")
increased its prevalence in the past few years all over the northern
hemisphere, and in many recreational lakes bathing had to be
prohibited. The main agent of cercarial dermatitis in Europe is the
trematode worm parasite of ducks, Trichobilharzia szidati. The duck and the human accidental hosts are actively invaded by the water-living cercariae of the parasites, which emerge from
the aquatic snail Lymnaea stagnalis.
Whereas the cercariae grow in the duck host to adult worms, they are
unable to develop in the human host. But in the human skin the
penetrated larvae cause erythema and urticaria of the invaded area, and
intense itching may last several days up to 3 weeks. The snail is
invaded by miracidia, which emerge from eggs, passed in feces of
infected ducks. We maintain the life cycle of T. szidati in the laboratory using ducks as final
hosts and the snail Lymnaea stagnalis
as its intermediate host. We have analysed how the cercariae find and
recognize their host's skin. In at least 5 host-identification phases,
the parasites respond with high sensitivity and specificity to chemical
host skin cues such as ceramides, cholesterol and defined fatty acids,
but also to temperature gradients, water currents and shadow stimuli.
As they respond to defined free fatty acids with penetration behavior
and transformation of their body wall, like Schistosoma-cercariae,
they might be controlled with the same methods as proposed for
schistosome cercariae. We have also studied the migration of
radiolabelled larvae in the duck and the abnormal murine host. In the
mouse host the parasites reached lungs, liver and intestine, and it may
be suggested, that this occurs also in humans. We presently also
investigate, how the miracidia
find and recognize their snail intermediate hosts. Main cues for the
chemo-orientation are host-species specific macromolecular
glycoconjugates.
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Wilfried Haas, Tina Loy, Cynthia Wulff, Jan Hertel, Alexander Holweg
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