Endocrine interactions between digenetic trematode parasites and their intermediate hosts, freshwater snails, with emphasis on the possible role of ecdysteroids

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Abstract
The possible involvement of ecdysteroids in the endocrine interaction between digenetic trematode parasites and their intermediate hosts, freshwater snails, was studied in the combination Lymnaea stagnalis-Trichobilharzia ocellata. Measurements of ecdysteroids by radioimmunoassay in Bemax-fed and starved non-parasitized and parasitized snails showed that all tissues examined contain ecdysteroids, albeit at low levels. The highest titers of immunoreactive molecules were found in the digestive gland and gonad, Infection appeared to have no effect on the ecdysteroid titers, but the state of nutrition is important. In tissues of starved snails of non-parasitized as well as of parasitized snails the ecdysteroid titer appeared to be 2–3 times as high as in Bemax-fed snails. It is not clear whether the high titers are caused by an increased synthesis of ecdysteroids in starved snails or by an inhibited metabolism/catabolism of ecdysteroids. The distribution of 3H-ecdysone over various body parts of both non-parasitized and parasitized snails was studied in injection experiments. The highest values were observed in tissue samples of digestive gland and gonad of non-parasitized snails; these were eight times as high as the corresponding levels in parasitized snails. The lower values of the latter are probably caused by a lower uptake or binding capacity of these tissues. With an antiserum against ecdysteroids the following positive reactions were found in daughter sporocysts containing (almost) mature cercariae: (1) nuclei of parenchymal cells in the body of the cercariae, (2) cytoplasm of cells in the parenchyma of the tails of cercariae, (3) nuclei of cells constituting the wall of daughter sporocysts. The possible role of these ecdysteroids (i.e. ecdysone or 20-hydroxy-ecdysone) is discussed.

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