How schistosomes profit from the stress responses they elicit in their hosts.

Results obtained with the model Trichobilharzia ocellata-Lymnaea stagnalis have confirmed the hypothesis that the physiological effects evoked by schistosomes in their snail host--castration and giant growth--are brought about by them interfering with the neuroendocrine systems (NES) regulating the physiological processes concerned. As soon as differentiating cercariae are present in the daughter sporocysts a factor can be detected in the haemolymph of the snail host, called schistosomin, which acts both at the central and the peripheral parts of the NES involved in regulation of reproduction and growth. Schistosomin appears to be a host-derived factor, which is probably released by cells of the internal defence system, the haemocytes, and by connective tissue cells, the telo-glial cells. It meets the criteria of having a cytokine-like function although its molecular structure does not show sequence homology with any of the vertebrate-type cytokines identified to date. Its cytokine nature explains why schistosomin can interfere with different neuroendocrine regulatory systems both at the central and peripheral--target--level, namely after binding to its own receptor. Schistosomin is probably not only responsible for the effects exerted by the parasite on female reproduction but also for those on male reproduction and on growth so that energy and space become available for the continuous production of cercariae. The nature of the humoral cercarial factor, which induces schistosomin release, is as yet unknown. Based on its hydrophobic character and on the fact that it can pass through the wall of the daughter sporocyst, it is supposed to be a diffusible molecule or a protonephridial excretion product. It does not seem to be a vertebrate-type steroid, an ecdysteroid or an eicosanoid. Results obtained in vitro have indicated that schistosomin might have a suppressive effect on haemocyte activity. Plasma from snails 5-6 weeks post-exposure showed a tendency to inhibit phagocytic activity of haemocytes from non-infected snails, that is preparatory to the escape and migration of cercariae. Once shedding has started this effect of schistosomin is overrruled by a strong activation of haemocyte activity coinciding with the tissue damage that the cercariae cause in the host. The cercariae escape from being attacked by masking their surface coat with host molecules. As the physiological effects caused by schistosomes resemble those observed during stress in mammals, experiments were carried out to find out whether schistosomin is also released in non-parasitized snails during stress resulting in an inhibiting effect on reproduction.(ABSTRACT TRUNCATED AT 400 WORDS)