Mechanism of pathogenesis of somatic gigantism of water snails infected with development stages of digenetic trematodes is a complex phenomenon. The manner of origination of these abnormalities is often dependent on accepted host-parasite relationship. In the article there are discussed essential foundations of hypotheses explaining snails' parasitological gigantism as a consequence of alterations in gonadal activity (e.g. the hypothesis of disturbances in energetic budget of a host) or of different aspects of the parasite effect on activity of the neurosecretory centres, involved in the control of somatic growth, as well as a possibility of production by parthenites some compounds, acting synergically with host's growth neurohormone.
The effect of exposure of juvenile individuals of Lymnaea stagnalis to infectious eggs of Opisthioglyphe ranae on dynamics of their growth was investigated under controlled conditions in laboratory. In infected snails enlargement of body weight and linear parameters of shell were observed. Any differences in shell shape, analysed on the base of regression of shell width towards to its height, were not ascertained in uninfected snails and those infected with parthenites of the trematode. The findings with this host-parasite system are discussed in relation to possible mechanisms of pathogenesis of somatic gigantism in other trematode-snail interactions.