The paper presents results of the studies on alterations in the structure and functions of mollusc organs under the effect of the trematode Orientobilharzia turkestanica (Skrjabin, 1913) larvae. Destructive changes were observed along with a vast necrosis of the hepatopancreatic gland. Parasitism of O. turkestanica parthenitae and cercariae in L. auricularia causes deep histopathological changes in the tissues. Protein, carbohydrate and fat metabolism disorders result in the weakening of molluscs and loss of their lability, which is essential for adaptation to environmental changes. The latter is confirmed by a high mortality of infected individuals.