Alterations in the internal defence system of the pond snail Lymnaea stagnalis induced by infection with the schistosome Trichobilharzia ocellata.

In order to investigate whether the schistosome Trichobilharzia ocellata interferes with defence activities in its snail intermediate host Lymnaea stagnalis, aspects of the immune system of infected snails and of non-infected controls were compared. The elimination of injected live Staphylococcus saprophyticus bacteria starts at a lower rate in infected snails 1 and 5 weeks after exposure to the parasite, but then proceeds faster than in control snails. During the first 3 weeks of infection, when only mother sporocysts are present, the haemocytes of the infected snails have an increased capacity to phagocytose rabbit red blood cells in vitro. From 5 weeks onwards, when mother and daughter sporocysts are present but cercariae are not yet mature, the phagocytic activity decreases to below control level. The number of circulating haemocytes is also higher in infected snails than in controls at this time. Moreover, the cells are larger, have more inclusions and an increased surface area with many long, branched, spiked pseudopods. The development of the parasite is retarded in a subpopulation of snails in which the haemolymph plasma agglutinates erythrocytes with high titres, compared to a subpopulation with low haemagglutinating activity. The haemagglutinating activity in infected snails of the first decreases significantly from 6 weeks onwards.