MODULATION OF THE BACTERIAL CLEARANCE ACTIVITY OF HEMOCYTES FROM THE FRESH-WATER MOLLUSK, LYMNAEA-STAGNALIS, BY THE AVIAN SCHISTOSOME, TRICHOBILHARZIA-OCELLATA

The ability of haemocytes, from the haemolymph of the gastropod mollus c Lymnaea stagnalis, to recognize and eliminate the bacterium Aeromona s salmonicida was shown using an in vitro bacterial clearance assay. T he assay employs a dye which is reduced by A. salmonicida in direct pr oportion to the number of viable bacteria resulting in a colour change which can be determined spectrophotometrically. Addition of cytochala sin B resulted in a marked decrease in bacterial clearance, implicatin g both intracellular and extracellular cytotoxicity of haemocytes. A c omparison of haemocytes from uninfected snails and snails infected wit h the avian schistosome parasite Trichobilharzia ocellata showed that both juveniles and adults of L. stagnalis were susceptible to infectio n with T. ocellata. After exposure to the trematode for 1.5 h the haem ocytes from these infected snails had an enhanced clearance capacity, whilst cells obtained from snails with 24-96 h infections showed decre ased clearance of the bacteria, indicating suppression by the parasite . Haemocytes, as well as plasma, which was tested on haemocytes from u ninfected snails, were used and hence a distinction was made between c ell and humoral-associated effects. The results show that both cellula r and humoral components of immunity were activated, then suppressed, following exposure to the parasite. Infection with T. ocellata seems t o have a modulating effect on the bactericidal activity of the interna l defence system of the snail host, L. stagnalis.