Epidemiological studies on the mechanism for persistence of Vibrio parahaemolyticus in some estuarine gastropods

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Vibrio parahaemolyticus was observed to overwinter in sediments and to be present in considerable number in sediments and a gastropod mollusc, Clithon retropictus, during summer months at Hashizu estuary in Tottori Prefecture. The highest level of the organism was 9.3 x <10^4> and 2.3 x <10^5> cfu/g in sediments and C. retropictus respectively. Production of Kanagawa hemolysin was detected in about 12 and 20% of strains isolated from sediments and C. retropictus respectively at 2 stations in the estuary but not detected at the other 3 stations. Levels of the organism in 20 molluscs varied between <10^3> to zero cfu per mollusc at the estuary even in summer. The organism was also detected from microalgal populations at the estuary in summer, in which the highest was 1.4 x <10^5> cfu/g.C. retropictus ingested V. parahaemolyticus strain D-3(Kanagawa positive; nal-r) or R-13(Kanagawa negative; rif-r) of Escherichia coli strain YS-2(nal-r) was held in aquariums with 28 liters of artificial seawater with salinities of 8.8 to 25 permil at 25゜C for 21 days. Every 5 molluscs were sacrificed at appropriate intervals to obtain homogenates of soft tissues for viable counts of the organisms. V. parahaemolyticus strain D-3 was detected for 21 days in levels of <10^3> to <10^5> cfu/g from the mollusc held in the artificial seawater with salinity of 15 or 20 permil. When the mollusc ingested strains D-3 and R-13 was held in 15 permil water, these strains were detected in similar levels to those ingested each strain, in which any conversion on their productivity of Kanagawa hemolysin as well as pathological changes were not observed. E. coli strain YS-2, though it was ingested in level of <10^6> cfu/g, decreased to <10^3> cfu/g within 4 days and preserved the level afterward in those held in any salinity.