ABSTRACT. An estuarine neritid gastropod, Clithon corona, maintained in UV-irradiated recirculating artificial seawater with a salinity of 15 per mil (‰) was found to retain thermostable direct hemolysin (TDH)-producing Vibrio parahaemolyticus in the gut at significantly higher levels than TDH-non-producing one for at least 14 days. Another estuarine neritid gastropod, C. sowerbianus, was not able to support the preferential survival of TDH-producing organisms. This evidence suggests that, if TDH-producing vibrios are brought to estuaries inhabited by C. corona, repeated ingestion of V. parahaemolyticus by this gastropod could lead to accumulation of TDH-producing vibrios in the estuaries. KEY WORDS: Clithon corona, reservoir, V. parahaemolyticus.J. Vet. Med. Sci. 67(8): 833-835, 2005 Vibrio parahaemolyticus is an autochthonous estuarine organism that can cause gastroenteritis in man after consumption of contaminated raw fish and shellfish [1,5,8] Another field survey at Sada Estuary found that TDH-producing vibrios grew in the gut of juvenile C. retropictus from a non-detectable level to 2.0 × 10 3 cfu/g (approximately 15% of V. parahaemolyticus organisms) in a twomonth period between July and September 1991 [3]. TDHproducing vibrios survived at a level of 10 4 -10 5 cfu/g maintained in UV-irradiated recirculating artificial seawater with a salinity of 20‰, while TDH-non-producing (non-pathogenic) vibrios survived there at a level of only 10 1 -10 2 cfu/ g [7]. V. parahaemolyticus can attach to estuarine diatom algae, Navicula spp. regardless of TDH-productivity [2]. This evidence suggests that V. parahaemolyticus would overwinter in muddy sediments at estuaries, grow there with rising water temperature, be released into the water column and attach to diatom algae, where it would be ingested by juvenile C. retropictus, thereby supporting selective growth of TDH-producing vibrios in the gut.The range of C. retropictus includes many estuaries of the main land of Southern Japan [6], though V. parahaemolyticus-gastroenteritis has been reported throughout the world including Southeast Asia [8]. Two neritid gastropods, C. corona and C. sowerbianus, predominate in the estuaries of the Southwest Islands (subtropical zone) of Japan, where C. retropictus seems to take refuge in freshwater areas above estuaries in competition with these gastropods [10]. A halophile, V. parahaemolyticus, could not survive in the gut of C. retropictus which inhabits freshwater areas. This raises the possibility that C. corona and C. sowerbianus are the reservoirs for TDH-producing strains of V. parahaemolyticus in the estuaries of tropical and sub-tropical zones.In the present study, two TDH-producing and two TDHnon-producing strains of V. parahaemolyticus were investigated. The TDH-producing strains, D5 (O3:K6) and D12 (O3:K6), are nalidixic acid-resistant derivatives of clinical strains, while TDH-non-producing strains, R15 (O8:K41) and R13 (untypable), are rifampicin-resistant derivatives of environmental strains. The bacteria were gr...
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