| Summary: | Knowledge that can be applied towards the diagnosis and prevention of Flounder Infectious Necrotizing Enteritis in summer flounder ( Paralichthys dentatus), caused by the bacterial pathogen Vibrio harveyi, are presented in this research. Biochemical profiles are insufficient to differentiate V. harveyi from other closely related species. Biochemical profiles can, however, be used in conjunction with 16S ribosomal DNA sequencing to provide a reliable method for identifying V. harveyi. A monitoring program for V. harveyi and other potential bacterial pathogens in cultured juvenile summer flounder found that V. harveyi was the most abundant and commonly found bacterial species in juvenile summer flounder at a commercial hatchery during the summer months. Presence of V. harveyi in summer flounder was not necessarily associated with large-scale mortalities, which only occurred after transport of juvenile fish. Temperatures of 25°C and transport stress resulted in highest mortalities in experimental infections of summer flounder with V. harveyi. The susceptibility to experimental infection by V. harveyi strain DN01, originally isolated from diseased summer flounder, was assessed for several economically or environmentally important fish species in the Northeast US including tautog, black sea bass, Atlantic salmon, rainbow trout, and mummichog. Vibrio harveyi strain DN01 was significantly more virulent to summer flounder than to the other species tested. A mutant strain of V. harveyi (SGM1) was created that was partially impaired in its ability to grow on a variety of media, including media with flounder intestinal mucus as a sole carbon source. The mutant also showed lower proteolytic activity in culture supernatants than the wild type. In experimental infections, the mutant was shown to be attenuated, however, it was not shown to be protective against subsequent exposure of summer flounder to wild type V. harveyi DN01. In summary, V. harveyi is a major bacterial pathogen of juvenile summer flounder, capable of causing mortalities in situations of stress. Management options include maintaining fish at temperatures at or near 18°C, reducing stress during transport, and timing production runs or transport in order to avoid the peak abundance of V. harveyi in incoming waters.
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