Betanodaviruses are the causative agents of viral nervous necrosis (VNN) or viral encephalopathy and retinopathy (VER) in cultured marine fish. A total of 131 apparently healthy fish from 30 species were collected in two geographically remote aquaculture areas, Yashima Bay (Kagawa Prefecture) and Tamanoura Bay (Nagasaki Prefecture), in Japan. The brains of fish were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and nested PCR to detect the coat protein gene of betanodavirus. In Yashima Bay, two and 13 of 20 cultured fish were positive for nodavirus in RT-PCR and nested PCR, respectively, and four of five wild fish were positive only in nested PCR. In Tamanoura Bay, 28 and 99 of 106 wild fish were positive for the virus in RT-PCR and nested PCR, respectively. All the sequences of the nested PCR products (177 nt) from 27 fish species (10 cultured and 17 wild) were highly homologous to each other (99-100%) and were closely related to that of the known betanodavirus, redspotted grouper nervous necrosis virus (RGNNV). These results illustrate that large populations of cultured and wild marine fish in aquaculture areas are subclinically infected with genetically closely related betanodaviruses, suggesting an importance of such infected fish as a carrier or reservoir of betanodaviruses.

Streptococcus sp. is gram-positive coccus that causes streptococcal infections in fish due to intensification of aquaculture and caused significant economic losses in fish farm industry. A streptococcal infection occurred from cultured diseased olive flounder (Paralichthys olivaceus) in May, 2005 at a fish farm in Jeju Island, Korea. The diseased flounder exhibited bilateral exophthalmic eyes and rotten gills; water temperature was 16~18℃ when samples were collected. Of the 22 fish samples collected, 3 samples were identified as Lactococcus garvieae and 18 samples were identified as Streptococcus parauberis by culture-based, biochemical test. Serological methods such as slide agglutination, hemolysis and antimicrobial susceptibility test were also used as well as multiplex PCR-based method to simultaneously detect and confirm the pathogens involved in the infection. S. parauberis and L. garvieae have a target region of 700 and 1100 bp., respectively. One fish sample was not identified because of the difference in the different biochemical and serological tests and was negative in PCR assay. In the present study, it showed that S. parauberis was the dominant species that caused streptococcosis in the cultured diseased flounder.

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Betanodaviruses are the causative agents of viral nervous necrosis (VNN) in cultured marine fish. A total of 237 apparently healthy aquarium fish, marine (65 species) and freshwater (12 species) fishes and marine invertebrates (4 species), which were stocked in a commercial aquarium in Seoul, South Korea, were collected from November 2005 to February 2006. The brains of the fish and other tissues of the invertebrates were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and nested PCR to detect betanodavirus. Positive nested PCR results were obtained from the brains of 8 marine fish species (shrimp fish Aeoliscus strigatus, milkfish Chanos chanos, three spot damsel Dascyllus trimaculatus, Japanese anchovy Engraulis japonicus, pinecone fish Monocentris japonica, blue ribbon eel Rhinomuraena quaesita, look down fish Selene vomer, yellow tang Zebrasoma flavesenes), 1 marine invertebrate species (spiny lobster Pamulirus versicolor), and 2 freshwater fish species (South American leaf fish Monocirrhus polyacanthus and red piranha Pygocentrus nattereri). The detection rate in nested PCR was 11/237 (4.64%). These subclinically infected aquarium fish and invertebrates may constitute an inoculum source of betanodaviruses for cultured fishes in the Korean Peninsula.