Fish nodaviruses are causative agents of viral nervous necrosis causing high mortality in cultured marine fishes around the world. The first successful isolation of fish nodavirus was made with SSN-1 cells, which are persistently infected with snakehead retrovirus (SnRV). In the present study, a BF-2 cell line persistently infected with SnRV (PI-BF-2) was established to evaluate the influence of SnRV on the production of fish nodavirus. The PI-BF-2 cells were slightly more slender than BF-2 cells, but no difference was observed in propagation rate between both cell lines. No difference was observed in production of SnRV between PI-BF-2 and SSN-1 cell lines. Although both PI-BF-2 and BF-2 cell lines showed no cytopathic effect (CPE) after inoculation of striped jack nervous necrosis virus (SJNNV) and redspotted grouper nervous necrosis virus (RGNNV), these fish nodaviruses could be amplified in BF-2 cells, and moreover, production of fish nodaviruses in the PI-BF-2 cell line was more than 40 times higher than in BF-2 cells. Thus, it was concluded that BF-2 cell permissiveness to fish nodaviruses was enhanced by persistent infection with SnRV. Furthermore, homologous cDNA to genomic RNA of SJNNV was detected from both PI-BF-2 and SSN-1 cell lines persistently infected with SnRV. The amount of nodavirus cDNA in SJNNV-inoculated PI-BF-2 cells was clearly lower than that in SJNNV-inoculated SSN-1 cells.
KEY WORDS: Persistent infection · Fish nodavirus · SJNNV · RGNNV · Retrovirus · SnRVResale or republication not permitted without written consent of the publisher
ABSTRACT-Kuchijirosho (snout ulcer disease) is an infectious disease of tiger puffer Takifugu rubripes in Japan and Korea. Little is known about detailed characteristics on the causative agent of kuchijirosho, except it is a filterable agent. In the present study, kuchijirosho associated proteins (KAPs) with molecular weight of 100 -120 kDa were detected in brain tissues of tiger puffer by western blot analysis with sera of tiger puffer which survived kuchijirosho, but not in the kidney or spleen tissues. Interestingly, KAPs existed in brain tissues of tiger puffer with no history of kuchijirosho, however, the KAPs were not recognized by those healthy fish sera. KAPs were fractionated in supernatant of brain tissue homogenate containing the kuchijirosho agent by ultra-centrifugation, while pathogenicity of the kuchijirosho agent was observed in the precipitated fraction rather than the supernatant. These results suggest that KAPs are not structural proteins of the kuchijirosho agent, and could be brain tissue proteins antigenically altered by infection of the kuchijirosho agent. It is also considered that the causative agent could be low in antigenicity to host fish because structural proteins of the causative agent was never recognized by host fish sera.
ABSTRACT-The causative agent of kuchijirosho (snout ulcer disease), an infectious disease of tiger puffer Takifugu rubripes, is filterable, detailed characteristics of the agent have not been clear. Here, susceptibility of eight fish species against kuchijirosho was studied by injection with the kuchijirosho agent. A hundred percent mortality with typical signs of kuchijirosho was observed in tiger puffer, grass puffer T. niphobles, panther puffer T. pardalis and blue-spotted boxfish Ostracion immaculatus, while no mortality occurred in Japanese parrotfish Oplegnathus fasciatus, largescale blackfish Girella punctata and red seabream Pagrus major. Moreover, yellowtail Seriola quinqueradiata was demonstrated to be susceptible to this disease, because cumulative mortalities of 100% and 60% were recorded in young (average 12 g in body weight) and adult (average 530 g in body weight) yellowtail, respectively. Affected young yellowtail exhibited convulsion with the curvature of the spine (scoliosis), although no typical disease sign was observed in the dead adult yellowtail. In experimental infection with the brain homogenate of the dead adult yellowtail, scoliosis was produced in young yellowtail.
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