High permissivity of the fish cell line SSN-1 for piscine nodaviruses

Seventeen isolates of piscine nodavirus from larvae or juveniles of 13 mari ne fish species affected with viral nervous necrosis (VNN) were examined fo r their infectivity to a fish cell line SSN-1. Based on cytopathic effects (CPE) and virus antigen detection by fluorescent antibody technique (FAT) a fter incubation at 25 degrees C, the infectivity of these virus isolates wa s divided into 4 groups. Group 1, including 9 virus isolates from 4 species of grouper, 2 species of sea bass, barramundi, rock porgy, and Japanese fl ounder showed CPE characterized by rounded, granular cells with heavy cytop lasmic vacuoles within 3 d post-incubation (p.i.), and the monolayer partia lly or completely disintegrated over 3 to 6 d p.i. Scattered FAT-positive c ells appeared at 3 h p.i. and spread through the cell sheet with an increas ing fluorescence signal over 24 h p.i. Group 2, consisting of 3 virus isola tes from striped jack, induced CPE with thin or rounded, granular, refracti le cells without conspicuous vacuole formation, and extensive FAT-positive reaction was observed in a time course similar to that of Group i. Cells in oculated with Group 3 (1 isolate from tiger puffer) developed no distinct C PE but viral infection was evidenced by localized FAT-positive cells. There were no FAT-positive cells in Group 4, which included 4 isolates from Japa nese flounder, Pacific cod and Atlantic halibut. However, when incubation w as per formed at 20 degrees C, the SSN-1 cells inoculated with the Group a isolate showed CPE similar to that of Group 1 and extensive FAT-positive re action. Evidence of virus proliferation at 20 degrees C was also obtained i n Group 4 isolates. The virus titers in the infected fish varied from 10(11 ) to 10(16) tissue culture infectious dose (TCID50) g(-1) of fish. There is a good correlation between these infectivities to the SSN-1 cells and the coat protein gene genotypes of the isolates. The present results indicate t hat SSN-1 cells are useful for propagating and differentiating genotypic va riants of piscine nodavirus.