Methods of producing, propagating and multiplying viruses, methods of identifying the presence of a virus in a sample, specifically methods using Japanese eel cell cultures. Also provided are an isolated Japanese eel cell culture, optionally infected with a virus, vaccines comprising viruses grown according to the methods of the invention, and a kit for propagating viruses comprising the Japanese eel cell culture. The virus may be a fish virus such as nodavirus, Infectious Pancreatic Necrosis Virus (IPNV) and Salmon Pancreas Disease Virus (SPDV).

Disclosed are interactions between different betanodaviruses during infection of cells in vitro and in European sea bass. More specifically, fish administered striped jack nervous necrosis viruses (SJNNV), which are then exposed to red-spotted grouper nervous necrosis viruses (RGNNV), have fewer symptoms of disease associated with RGNNV and/or increased survival as compared to fish not administered SJNNV prior to exposure to RGNNV.

The disclosure provides recombinant viral vectors and attenuated viruses and viral vaccines comprising a virus lacking a functional viral RNA suppressor of host RNAi.