Molecular characterization of thermo-adapted Novirhabdovirus variants and virulence in vivo in rainbow trout and zebrafish at different temperatures.

IHNV thermo-adapted to 25{degrees}C (IHNV25) retains efficient low-temperature replication and uses only 9 amino acid substitutions across 4 of 6 viral genes for adaptation.

• Why it matters: Aquaculture needs better prediction of how infectious hematopoietic necrosis virus will change with warming, since fish body temperatures track the environment and IHNV loses infectivity above 20{degrees}C.
• What they did: The virus was progressively thermo-adapted to 25{degrees}C, then genome sequencing plus reverse genetics identified 9 amino acid substitutions across 4 of 6 viral genes as essential for thermo-adaptation.
• The result: In rainbow trout, IHNV25 is highly attenuated at any temperatures, but in zebrafish larvae at 20{degrees}C and 25{degrees}C it is highly virulent, with mortality preceded by massive replication and delayed, inefficient interferon and inflammatory responses.

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