Temperature impact on replication and virulence of European infectious hematopoietic necrosis viruses

Sporadic acute IHN outbreaks at water temperatures above 14 °C have been reported, challenging the temperature upper limit for IHN epizootics. The influence of temperature on European Infectious Hematopoietic Necrosis Virus (IHNV) was tested in vitro on five selected isolates of IHNV. Each isolate was tested by end-point titration onto EPC monolayers incubated at 15 °C, 20 °C, 22 °C and 25 °C. Replication was hindered at 22 °C for IHNV isolates IT-217A (IHNV 217/A), IHNV/O.mykiss/I/TN/224–2/Mar18 (IHNV 224–2), IHNV/O.mykiss/I/TN/534/Dec11 (IHNV 534), whereas isolates IHNV DK-FF1/21–4070–1 (IHNV DK) and IHNV/O.mykiss/I/BG/67–1/Feb22 (IHNV 67–1) were not affected. Further increase of incubation temperature to 25 °C significantly limited replication for all the isolates. IHNV 67–1, IHN 224–2 and IHNV DK were selected for the in vivo trial. Rainbow trout juveniles were exposed to IHNV and housed at water temperatures of 12 °C and 19 °C for 28 days. IHNV 67–1 induced clinical disease with reduced survival both at 12 °C and 19 °C. Isolate IHNV 224–2, which showed limited replication at 22 °C in vitro, reduced survival only at 12 °C. Isolate IHNV DK, capable of replication at 22 °C in vitro, severely reduced survival only at 12 °C. Isolates IHNV DK and IHNV 67–1 showed an insertion of seven amino acids in the 3′ end part of the coding region of the L gene. This mutation can potentially play a role in their adaptation at warm temperature. Characterization of further IHNV isolates, analysis of other genes and evaluation of the host immune response at different temperatures are necessary to investigate the temperature influence on IHNV virulence.