The first detection of infectious hematopoietic necrosis virus (IHNV) genogroup U in western Europe in the first IHNV outbreak in Finland.

Infectious hematopoietic necrosis virus (IHNV) was detected for the first time in Finland in November 2017. The virus was isolated from rainbow trout (Oncorhynchus mykiss) from a sea-cage farm in the Gulf of Bothnia. Sampling was carried out as part of a routine risk-based surveillance program. During inspections based on epidemiological investigations, five other IHNV-positive rainbow trout establishments were detected in 2017-2018. No symptoms indicative of IHN were reported, and only a few macroscopic and histopathological findings were recorded. To assess the pathogenicity of the Finnish isolates, experimental trials were performed using an isolate from the index case in November 2017 and one from a positive case in January 2018. A significant difference in the survival rate was found between rainbow trout exposed to the two isolates, with the isolate from 2018 having higher virulence. Similarly, the two isolates displayed genetic heterogeneity, particularly in their G-gene sequence. According to phylogenetic analysis, the Finnish isolates belonged to IHNV genotype U. Based on N (nucleocapsid) gene sequence analysis, the isolates were closely related to either Russian IHNV isolates from 2000 to 2002 or older isolates from the coast of Washington and British Columbia. This is the first report of IHNV genotype U in Europe west of Russia.