Virulence Change of Infectious Hematopoietic Necrosis Virus against Rainbow Trout Oncorhynchus mykiss with Viral Molecular Evolution

Infectious hematopoietic necrosis virus (IHNV) is the causative agent of IHN, one of the most serious viral diseases of salmonid fish. A total of five major genogroups including JRt for Asian isolates were confirmed among worldwide isolates based on glycoprotein (G) gene nucleotide (nt) sequences. The present study revealed existence of new two lineages, JRt Shizuoka and JRt Nagano, in the genogroup JRt by addition of new isolates obtained in 2006. The maximum nt diversity of G gene within JRt Shizuoka or JRt Nagano lineage was 6.3% or 3.5%, while that between JRt Shizuoka and JRt Nagano lineages was 70%. To evaluate influence of the evolutional divergence to virulence of IHNV, experimental challenges to rainbow trout Oncorhynchus mykiss were conducted by bath exposure at 10(4) TCID50/mL of RtShiz06s and RtShiz06a (JRt Shizuoka lineage), RtNag96 and RtNag06a (JRt Nagano lineage), and ChAb76 (a representative of the genogroup U). Distinct difference was observed in IHNV virulence to rainbow trout, i.e. the highest virulence was in RtShiz06s and RtShiz06a (>= 76% of mortalities), and subsequently in RtNag96 and RtNag06a (20-40%), but scarcely any virulence in ChAb76 (<= 10%). Thus it was suggested that nt diversity of Japanese IHNV continued rapidly with changing its virulence in rainbow trout farm environments.