Susceptibility of rainbow trout to three different genogroups of infectious pancreatic necrosis virus.

Infectious pancreatic necrosis (IPN) is a globally distributed viral disease that is highly prevalent in rainbow trout Oncorhynchus mykiss farms in Finland. Seven genogroups (1-7) of infectious pancreatic necrosis virus (IPNV) exist, of which genogroup 5 (serotype Sp) is generally considered to be the most virulent in European salmonid farming. In Finland, 3 genogroups (2, 5 and 6) have been detected. Genogroup 2 is the most widespread and to date is the only genogroup associated with clinical disease in field observations. A bath challenge model infection trial was conducted to investigate the potential pathogenicity of the existing Finnish IPNV genogroups on IPNV-negative rainbow trout fry. Three Finnish IPNV isolates, a positive control (a Norwegian genogroup 5 isolate previously associated with high virulence in Atlantic salmon Salmo salar) and a negative control were used, and mortality was recorded daily for 8 wk. The Finnish IPNV genogroup 5 isolate caused the highest cumulative mortality, and the genogroup 2 isolate also caused elevated mortalities. The genogroup 6 isolate caused only low mortality, and the positive control treatment showed negligible mortality. Fish exposed to the Finnish genogroup 2 and 5 isolates had IPN-associated lesions, while no lesions were noted in the other treatment groups. These results indicate that Finnish IPNV genogroup 5 is potentially the most virulent IPNV genogroup for Finnish rainbow trout. Interestingly, the Norwegian IPNV genogroup 5 isolate caused only a subclinical IPN infection, providing further evidence for a host species-dependent, virus isolate-related difference in virulence in IPNV genogroup 5. The results also support the continuation of legislative disease control of IPNV genogroup 5 in Finnish inland waters.