In vivo correlates of infectious salmon anemia virus pathogenesis in fish.

The phenotypic correlates of pathogenicity for Infectious salmon anemia virus (ISAV) in salmonid fishes have not been thoroughly studied to date. In this study, a comparison was made of 13 different strains of ISAV, isolated from different geographical regions between 1997 and 2004, for their infectivity in three fish species [Atlantic salmon (Salmo salar), coho salmon (Oncorhynchus kisutch) and rainbow trout (Oncorhynchus mykiss)]. When the different virus isolates were used at an approximate inoculum dose of 10(6) TCID(50) in 0.2 ml per fish, it was found that the most virulent strains had an acute mortality phase in Atlantic salmon that started at 10-13 days post-inoculation and lasted for 9-15 days with a cumulative mortality of >/=90 %. These highly pathogenic strains also caused low mortality in rainbow trout, albeit later in infection. Viruses with a more delayed or protracted mortality phase resulting in cumulative mortalities of 50-89 % in Atlantic salmon were considered to be of intermediate pathogenicity and isolates with cumulative mortalities of </=49 % were considered to be of low pathogenicity. On this basis, three of the ISAV isolates showed a high-, eight an intermediate- and two a low-pathogenicity phenotype in Atlantic salmon. Coho salmon were resistant to all ISAV isolates. These results confirmed that there is variation in pathogenicity among ISAV strains for Atlantic salmon and rainbow trout, and that other salmonid species such as coho salmon can carry highly pathogenic strains of ISAV without showing signs of disease. The identified pathogenicity phenotypes may aid in the identification of molecular markers of ISAV virulence.