It is recognized that asymptomatically ISAV-infected fish exist. ISA outbreaks have only been reported in farmed Atlantic salmon, although ISAV has been detected by RT-PCR in wild fish. To date, all of the ISAV isolates from the clinical disease, ISA, have had deletions in the highly polymorphic region (HPR) on ISAV segment 6 (ISAV-HPRΔ) relative to ISAV-HPR0, named numerically from ISAV-HPR1 to over ISAV-HPR30. The European genotype is more widespread and shows greater genetic variation and greater virulence variation than the North American genotype.
ISAV occurs in two basic genotypes, North American and European. Infectious salmon anaemia (ISA) virus (ISAV) belongs to the genus Isavirus, family Orthomyxoviridae.
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