Samples for targeted surveillance for pathogenic infectious salmon anaemia virus (ISAV HPRdel) was recorded for 253 unique farms located within 32 containment areas for infectious salmon anaemia. Eight farms had ISAV HPRdel detected in surveillance samples, all of which received an official ISA diagnosis the same month or the month after the positive surveillance sample.

Infectious salmon anaemia (ISA) is a serious disease in salmon caused by ISA virus (ISAV), a virus within the Orthomyxoviridae family. The disease was first described in Atlantic salmon (Salmo salar) in Norway in 1984 and has since been reported in several countries (USA, UK, Canada, Faroe Islands and Chile). In Norway, the number of outbreaks peaked in 1990 with 80 cases per year. In the late 1980s and early 1990s several measures were implemented in order to combat and limit the spread of the disease. Since 1993, the number of annual outbreaks has varied between 1 and 20, and ISA is still a recurring challenge to the salmon farming
industry in Norway.

There are two main types of ISAV. The pathogenic type, termed ISAV HPR-deleted (ISAV HPRdel), is associated with ISA outbreaks, while the non-pathogenic type, termed ISAV HPR0, causes subclinical infections only. ISAV HPR0 is now regarded as the origin of the virulent ISAV HPRdel through differential mutations in at least two virus genes. Positive PCR-tests for ISAV HPR0 have so far not been considered notifiable by the Norwegian regulations.