Herring (Clupea harengus):: A host for infectious salmon anemia virus (ISAV)

The emergence of infectious salmon anaemia virus (ISAV) in several countries outside Norway and frequent new outbreaks of the disease within Norway, strongly suggests that there are natural reservoirs for the virus, probably in fish occurring in the coastal waters. Both in Norway and Canada fish farmers have claimed that there could be a possible connection between wild herring (Clupea harengus) migrating through fish farms and an outbreak of ISA in the same farms. It has also been claimed that wet feed made from herring could contain the ISA virus and, hence, transmit the disease to salmon (Salmo salar). Both these claims are "mythical" in that respect that they are not based on any scientific study or verification that the ISA virus may propagate in herring. Hence, the aim of this study was to challenge herring with the ISA virus, check for virus replication and see if the virus could be transmitted from challenged herring to disease-free Atlantic salmon. With the help of RT-PCR it was shown that the herring became infected with the ISA virus after bath challenge. A drop in haematocrit towards day 20 followed the infection. One salmon that was challenged with filtered homogenate made from ISA challenged herring died. However, most of the salmon survived, but they were positive in the RT-PCR test. It is concluded that the ISA virus is able to propagate in herring and that the herring may be an asymptomatic carrier of the virus.