Some interesting points from the link I posted earlier:
Importance:
Infectious salmon anemia (ISA) is one of the most important viral diseases of farmed Atlantic salmon. This highly contagious disease can be insidious, with an initially low mortality rate; however, the cumulative mortality can sometimes exceed 90% if the disease remains unchecked.
Understanding of the epidemiology of ISA is still incomplete, which complicates its control. The reservoirs for the virus are not known, but experiments have shown that several species of salmonids can carry virulent ISA viruses asymptomatically. These viruses might cause outbreaks if they are transmitted to farmed Atlantic salmon. Noncultivable, apparently nonpathogenic, isolates have also been detected in wild salmonids. Small changes in these viruses, analogous to the mutations that allow low pathogenicity avian influenza viruses to become highly pathogenic, may allow them to become more virulent.
Transmission:
ISAV probably infects fish through the gills, but ingestion has not been ruled out. This virus is shed in epidermal mucus, urine, feces and gonadal fluids. In one study, virus shedding was first detected 7 days after inoculation, and rose above the minimum infective dose on day 11, two days before the first deaths occurred. Shedding peaked approximately 15 days after inoculation, when mortalities were high. ISAV also occurs in blood and tissues; tissue wastes from infected fish are infectious. Fish that survive the illness can shed the virus for more than a month.
Sea lice (Lepeophtheirus salmonis and Caligus sp.) may be mechanical vectors. These parasites could also increase the susceptibility of fish by increasing stress.