Viruses of Fish

Release and survival of infectious salmon anaemia (ISA) virus during decomposition of Atlantic salmon (Salmo salar L.)

“…It is known that low virulent virus in nature can become more virulent when they are exposed to artificially high densities of the host (Kurath and Winton, 2011;Nerland et al, 2011). This is also suggested for the ISA virus, as there is strong support for the hypothesis that HPR0 ISA virus can mutate into virulent variants (Nylund et al, 2006;McBeath et al, 2009;Christiansen, 2011;Snow, 2011;EFSA, 2012;Lyngstad et al, 2012;Plarre, 2012;Plarre et al, 2012).…”

“…A continuous production with high fish densities and fish numbers, and with three salmonid species in the same area provided excellent conditions for horizontal transmission of pathogens (Groff and LaPatra, 2000;Murray and Peeler, 2005;Kurath and Winton, 2011). The fry were too small to be vaccinated (Nerland et al, 2011) before entering the uncontrolled environment in the lake and was exposed to a number of different pathogens (pers. obs.).…”

“…Several farming companies increased their number of smolt transferred to sea, to compensate for the economic losses due to weak biological performance (Alvial et al, 2012;Kibenge et al, 2012). However, it has been shown that intensive production with high fish density may result in higher prevalence as well as higher virulence of pathogens, and consequently a higher risk for disease development (Groff and LaPatra, 2000;Christiansen, 2011;Kurath and Winton, 2011;Nerland et al, 2011;Rimstad, 2011).…”

“…In nature this stress stimulus can be maturation or escaping from predators, while in farming stress is induced through for instance: high rearing densities, exposure to bath treatments, periods with low oxygen, change in feeding regimes, transportation, handling, vaccination, forced hierarchy, infections with parasites and other pathogens etc. Taken together these may lead to chronic stress, which may make the fish more susceptible to pathogens (Kurath and Winton, 2011;Nerland et al, 2011;Kibenge et al, 2012) and may also promote replication and mutations of HPR0 ISA virus resulting in virulent strains. Many production related stressors were present in Chile before the ISA crisis, probably giving the ISA virus an optimal environment and might explain the rapid development of the epizootic.…”

“…The possible mechanisms for horizontal transmission of ISA virus between marine production sites are not well documented, but it has been suggested that passive transmission through water or by vectors/fomites is important (Nylund et al, 1993;Lyngstad et al, 2008;Mardones et al, 2009). There is no definitive proof of fish virus being obligate users of arthropod vector transmission (Nerland et al, 2011), but…”