Wild and farmed salmon (Salmo salar) as reservoirs for infectious salmon anaemia virus, and the importance of horizontal- and vertical transmission

Abstract

The infectious salmon anaemia virus (ISAV) is an important pathogen on farmed salmon in Europe. The virus occurs as low- and high virulent variants where the former seem to be a continuous source of new high virulent ISAV. The latter are controlled in Norway by stamping out infected populations while the former are spreading uncontrolled among farmed salmon. Evidence of vertical transmission has been presented, but there is still an ongoing discussion of the importance of circulation of ISAV via salmon brood fish. The only known wild reservoirs are in trout ( Salmo trutta ) and salmon ( Salmo salar ). This study provides the first ISAV sequences from wild salmonids in Norway and evaluates the importance of this reservoir with respect to outbreaks of ISA among farmed salmon. Phylogenetic analyses of the surface protein hemagglutinin-esterase gene from nearly all available ISAV from Norway, Faeroe Islands, Scotland, Chile and wild salmonids in Norway show that they group into four major clades. Including virulent variants in the analysis show that they belong in the same four clades supporting the hypothesis that there is a high frequency of transition from low to high virulent variants in farmed populations of salmon. There is little support for a hypothesis suggesting that the wild salmonids feed the virus into farmed populations. This study give support to earlier studies that have documented local horizontal transmission of high virulent ISAV, but the importance of transition from low- to high virulent variants has been underestimated. Evidence of vertical transmission and long distance spreading of ISAV via movement of embryos and smolt is presented. We recommend that the industry focus on removing the low virulent ISAV from the brood fish and that ISAV-free brood fish salmon are kept in closed containment systems (CCS).

Synthetic Peptides as a Promising Alternative to Control Viral Infections in Atlantic Salmon

“…Nowadays it is known that the highest prevalence corresponds to the apathogenic variant HPR0, which is present in practically all countries that grow Atlantic Salmon [ 27 ], and sporadically (2 to 3 cases with HPRΔ per year in Chile) due to deleterious variants that cause the disease. The current regulations in Chile do not allow the treatment of animals infected with pathogenic variants of the virus and sanitary slaughter is enforced; however, animals infected with apathogenic variants (HPR0) can continue in the production cycle under intensive surveillance being aware that the situation can change at any time [ 8 , 28 ]. The results obtained in our study, show that the peptides can decrease the viral load of the variants tested (HPR3a and HPR14a).…”

Beyond hybridization: the genetic impacts of nonreproductive ecological interactions of salmon aquaculture on wild populations

“…Second, Madhun et al (2018) also document the presence of piscine ortho reo virus (PRV) in returning wild adult Atlantic salmon in Norway, and that the frequency of infection increased with body size and displayed no geographic signal, suggesting infection was occurring between escapees and wild salmon at marine feeding areas. Nylund et al (2019) report that infectious sal mon anemia virus (ISAV) variants in farmed sal mon are increasing in prevalence in the wild consistent with horizontal transmission from farmed sal mon to wild populations. Similarly, Garseth et al (2013) examine pathogen transfer between wild and farmed salmon using analysis of protein coding sequences in PRV in Norway and suggest occurrence in the wild is due to long distance transmission likely associated with the aquaculture industry.…”

Where the salmon roam: fjord habitat use of adult Atlantic salmon

“…Viral diseases constitute the dominate disease vector affecting aquaculture. However, direct evidence of disease transfer from aquaculture to wild conspecifics is limited (but see Nylund et al 2019). Yet, horizontal transmissions have been documented to be introduced via aquaculture farms and subsequently detected in wild populations (Nylund et al 2019) and recaptured escaped farmed salmon have been observed with high densities of pathogens in or near rivers (Madhun et al 2017).…”

“…However, direct evidence of disease transfer from aquaculture to wild conspecifics is limited (but see Nylund et al 2019). Yet, horizontal transmissions have been documented to be introduced via aquaculture farms and subsequently detected in wild populations (Nylund et al 2019) and recaptured escaped farmed salmon have been observed with high densities of pathogens in or near rivers (Madhun et al 2017). Still, broad scale infection rates are largely unknown due to the difficulty of detecting infected individuals as they tend to be less catchable and more susceptible to predation events (Bakke and Harris 1998).…”