ISA virus in Chile: evidence of vertical transmission

Abstract

Infectious salmon anaemia virus (ISAV), genus Isavirus (family Orthomyxoviridae), is present in all large salmon (Salmo salar)-producing countries around the North Atlantic. The target species for this virus are members of the genus Salmo, but the virus may also replicate in other salmonids introduced to the North Atlantic (Oncorhychus spp.). Existing ISA virus isolates can be divided into two major genotypes, a North American (NA) and a European (EU) genotype, based on phylogenetic analysis of the genome. The EU genotype can be subdivided into several highly supported clades based on analysis of segments 5 (fusion protein gene) and 6 (hemagglutinin-esterase gene). In 1999 an ISA virus belonging to the NA genotype was isolated from Coho salmon in Chile, and in 2007 the first outbreaks of ISA in farmed Atlantic salmon was observed. Several salmon farms in Chile were affected by the disease in 2007, and even more farms in 2008. In this study, ISA virus has been isolated from salmon in a marine farm suffering an outbreak of the disease in 2008 and from smolts with no signs of ISA in a fresh water lake. Sequencing of the partial genome of these ISA viruses, followed by phylogenetic analysis including genome sequences from members of the NA and EU genotypes, showed that the Chilean ISA virus belongs to the EU genotype. The Chilean ISA virus groups in a clade with exclusively Norwegian ISA viruses, where one of these isolates was obtained from a Norwegian brood stock population. All salmonid species in the southern hemisphere have been introduced from Europe and North America. The absence of natural hosts for ISA viruses in Chile excludes the possibility of natural reservoirs in this country, and the close relationship between contemporary ISA virus strains from farmed Atlantic salmon in Chile and Norway suggest a recent transmission from Norway to Chile. Norway export large amounts of Atlantic salmon embryos every year to Chile; hence, the best explanation for the Norwegian ISA virus in Chile is transmission via these embryos, i.e. vertical or transgenerational transmission. This supports other studies showing that the ISA virus can be transmitted vertically.

Epidemiological investigation into the re-emergence and control of an outbreak of infectious salmon anaemia in the Shetland Islands, Scotland

“…These plants were also visited by FHIs to assess biosecurity and disinfection regimes as a failure of these could have the potential to contaminate the surrounding waters and hence, potentially, visiting vessels. Vertical transmission: There is disagreement as to whether ISAV is vertically transmitted (Lyngstad et al 2008, Vike et al 2009). If vertical transmission did occur then this could be significant both as a potential source of the Shetland outbreak, since imported ova are used (although these are certified ISAV-free), and as a potential route of spread within Scotland and beyond via Shetland-reared broodstock.…”

“…Evidence for the occurrence of vertical transmission of ISAV is ambiguous (Lyngstad et al 2008, Vike et al 2009) and the broodstock in MA 1b exhibited no signs of excessive mortality when inspected. Eggs were disinfected on 3 occasions during production and transfer; in these circumstances, and given the extremely low risk broodstock were exposed, the overall risk of vertical transmission is negligible assuming good biosecurity was practiced.…”

“…Vertical transmission might potentially spread ISA (Vike et al 2009). The only broodstock site on Shetland is in MA 1b (Fig.…”

Environmental issues of fish farming in offshore waters: perspectives, concerns and research needs

“…Further, the collapse in salmon farming in Chile has been forecasted as a motivation factor to move farms further out to sea. Uncontrolled and rapid spread of infectious salmon anaemia in the intensively farmed Chiloe Sea was the main reason for the collapse of the Chilean industry in 2008-2009, leading to an economic loss exceeding the total earnings since farming started in Chile (Vike et al 2009). Other countries like Spain, Ireland and the USA do not have sheltered fjords and are developing aquaculture under offshore conditions (e.g.…”

Potency of Barnacle in Aquaculture Industry

“…This is a consequence of reduced demand for workers in the salmonid production sector, which has been negatively affected by the presence of viral disease (Mardones et al, 2009;Vike et al, 2009). This same causal factor has increased the availability of maritime concessions, processing plants, transport systems and other facilities for new aquacultures activities.…”