Abstract
Infectious salmon anemia virus (ISAV) is an orthomyxovirus responsible for a significant disease of farmed Atlantic salmon. Fallowing and re-establishment of the Atlantic salmon farming industry in the Faroes following a recent devastating infectious salmon anaemia (ISA) disease epidemic provided a unique opportunity to study the risk of re-emergence of disease. Over 53 months, 2787 of 34 573 (8.1%) apparently healthy Atlantic salmon analysed tested positive for ISAV by RT-PCR. Sequence analysis revealed the putative low-pathogenic ISAV-HPR0 subtype in all cases. Results demonstrated that ISAV-HPR0 appeared as a seasonal and transient infection without detectable ISA mortality or pathology. This finding, coupled to an apparent gill tropism of ISAV-HPR0, suggests ISAV-HPR0 causes a subclinical respiratory infection more like seasonal influenza, as opposed to the systemic infection and serious disease caused by highly pathogenic ISAV. The mean time before marine sites became infected was 7.7 months after transfer to seawater of the fish, suggesting a potentially unknown marine reservoir of infection. Sequence analysis identified two main subtypes of ISAV-HPR0 sequences, one of which showed close genetic association with ISAV isolates responsible for the disease outbreak in the Faroes. Thus ISAV-HPR0 might represent an ancestor of pathogenic variants and thus be a potential risk factor in the emergence of new strains of disease-causing ISAV. Our data, however, suggest that the risk of emergence of pathogenic ISAV variants from a reservoir of ISAV-HPR0 is low. This risk is probably being further reduced by practical management strategies adopted in the Faroes and aimed at reducing the potential for maintenance and adaptation of ISAV-HPR0.
Highlights
Infectious salmon anaemia (ISA) is one of the most important diseases of farmed Atlantic salmon (Salmo salar L.) and predominantly affects fish at marine grow-out sites
The current study demonstrates a high prevalence of detection of the Infectious salmon anemia virus (ISAV)-HPR0 variant in association with Faroese Atlantic salmon aquaculture
While the presence of ISAV-HPR0 has previously been reported in other significant Atlantic salmon-growing nations including Norway (Nylund et al, 2007), Canada (Cook-Versloot et al, 2004), Scotland (Cunningham et al, 2002; McBeath et al, 2009) and Chile (Kibenge et al, 2009), this represents the first systematic and detailed study of its prevalence in Atlantic salmon aquaculture
Summary
Infectious salmon anaemia (ISA) is one of the most important diseases of farmed Atlantic salmon (Salmo salar L.) and predominantly affects fish at marine grow-out sites. Disease control measures subsequently implemented by the Norwegian government in collaboration with the industry reduced the spread of ISA significantly, and the prevalence of farms experiencing ISA has since been less than 5 % per year (Johansen et al, 2008). Eradication of ISA has only been achieved in Scotland (Stagg, 2003) and the Faroe Islands. In the latter case, eradication was at the cost of an almost complete break in production in the Faroese salmon-farming industry
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