A comparative study of marine salmonid alphavirus subtypes 1–6 using an experimental cohabitation challenge model

Abstract

A comparative challenge study of six marine isolates representing subtypes 1-6 of salmonid alphavirus (salmon pancreas disease virus, Genus Alphavirus, Family Togaviridae) was conducted in Atlantic salmon in a fresh water cohabitation trial. Histopathological lesions typical of pancreas disease were observed with all subtypes, and virus was re-isolated from serum of cohabitant fish in each case. Using a virus neutralization (VN) test neutralizing salmonid alphavirus (SAV) subtype 1 strain F93-125, VN antibodies were detected in all challenge groups, consistent with serological cross-reactivity between these subtypes. Using real-time RT-PCR, SAV RNA was detected in heart tissue from 2 to 3 weeks post-challenge (wpc) in all cohabitant groups excluding controls. The results obtained suggested differences in the dynamics of infection between strains of SAV and potentially between subtypes. Results for SAV subtypes 1 and 3 suggested essentially synchronous infection of cohabitant fish. These two study groups also had the highest virus load in heart tissue as measured by quantitative RT-PCR and also had the most extensive histopathological changes. In contrast, results for SAV subtypes 2 and 6 strains were consistent with asynchronous infection in the cohabitant fish and were characterized by slow spread, low virus loads and mild histopathological changes. The SAV subtype 4 and 5 strains occupied an intermediate position in this regard. Despite the use of concentration procedures, it was not possible to detect SAV RNA in water samples from selected study tanks. However, testing of faeces from the SAV subtypes 1, 3 and 6 challenge groups found positive signals in each beginning at 1-3 wpc and remaining detectable for a further 2-3 weeks. Parallel testing of mucus samples found these became positive at 2-3 wpc and remained positive for a further 1-3 weeks. These results demonstrate for the first time that shedding and transmission of virus may occur by both these routes and suggest that dispersal in these matrices should be included in any disease transmission models.

Identification of a wild reservoir of salmonid alphavirus in common dab Limanda limanda, with emphasis on virus culture and sequencing

“…We hypothesise that viruses other than SAV could be targeting this tissue and may have contributed to the lesions recorded, as no persistent subclinical SAV infection would be detectable by RT-qPCR. A comprehensive cohabitation challenge study by Graham et al (2011) compared isolates from all 6 subtypes using parameters that included lesion score and virus load. In that study the authors demonstrated that there are differences between isolates with regards to replication and shedding dynamics; for example, Subtype V fish were found at a lower frequency than Subtype I and III fish, and virus load was initially lower than in the latter subtypes, but exceeded Subtype I at later times.…”

Natural infection of Atlantic salmon (Salmo salar L.) with salmonid alphavirus 3 generates numerous viral deletion mutants

“…Recently, a comparative experimental study in Atlantic salmon, conducted as a freshwater cohabitation trial, showed that all subtypes caused pathological changes typical of PD, although the relative virulence of the strains varied (Graham et al, 2011).…”

Next-generation salmonid alphavirus vaccine development

“…This may suggest that SAV is not a virus specifically infecting salmonids. Cohabitation trials with infected shedders have shown that SAV is certainly horizontally transmitted from fish to fish, so high density aquaculture tanks are ideal for horizontal SAV transmission (Graham et al, 2011 (Fig. 2), and contains a positive, single-stranded RNA genome of around 12 kb, encoding two polyproteins (Villoing et al, 2000).…”

“…At present, at least six different SAV subtypes have been distinguished based on genotype and geographic distribution (Fringuelli et al, 2008;Graham et al, 2012;Graham et al, 2011). Infections may lead up to 48% mortality and deteriorated fillet quality (Lerfall et al, 2012).…”

“…Furthermore, no evidence of vertical transmission of SAV is available (Kongtorp et al, 2010). SAV was also shown to be effectively transmitted horizontally in cohabitation experiments and this is assumed to be the main route of transmission in high-density aquaculture settings (Graham et al, 2011). Nonetheless, the haematophagous salmon louse Lepeophtheirus salmonis was found to contain the virus when caught feeding on the skin of an infected salmon, though no direct evidence of active viral replication within these arthropods was provided (Karlsen et al, 2006;Petterson et al, 2009).…”