Latest Update and Implications of the Emergence of Salmon Pancreas Disease Virus subtype 2 in Norway


1 January 2013
Norway

Dr Marian McLoughlin & Dr David Graham & Dr Petter Frost, January 2012

There are two distinct genotypic branches of SAV subtype 2 (SAV 2) currently recognised from sequencing data. There is the original SAV 2 group which was isolated from freshwater rainbow trout in France, and subsequently in Italy, Spain, Germany, England & Scotland (McLoughlin & Graham 2007). These are associated with sleeping disease (SDV) in freshwater trout, a condition very similar to pancreas disease in marine farmed Atlantic salmon. Sleeping disease has not been diagnosed and SAV 2 has not been detected in Ireland to date. However infection and disease caused by SAV subtype 2 strains belonging to this original ”SDV‐like” group have been recently found in Scotland in rainbow trout in freshwater and Atlantic salmon in marine environments. 

With  sleeping  disease,  trout  get  similar  pancreatic,  heart  and  muscle  lesions  to  those  seen  in pancreas disease in Atlantic salmon. The red muscle damage is particularly severe, hence the reason the fish appear to be sleeping on their sides as the propulsion muscles are severely damaged. All sizes and ages of freshwater rainbow trout are susceptible to SAV infection and significant mortality can be recorded up to 50% in individual tanks/net‐pens. 

As mentioned above, SDV‐like strains of SAV subtype 2 can also infect and cause PD–like lesions in Atlantic salmon in marine pens and we have seen evidence of this in marine sites on the west coast of mainland Scotland, which are adjacent to the outlets of freshwater lochs containing rainbow trout (Marian McLoughlin Pers. Comm.). There is no current information about the SAV status of rainbow trout reared in sea water in Scotland and there are no reports of SAV infection or disease in sea reared rainbow trout in Ireland, although these are stocked on a very small number of sites. 

In recent years SAV 2 strains, genetically distinguishable from the freshwater SAV 2 strains, has been regularly detected in marine phase Atlantic salmon in Scotland. Indeed the original PD infective homogenates used by Raynard & Houghton in 1993 were subsequently found to contain a SAV 2 subtype (P42p) belonging to this cluster, indicating that these “marine” SAV subtype 2 strains have been present for an extended period. Recent analysis has shown that they are detected more commonly than the SDV‐like subtype strains in farmed Scottish salmon being particularly common in the Shetland Islands and Orkney islands (Graham et al. 2012 in preparation). 

Indeed these marine strains of SAV subtype 2 are the predominant viruses detected in Shetland over the last several years (only one SAV 5 detected) and are associated with pancreas disease in this region. There are very few published descriptions of the clinical or pathological outcome of different SAV subtypes in natural outbreaks from Scotland, so it is difficult to say how disease caused by the recently detected SAV 2 isolates in Atlantic salmon in Norway will develop. We know that SAV 2 isolates cause PD similar to SAV1 and SAV3 isolates, and can cause mortality in Atlantic salmon, but as for all SAV isolates, factors other than SAV subtype determines the clinical outcome.

In  experimental  SAV  2  infection  compared  with  SAV  3  and  SAV  1  infection,  using  only  one isolate/subtype, it was indicated that the SAV 2 isolate spread slightly more slowly than the tested SAV 3 and SAV 1 isolates and caused somewhat less severe lesions (Graham et al 2011). Overall, this corresponded  to  the  mortality  patterns  of  the  outbreaks  from  which  those  isolates  originated.  
Therefore, considering the major variation in clinical outbreaks within a given subtype, it may be a different story if other isolates of the different subtypes had been compared.  

A SAV 2 subtype has also recently been detected in Atlantic salmon with PD lesions in the sea on a number of sites in Norway. It is not confirmed in which branch of the SAV 2 group the recent SAV 2 detections are located.  NVI is consulting with Dr. Fringuelli at the Fish Diseases Unit in Belfast, but it most likely belongs to the Atlantic salmon marine branch.

SAV 3 infection and PD‐like lesions have been recorded in rainbow trout in sea net‐pens in Norway for many years and indeed the number of confirmed cases of PD in rainbow trout has significantly increased in the last 2 years (from 4‐5 cases/year to ~20 in 2011). [Taksdal et al 2007, NVI personal communication]. 

Regarding the recent detection of SAV 2 in Norway, it is too early to say what the full impact will be in either the short or the long‐term. For farms outside the endemic zone PD, the detection of SAV, independent of the subtype causing it has serious consequences as current regulations will require culling of affected populations. At this stage, it is not clear what the origin of the SAV 2 subtype isolates in Norway has been. Most of the recent SAV 2 findings have been close to the PD zone and 
in mid‐Norway but the most recent detection has been further north. So there are at least three possibilities; one that the SAV 2 has been imported with smolts, or two that it has “always” been there but only recently “discovered” due to genotyping analysis not consistently being done in the past, or three it has been introduced by wild fish.

Further  surveillance  and  sequencing  of  future  and  historical  SAV  isolates  will  be  required  to determine the introduction, prevalence and severity of non‐SAV 3 infections in Norway. Moving smolts or fish is not recommended as once introduced into an area, the new infection may persist even in absence of farmed fish via, as yet unidentified, natural reservoirs, thus supporting the argument for culling in a newly infected area.  

Unless the SAV 2 infection is endemic and not causing serious disease outside the PD zone it is unlikely that the Norwegian authorities will change the culling or stamping out policy, as it has succeeded in limiting the number of ISA outbreaks by this method. However, they did not allow vaccination  for  ISA  so  no  commercial  vaccine  was  made  available.  With  PD  there  is  a  vaccine (NORVAX® COMPACT PD) which can reduce infection pressure and reduce the impact of the SAV infection, (NVI Report 14  2011)  so there  is  another tool which, alongside  good husbandry  and biosecurity practice, could help to control and limit the spread and cost of disease. Efficacy of Norvax® Compact PD is documented primarily using Norwegian SAV 3 isolates (lab and field trials) but also with the Irish SAV 1 isolate. The vaccine has also been used in Shetland over the past two years where SAV 2 infection is endemic, with good results (data on file). While infection has been shown on some vaccinated sites no significant disease has occurred, indicating good vaccine efficacy independent of subtype.  

Fringuelli E., Rowley H.M, Wilson J.C., Hunter R., Rodger H. & Graham, D.A. (2008) Phylogenetic analyses and molecular epidemiology of European salmonid alphaviruses (SAV) based on partial E2 and nsP3 gene nucleotide sequences. Journal of Fish Diseases31, 811‐824

Graham, D.A., Frost, P., McLaughlin, K., Rowley, H.M, Gabestad, I. Gordon, A & McLoughlin, M.F. (2011)  A comparative study of marine salmonid alphavirus subtypes 1‐6 using an experimental cohabitation challenge. Journal of Fish Diseases 34 273‐286.

Graham, D.A, Fringuelli, E. Rowley H.M., Cockerill, D., Hoare, J. Turnbull, T., Rodger, H., Morris, D. & McLoughlin,  M.F.  (  2012)  Geographical  distribution  of  salmonid  alphavirus  subtypes  in  marine farmed Atlantic salmon Salmo salar L in Scotland and Ireland. ( in preparation)

McLoughlin, M.F. And Graham, D.A. (2007) Alphavirus Infections in salmonids‐a review. Journal of Fish Diseases 30, 511‐531 

Taksdal, T., Olsen, A.B., Bjerkas, I., Hjortaas, M.J., Dannevig, B.H., Graham, D.A. and McLoughlin, M.F. (2007) Pancreas disease in farmed Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in Norway. Journal of Fish Diseases 30, 545‐558. 

For more information about NORVAX COMPACT PD, go to http://aqua.merck‐animal‐health.com/, e‐mail aqua@merck.com or contact your local MSD Animal Health representative. 

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REFERENCE 040CPD