Atlantic salmon (Salmo salar L.) post-smolts challenged two or nine weeks after seawater-transfer show differences in their susceptibility to salmonid alphavirus subtype 3 (SAV3).

J Jarungsriapisit,G L Taranger,P G Fjelldal,S Patel,T O Nilsen,H C Morton,Ø Evensen,I U Fiksdal,L J Moore,S Stefansson

doi:10.1186/s12985-016-0520-8

J Jarungsriapisit, G L Taranger + Show 8 more

Open Access

https://doi.org/10.1186/s12985-016-0520-8

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Abstract

BackgroundPancreas disease (PD), caused by salmonid alphavirus (SAV), is an important disease affecting salmonid aquaculture. It has been speculated that Atlantic salmon post-smolts are more prone to infections in the first few weeks following seawater- transfer. After this period of seawater acclimatization, the post-smolts are more robust and better able to resist infection by pathogens. Here we describe how we established a bath immersion (BI) model for SAV subtype 3 (SAV3) in seawater. We also report how this challenge model was used to study the susceptibility of post-smolts to SAV3 infection in two groups of post-smolts two weeks or nine weeks after seawater - transfer.MethodsPost-smolts, two weeks (Phase-A) or nine weeks (Phase-B) after seawater- transfer, were infected with SAV3 by BI or intramuscular injection (IM) to evaluate their susceptibility to infection. A RT-qPCR assay targeting the non-structural protein (nsP1) gene was performed to detect SAV3-RNA in blood, heart tissue and electropositive-filtered tank-water. Histopathological changes were examined by light microscope, and the presence of SAV3 antigen in pancreas tissue was confirmed using immuno-histochemistry.ResultsVirus shedding from the Phase-B fish injected with SAV3 (IM Phase-B) was markedly lower than that from IM Phase-A fish. A lower percentage of viraemia in Phase-B fish compared with Phase-A fish was also observed. Viral RNA in hearts from IM Phase-A fish was higher than in IM Phase-B fish at all sampling points (p < 0.05) and a similar trend was also seen in the BI groups. Necrosis of exocrine pancreatic cells was observed in all infected groups. Extensive histopathological changes were found in Phase-A fish whereas milder PD-related histopathological lesions were seen in Phase-B fish. The presence of SAV3 in pancreas tissue from all infected groups was also confirmed by immuno-histochemical staining.ConclusionOur results suggest that post-smolts are more susceptible to SAV3 infection two weeks after seawater-transfer than nine weeks after transfer. In addition, the BI challenge model described here offers an alternative SAV3 infection model when better control of the time-of-infection is essential for studying basic immunological mechanisms and disease progression.Electronic supplementary materialThe online version of this article (doi:10.1186/s12985-016-0520-8) contains supplementary material, which is available to authorized users.

Highlights

Pancreas disease (PD), caused by salmonid alphavirus (SAV), is an important disease affecting salmonid aquaculture
Fish and rearing conditions The fish used in the present study were non-vaccinated Atlantic salmon post-smolts from the Aquagen strain, which had been produced as under-yearling smolts [25, 26] at the Institute of Marine Research (IMR) in Matre, Western Norway
Quantification of virus stock was performed using the end-point dilution assay, and 50 % tissue culture infectious dose (TCID50) was calculated [28]. After this experiment was completed we discovered that the SAV3 isolate used was contaminated with low levels of infectious pancreatic necrosis virus (IPNV), as the isolate had earlier been passed through one round of infection in fish

Summary

Introduction

Pancreas disease (PD), caused by salmonid alphavirus (SAV), is an important disease affecting salmonid aquaculture. It has been speculated that Atlantic salmon post-smolts are more prone to infections in the first few weeks following seawater- transfer. After this period of seawater acclimatization, the post-smolts are more robust and better able to resist infection by pathogens. Pancreas disease (PD) adversely affects the production of salmonids in many countries in Europe and North America, and is caused by an alphavirus commonly referred to as salmon pancreas disease virus (SPDV) or salmonid alphavirus (SAV). In Norway, PD outbreaks are mainly seen in the seawater phase of Atlantic salmon and rainbow trout aquaculture [6]. The number of SAV2 outbreaks has been increasing, the severity of the disease associated with SAV2 is reportedly less severe than that caused by SAV3 [11, 12]

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