Viral encephalopathy and retinopathy (VER) in Atlantic salmon Salmo salar after intraperitoneal challenge with a nodavirus from Atlantic halibut Hippoglossus hippoglossus

Abstract

Homogenate of tissue from juveniles of Atlantic halibut Hippoglossus hippoglossus suffering from viral encephalopathy and retinopathy (VER) was used to challenge smolt of Atlantic salmon Salmo salar with an initial average weight of 110 g. The nodavirus was administered in the form of an intraperitoneal injection, and the fish were kept for 134 d post challenge. Genotype characterisation of the nodavirus was performed by sequencing the RNA1 and RNA2 segments, and a quantitative real-time PCR (Q-PCR) assay was developed. Tissues from different organs were stained by immunohistochemistry (IHC). Samples were collected at random on Days 7, 25, 45, 69, 125 and 134 after challenge. Mortality, clinical signs and pathology of VER were observed only in the challenged group. The Q-PCR detected positive fish only in the challenged group, all of which were positive on all days of sampling. An increase in relative virus concentrations was observed from Day 7 to Day 25 post challenge. The increased level of virus concentration was maintained in the medulla oblongata throughout the experiment, suggesting persistence or slow elimination of the virus over time. The IHC detected positive cells on Days 34, 70 and 74. These results suggest that the nodavirus is transported to the medulla oblongata from the intraperitoneal injection site and is able to replicate in salmon. When injected, this nodavirus isolate caused mortality and established a persistent infection in the challenged salmon throughout the experiment. This susceptibility suggests that co-location of salmon and marine species should be avoided until further studies of possible transmission have been carried out. KEY WORDS: Nodavirus · Atlantic salmon · Atlantic halibut · Experimental infection · Q-PCR · Immunohistochemistry · Viral encephalopathy · RetinopathyResale or republication not permitted without written consent of the publisher Dis Aquat Org 68: 7-15, 2005 such as cod Gadus morhua (Starkey et al. 2001, Johnson et al. 2002, Gagne et al. 2004, Atlantic halibut Hippoglossus hippoglossus (Grotmol et al. 1995, Grotmol et al. 1997b) and spotted wolffish Anarhichas minor , Sommer et al. 2004. In Norway, Atlantic halibut hatcheries have suffered several outbreaks of VER during the past few years (Dannevig et al. 2000, Johansen et al. 2004a, suggesting the persistence of virus in cultured populations. Although VER has been considered to be a disease of juveniles, individuals weighing several kg have also shown clinical signs of VER (Aspehaug et al. 1999). This observed persistence of nodavirus in adult fish suggests that the virus could be present in farmed halibut for a long time and may represent a risk of spreading nodavirus with movement of persistently infected animals. Transportation of infected animals into areas with potential susceptible hosts may represent one important way of spreading diseases. In Norway, Atlantic salmon Salmo salar is the dominant farmed species, but farming of marine fish such as Atlantic cod and halibut is growing. Although...

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