Nodavirus infections in farmed and wild Atlantic cod (Gadus morhua L.) in Norway

Nodavirus are causative agents for the disease viral nervous necrosis (VNN) in fish. In Norway, nodavirus (NNV) infections is known to cause severe mortalities in hatchery reared Atlantic halibut (Hippoglossus hippoglossus) and turbot (Scopthalmus maximus). Phylogenetic studies have shown that there are different genotypes of nodaviruses infecting fish. Nodaviruses infecting halibut and turbot in Norwegian fish farms belongs to two distinct groups described as Atlantic halibut nodavirus (AHNNV) and turbot nodavirus (TNNV) respectively. More recently, presence of a new nodavirus, cod nervous necrosis virus (CNNV) from wild Atlantic cod (Gadus morhua) in Norway, has been reported. The rising Atlantic cod production in Norway might have influence on the natural reservoir of nodaviruses in wild fish, even favouring more virulent types to establish in wild fish populations in the future. For these reasons there are ongoing research projects on wild cod in Norway. In our study, farmed and wild fish have been collected and screened for presence of nodavirus during the period 2004-2007. Tissue samples from CNS (medulla oblongata) and kidney have been analysed for the presence of nodavirus using a real time RT PCR assay. When possible, virus isolates from positive samples have been sequenced and genotyped. A total of 558 Atlantic cod from 5 different counties in Norway have so far been tested for nodavirus. The farmed fish have been collected from Atlantic cod farms with and without clinical signs of VNN. The results of nodavirus screening of Atlantic cod from fish farms that were not suspected to be infected with VNN or other severe diseases, revealed 32 nodavirus positive fish of a total of 310 examined (10 %). Except one fish (ct-value = 17.5), the ct-values were all more than 35. Cod farms with suspicion of infections (i.e. clinical signs of disease, moribund fish) showed a prevalence of 91 % (N=134). Most of the positive fish had ct values less than 35. Wild Atlantic cod have been collected from coastal areas reaching from southern to northern parts of Norway. Among these, prevalence of nodavirus infection was 22 % (N=114). There were no clinical signs of VNN and only 4 fish had a ct-value less than 35. In addition to cod, we have also detected nodavirus in 7 different species and 26 specimens of wild marine fish that have been collected from a few sites in Hordaland County. These species are saithe (Pollachius virens), pollack (Pollachius pollachius), Atlantic mackerel (Scomber scombrus), lemon sole (Microstomus kitt), plaice (Pleuronectes platessa), angler fish (Lophius piscatorius) and sea bass (Dicentrarchus labrax). Only one of these specimens had ct-values less than 35 (mackerel, ct = 29.5). Our results show that nodavirus infections are common in farmed and wild marine fish in Norway. Still, nodaviruses from marine fish species in different geographical regions need to be isolated and sequenced to distinguish between different groups of nodaviruses. Phylogenetic analyses of the collected fish have so far revealed two different nodavirus genotypes in Atlantic cod in Norway. Further work in this study will reveal the geographical distribution of the different genotypes and increase the knowledge regarding host specificities.