Introduction
With the expansion of mariculture industry, Atlantic cod ( Gadus morhua L.), which is previously mainly acquired by wild fisheries, has been identified as a possible candidate for Capture B ased Aquaculture (CBA) . In Norway, 3 to 5 years old wild Atlantic cod, is captured, graded and transferred to sea cages. Then, after a weaning period, which consists of starvation and wet feeding, the fish are allowed to adapt to captivity. They are usually cultured for 6-8 months with feeding, before marketed as fresh cod products (Dreyer et al, 2008) . However, introducing wild caught cod to aquaculture stations of Norway may lead to adverse consequences with the possibility of disease spreading specially to Atlantic salmon (Salmo salar), which is the main mariculture fish species farmed in Norway . Therefore, appropriate screening of wild caught c od for the prevalence of notifiable fish diseases is necessary . Viral Hemorrhagic Septicemia (VHS) is a notifiable list 2 disease according to OIE (OIE, 2019), and has led to mass mortalities in both freshwater and marine water aquaculture species causing serious economic losses. In the present study, we focused on screening wild cod population in Norwegian waters to estimate the prevalence of VHSV. Furthermore, to evaluate pathogenicity and the transmission risk of VHSV from Atlantic cod to Atlantic salmon, artificial infection trial was performed.
Materials and methods
For VHSV screening, about 2800 wild Atlantic cod from 3 main locations along the Norwegian coastline namely Vesterålen (at small community Myre) , Båtsfjordand in Finnmark and Alesund were sampled to collect brain, heart and head kidney tissues, throughout the main c od fishing seasons from 2019 to 2021 . RNA was extracted from 2200 brain tissues so far a nd screening for VHS virus was done using Taqman -based real-time PCR assay, which was previously validated and published (Jonstrup et al, 2013).
Artificial infection trial started in April in 2021 and is currently ongoing for 8 weeks, using 2 VHS virus isolates belonging to 2 main genotypes of VHSV, namely VHSV genogroup III (Storfjorden , 2007) and VHSV 1b (Baltic Sea , 2016) . In individual challenge experiments, naïve Atlantic Cod and Atlantic Salmon juveniles (~80-100 g) were infected with the 2 virus strains by immersion and intra-peritoneal injection . In the cohabitation challenge experiment, infected Atlantic cod juveniles are cohabited with naïve Atlantic salmon juveniles to analyze the transmission risk of the tested virus strains from Atlantic cod to Atlantic salmon. Tissue samples will be collected at different time points and analyzed by cell culture ( Ex: IFAT staining), histopathology , molecular (Ex: RT-PCR) and genetic (Ex: transcriptome) methods . Water samples from each tank will also be collected, to analyze the amount and rate of virus shedding.
Results and discussion
There was no VHSV positive sample detected, out of 2200 cod brain tissue samples which were screened so far. Therefore, according to current results, we conclude that prevalence of VHSV in the main Atlantic cod populations in Norwegian waters is very low which is consistent with previous screening studies as only two positive cases of VHSV from 8395 different species of wild marine fish have been reported (Brudeseth and Evensen, 2002).
We expect to graphically visualize mortality rates and tissue distribution of virus, and also include microscopic images of histopathological changes inflicted by VHSV in main organs of both Atlantic cod and Atlantic salmon. Furthermore, transcriptome analysis will be focused on differential gene expression to understand host specificity of VHS virus and immunological responses in two fish species, and will be analyzed statistically. Considering disease transmission, European VHSV isolates in genotype Ib , II or III which often originated from marine environment generally has shown low pathogenicity to freshwater fish (Skall et al, 2004) . However, VHSV isolate from r ecent VHSV outbreaks in marine cultured rainbow trout in Storfjorden, Norway (Dale et al, 2009) was discovered as genetically closer to VHSV III. Therefore, we expect ongoing challenge trials which includes a co-habitation element will give us new insight to understand the transmissibility of marine originated European VHSV isolates.
References
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