Tissue tropism of nervous necrosis virus (NNV) in Atlantic cod, Gadus morhua L., after intraperitoneal challenge with a virus isolate from diseased Atlantic halibut, Hippoglossus hippoglossus (L.) K Korsnes 1,2 , E Karlsbakk 1 , M Devold 3,4 , A H Nerland 1 and A Nylund 3 1 Institute of Marine Research, Bergen, Norway 2 Bodø University College, Bodø, Norway 3 Department of Biology, University of Bergen, Bergen, Norway 4 PatoGen Analyse AS, A ˚ lesund, Norway Abstract Atlantic cod, Gadus morhua, averaging 100 g, were experimentally challenged by intraperitoneal injec- tion of nervous necrosis virus (NNV) originating from Atlantic halibut. Cod tissues, including blood, gill, pectoral fin, barbel, ventricle, atrium, spleen, liver, lateral line (including muscle tissue), eye (retina) and brain, were sampled at day 25 and 130 and investigated by real-time RT-PCR for the presence of NNV. Relative quantifications at day 130 were calculated using the 2 )DDCt method. Immunosuppression by injection of prednisolone- acetate was introduced for a 30-day period, and tissue sampled at day 180 and relative quantifica- tion estimated. No mortality or clinical signs of disease were observed in the challenged group. The challenge resulted in detection of NNV in blood, spleen, kidney, liver, heart atrium and heart ven- tricle at day 25, and by the end of the experiment NNV showed a clear increase in brain and retina, suggesting these to be the primary tissues for viral replication. There was no increase in the relative amount of NNV in blood, atrium, ventricle, spleen, liver and kidney. Corticosteroid implants resulted in a weak increase in virus RNA in spleen, kidney, liver and brain. These findings suggest that Atlantic cod is susceptible to infection with NNV from halibut. The observed tissue tropism patterns sug- gest an initial viraemic phase, followed by neuro- trophy. Head-kidney is the best tissue identified for possible NNV detection by non-lethal biopsy, but detection was not possible in all injected fish. Keywords: biopsy, Gadus morhua, NNV, nodavi- rus, real-time RT-PCR, tropism. Introduction Farming of Atlantic cod, Gadus morhua L., is a growing industry in Norway and in 2006, 10 384 metric tons were produced (Directorate of Fisheries, Norway). This is almost a doubling of volume produced since 2005 and demonstrates rapid growth in this industry. One of the major concerns are disease outbreaks and mortalities, e.g. the bacterial disease francisellosis (Nylund, Ottem, Watanabe, Karlsbakk & Krossøy 2006; Olsen, Mikalsen, Rode, Alfjorden, Hoel, Straum-Lie, Haldorsen & Colquhoun 2006) and viral nervous necrosis (VNN), also named viral encephalopathy and retinopathy (Patel, Korsnes, Bergh, Vik-Mo, Pedersen & Nerland 2007). Betanodavirus in the Nodaviridae family causes VNN, and these viruses are also referred to as nervous necrosis viruses (NNV) with a large number of marine fish species reported as hosts worldwide (Munday, Kwang & Moody 2002). The first case of VNN in farmed Atlantic cod in Norway was reported by Patel et al. (2007), in fish ranging from 5 to 24 g in size. Although NNV has been detected in Norway in farmed turbot, Scopthalmus maximus (L.), and Journal of Fish Diseases 2009, 32, 655–665 doi:10.1111/j.1365-2761.2009.01035.x Correspondence K Korsnes, Bodø University College, Bodø, Norway (e-mail: kjetil.korsnes@hibo.no) 655 Ó 2009 The Authors. Journal compilation Ó 2009 Blackwell Publishing Ltd