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This review summarises the state of knowledge of both viral and bacterial diseases of Atlantic cod Gadus morhua, and their diagnosis, prophylaxis and treatment. The most important losses have been at the larval and juvenile stages, and vibriosis has long been the most important bacterial disease in cod, with Listonella (Vibrio) anguillarum dominant among pathogenic isolates. Vaccination of cod against pathogens such as L. anguillarum and Aeromonas salmonicida clearly demonstrates that the cod immune system possesses an effective memory and appropriate mechanisms sufficient for protection, at least against some diseases. Well-known viruses such as the nodavirus that causes viral encephalopathy and retinopathy (VER), infectious pancreatic necrosis virus (IPNV) and viral haemorrhagic septicaemia virus (VHSV) have been isolated from Atlantic cod and can be a potential problem under intensive rearing conditions. No commercial vaccines against nodavirus are currently available, whereas vaccines against IPNV infections based upon inactivated virus as well as IPNV recombinant antigens are available. A number of investigations of the pharmacokinetic properties of antibacterial agents in cod and their efficacy in treating bacterial infections have been reviewed.

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A new immunoglobulin heavy-chain gene with some homology to mammalian IgD was recently cloned from the channel catfish and Atlantic salmon, two species of teleost fish. We have cloned and sequenced a new H-chain gene from Atlantic cod (Gadus morhua L.) which has clear similarities to these genes, but which also differs in several ways. The similarities of catfish, salmon, and cod delta to the mammalian delta genes are sequence homology, location immediately downstream of IgM (mu), and expression by alternative splicing rather than class switching. A unique feature of catfish, salmon, and cod delta is the chimeric nature of the gene product, as the mu1 exon is spliced to the delta1 exon. Several unique features of cod IgD were found: (1) a deletion of the delta3, delta4, delta5, and delta6 domains described in catfish and salmon IgD, (2) a tandem duplication of a part of the delta locus including the delta1 and delta2 domains, (3) the presence of a truncated delta7 domain downstream of the deltaTM exons, and (4) the separation of the duplicated domains by a short exon (deltay) which has homology to a conserved part of the transmembrane exon 1 (TM1) of some H-chain isotypes. This unique organization of the delta locus of cod probably developed after the evolutionary split from the catfish and salmon branches.

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