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Reared larval and juvenile Atlantic halibut H~ppoglossus hippoglossus collected during incidences of mass mortality in 2 hatcheries were studied by light and electron microscopy. A vacuolating encephalopathy and retinopathy as well as endocardial lesions were observed. By transmission electron microscopy isometric, spherical, unenveloped virus particles with mean capsid diameters of approximately 25 nm were detected. The virions were found intracytoplasmically in semicrystalline arrays or as membrane-associated aggregates and single particles. Infection could be observed in neurons, astrocytes, oligodendrocytes, microglia/macrophages, lymphocyte-like cells, vascular endothelium, endocardial endothelium, myocardial myocytes and in the mesothelium of the epicardium. Specific immunolabeling was obtained in affected tissues on immunohistochemistry applying primary antisera against nodaviruses isolated from the striped jack Pseudocaranx dentex and the sea bass Dicentrarchus labrax. We suggest that the mortality was caused by a nodavirus-like agent and that this disease of the Atlantic halibut is closely related to diseases termed viral nervous necrosis (VNN), viral encephalopathy and retinopathy, and fish encephalitis, which have been associated with mass mortality in numerous cultured marine teleost species. In Atlantic halibut, viral endocardial lesions were a constant finding, suggesting that viremia may be an important factor in the pathogenesis of the infection.

Body malformation due to shortness of the vertebral column, in most cases of unknown cause, has been observed in fish for more than 100 yr. It periodically occurs with high prevalence in farmed Atlantic salmon Salmo salar in Norway, and this paper describes the results of macroscopic, radiographic and histologic examination of parr and seawater-transferred fish. The vertebral bodies in both age groups did not acquire the length that they normally should due to a growth disturbance leading to the condition of platyspondyly and shortness in the column. The pathologic changes became visible at different ages in both groups and the process apparently starts in intervertebral tissues. There was proliferation of connective tissue and blood vessels, and sometimes infiltration with inflammatory cells, around affected vertebrae, especially in seawater-transferred fish. This is the first description of inflammation in abnormally short-spined fish, and it may indicate an infectious etiology, at least in farmed seawater-transferred salmon.

Infectious salmon anemia (ISA) is an emerging disease in farmed Atlantic salmon with important commercial consequences. The pathogenicity of the ISA virus (ISAV; an orthomyxovirus) varies, observed as differences in disease development and clinical signs. A small polymorphic region (PR) in the ISAV genomic segment encoding the hemagglutinin (HA) has been described. An analysis of 33 HA gene sequences from historical and recent ISA outbreaks was performed, added to a selection of previously published HA sequences. A differential deletion model explaining the generation of HA polymorphism is proposed. The European ISAV sequences could be grouped according to deletion patterns in PR. Cell-culture replication and cytopathic effect varied between viruses from different PR groups. A rather complex epidemiology is suggested, as (a) HA sequences representing several PR variants were detected in three samples; (b) identical mutations occurred in different genetic lineages; and (c) large genetic differences were present in closely related viruses.

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