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Arctic charr (Salvelinus alpinus L.) were fed diets based on a commercial recipe supplemented with either linseed, soybean or marine oil prior to cohabitant challenge with Aeromonas salmonicida ssp. salmonicida. Mortality varied significantly between the three dietary groups. Highest mortality (48%) was observed in fish fed the marine oil and the lowest mortality (20%) was in the group fed soybean oil. Transmission electron microscopy (TEM) examination of the digestive tract of uninfected fish demonstrated substantial numbers of bacterial cells between microvilli. However, only a few bacteria were recovered that were associated with the microvilli of infected fish. Immunocytochemical staining/labelling investigations using TEM and an immunogold method were performed on mid‐gut segments of fish fed the marine oil diet and showed augmentation of goblet cells and the presence of A. salmonicida ssp. salmonicida in the gastrointestinal tract of diseased fish after challenge with the pathogen. It is suggested that the gastrointestinal tract could be an infection route of A. salmonicida ssp. salmonicida. The greater prevalence of goblet cells supports the suggestion that sloughing off mucus is a protective response against bacterial infections. These results make an important contribution to our understanding of how nutrition can affect the disease resistance of fish.

. Juvenile Atlantic salmon, Salmo salar L. were sampled from a commercial Norwegian fish farm during an outbreak of ‘Hitra‐disease’. One group of fish subjectively judged as healthy and another as diseased were defined on the basis of the classical apathetic behaviour seen in‘Hitra‐disease ? salmon. Haematological and biochemical analyses were carried out from blood and organs in 10 fish from each group. The diseased fish were severely anaemic. The blood indices MVC, MCH and MCHC indicated active erythropoiesis to compensate for the loss. Alkaline phosphatase (AP) activity, total protein, albumin, creatinine, triglycerides and total cholesterol were significantly reduced in the serum of diseased fish, while the activities of aspartate aminotransferase (ASAT) and alanine aminotransferase (ALAT) showed normal and significantly increased values respectively. Liver and spleen weight relative to the body weight and the content of water and lipid in the liver were significantly elevated in diseased fish. Furthermore, the iron content of the spleen was increased, whereas the zinc content showed no changes. The branched‐chain amino acids valine, leucine and isoleucine were significantly higher and serine significantly lower in muscle extracts of diseased fish.

Abstract. Vibrio anguillarum isolated from diseased cod, Gadus morhua L., were serotyped by use of a panel of monoclonal antibodies. Four serotypes could be distinguished, having different lipopolysaccharide determinants. These phenotypic differences were also reflected in the genetic map, as revealed by fingerprinting of bacterial DNA. Antisera were raised in cod after immunization with the V. anguillarum serotypes, and Western blot techniques demonstrated production of specific antibodies mainly to LPS‐antigens. The immune system in cod discriminates to a eertain degree between the four serotypes as shown by crossreactions of the immune sera in elisa. Moreover, it was also shown that natural antibodies to bacterial antigens are present in non‐immune sera, but these specificities are non‐LPS in nature. As a consequence of the heterogeneity of the V. anguillarum strains, vaccination experiments were performed under laboratory conditions to compare the effectiveness of bacterins based on either single vaccines or polyvaccines. The results from these experiments were promising since challenge with one strain demonstrated 100% protection both in fish vaccinated with the homologous serotype as well as a mixture of all the four serotypes.

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