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Chapter 20 : Vibrio salmonicida
Resembling a generalized hemorrhagic septicemia, coldwater vibriosis is characterized by hemorrhaging around the abdomen. Internally, anemia and hemorrhaging of the organs, swim bladder, abdominal wall, and posterior gastrointestinal tract may be seen. Using isolated macrophages from Atlantic salmon and rainbow trout coupled with immunofluorescence techniques, the pathogen was observed to be internalized. Cultures have been recovered from blood and kidney on tryptic soy agar supplemented with 1.5% (wt/vol) NaCl and incubation at 15ºC for up to 5 days. Cultures, which were subsequently named as a new species, Vibrio salmonicida, were recovered from diseased salmon. DNA hybridization of four cultures confirmed homogeneity (DNA homology was 82 to 100%) but with low relatedness to Vibrio anguillarum (30%), Vibrio ordalii (34%), and Vibrio parahaemolyticus (40%). Experimental challenge by means of intraperitoneal injection, immersion, and cohabitation of Atlantic salmon with V. salmonicida has led to the development of clinical disease with pathogenicity related to water temperature. The dihydroxamate siderophore bisucaberin was produced significantly only at ≤10°C, which is when the disease is most troublesome to Atlantic salmon. A proposal was made that temperature-sensitive iron sequestration could well constitute an important virulence mechanism for V. salmonicida. There has been success with the development of whole-cell formalized vaccines, leading to the commercialization of more than one polyvalent product. Immersion of Atlantic salmon in these formulations resulted in protection for at least 6 months, with humoral antibodies being developed to the lipopolysaccharide (LPS) component, particularly the O-side chain.