Plasmids in Vibrio salmonicida isolated from salmonids with hemorrhagic syndrome (Hitra disease)

Vibrio-like isolates from Atlantic salmon (Salmo salar Linnaeas) and a few from rainbow trout (S. gairdneri Richardson) suffering from hemorrhagic syndrome (Hitra disease), also called cold-water vibriosis, a disease of great importance in Norwegian fish farming, were examined for plasmid content. Of 84 strains isolated from 1982 to 1984, 70 (83.3%) had a common 21-megadalton (MDa) plasmid. A 3.4-MDa plasmid was found in 58 of the strains with the 21-MDa plasmid, and a 2.8-MDa plasmid was found in 23 of the strains with both the 21- and 3.4-MDa plasmids. The strains were isolated from fish farms along the western and northern coasts of Norway. Ten (11.9%) of the strains possessed a 61-MDa plasmid in addition to a 21-MDa plasmid. Two strains had only a 21-MDa plasmid. Of the 84-Vibrio-like isolates, 14 did not harbor plasmids identical in mass to any other plasmids found in this material. Vibrio salmonicida strains, 257 in all, isolated from salmonids with the same disease from the same area from July 1986 to July 1987, all possessed a 21-MDa plasmid, either alone or in addition to a 3.4-MDa plasmid, or a combination of 3.4- and 2.8-MDa plasmids. Six of the strains had a 5.5-MDa plasmid instead of the 3.4-MDa plasmid. The restriction endonuclease patterns of the plasmids of similar molecular mass reflected similar nucleotide sequences. The plasmid content detected in isolates of V. salmonicida obtained from a coastline of more than 2,000 km and over a period of almost 6 years is stable.