Indole-positive Vibrio vulnificus isolated from disease outbreaks on a Danish eel farm

Vibrio vulnificus was isolated in 1996 from 2 disease outbreaks on a Danish eel farm which used brackish water. A characteristic clinical sign was ext ensive, deep muscle necrosis in the head region. V. vulnificus was isolated from kidney, mucus, spleen, gill and intestine of diseased eels. Thirty-tw o isolates were examined phenotypically and serologically for pathogenicity to eels and for correlation to ribotype and plasmid profile. Biochemically , the isolates showed properties similar to those described previously for eel-pathogenic strains of V, vulnificus, with the exception of indole produ ction. Virulence was evaluated by LD50 (the 50 % lethal dose), which ranged from <9.4 x 10(3) to 2.3 x 10(5) CFU (colony-forming units) per fish. The isolates which were lethal for eels showed identical ribotypes and serotype s, A relationship between certain plasmids and virulence was not found. A s erotyping system based on lipopolysaccharide (LPS)-associated O antigen typ e and on carbohydrate capsule antigens showed that the eel-virulent isolate s shared a common LPS-based homogeneous O serogroup and a capsule antigen. V. vulnificus serovar O4 and capsule type 9 was identical serologically to the Japanese isolate ATCC 33149 and was the agent responsible for the disea se outbreaks that occurred on the Danish eel farm. Despite absence of antib iotic resistance, treatment had little effect and disease reoccurred.