DAO 35:187-194 (1999)  -  doi:10.3354/dao035187

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

Inger Dalsgaard1,*, Lise Høi2, Ronald J. Siebeling3, Anders Dalsgaard2

1Danish Institute for Fisheries Research, Fish Disease Laboratory, The Royal Veterinary and Agricultural University, Bülowsvej 13, DK-1870 Frederiksberg C, Denmark
2Department of Veterinary Microbiology, The Royal Veterinary and Agricultural University, DK-1870 Frederiksberg C, Denmark
3Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana, USA

ABSTRACT: Vibrio vulnificus was isolated in 1996 from 2 disease outbreaks on a Danish eel farm which used brackish water. A characteristic clinical sign was extensive, deep muscle necrosis in the head region. V. vulnificus was isolated from kidney, mucus, spleen, gill and intestine of diseased eels. Thirty-two 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 production. Virulence was evaluated by LD50 (the 50% lethal dose), which ranged from <9.4 x 103 to 2.3 x 105 CFU (colony-forming units) per fish. The isolates which were lethal for eels showed identical ribotypes and serotypes. A relationship between certain plasmids and virulence was not found. A serotyping system based on lipopolysaccharide (LPS)-associated O antigen type and on carbohydrate capsule antigens showed that the eel-virulent isolates 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 disease outbreaks that occurred on the Danish eel farm. Despite absence of antibiotic resistance, treatment had little effect and disease reoccurred.


KEY WORDS: Vibrio vulnificus · Fish pathogen · Identification · Virulence


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