Atypical Vibiriosis in Chile

22 May 2004

During the winter and spring of 2003, Atlantic salmon (Salmo salar) reared in sea and brackish water net pen sites on the coast of Southern Chile were affected by several unusual disease outbreaks. The first clinical cases were reported by a private Chilean fish health lab, ADL Diagnostic Chile Ltda., who first described the clinical signs of the disease and the bacterial genus. The external clinical symptoms were characterized by the presence of lesions in skin, haemorrhaging ulcers and mortality of the affected fish, while the internal signs were pericarditis, peritonitis and multiple necrotic foci in the liver, and signs of systemic septicaemia. The mortality level of fish at different farms ranged from 2 to 22% before treatment with antibiotics was instigated. In all clinical cases, bacteria were isolated from the lesions and internal organs of the affected fish and, by using conventional bacteriological identification assays, a Vibrio sp. was suspected to be the agent of the disease. Vibrio isolates have been described in Chile prior to these new clinical cases, but they have so far not been related to disease outbreaks of farmed fish. At present, the disease seems to cause mortality in Atlantic salmon in hatcheries that use seawater to optimize smolting and in Atlantic salmon during the period from sea transfer until a size of 1 kg; however, the problem will most likely become bigger in the future as bacterial diseases typically affect fish of all sizes. Indeed, there are already reports of the bacterium being found in 4- to 5-kg Atlantic salmon, but without causing mortality. At present, sick fish are treated with antibiotics; a good response is observed if it is applied in the early stages of the disease outbreak. The bacteria seem to be sensitive to the various antibiotics registered for fish in Chile. 

In order to obtain the final identification of the agent, Intervet initiated a co-operation programme with ADL, which provided Intervet’s R&D Centre in Bergen (Norway), Intervet Norbio, with the bacterial isolates collected from the field outbreaks. Two researchers from ADL spent a week working with the bacteriological team of Intervet Norbio, with the bacterial isolates collected fromthe field outbreaks. Two researchers from ADL spent a week working with the bacteriological team of Intervet Norbio in order to get familiarized with the laboratory methods employed. The final identification of the bacterial isolates was made after amplification and characterization of the 16S rRNA of the isolates using polymerase chain reaction (PCR), followed by comparison of the nucleotide sequence with previously submitted bacterial 16S rRNA sequences in a gene bank. The results from these analysis showed that it was possible to identify the bacterium as belonging to the group Vibrio ordalii.