Coldwater Vibriosis in Salmonids

14 March 2003

Coldwater vibriosis, also called “Hitra” disease, is a problem in farmed salmon. It has caused heavy losses in Norway since the 1970`s. The disease is caused by the bacterium Vibrio salmonicida (V. salm).

Clinical signs are inappetence and erratic swimming. Pathology varies with stage of the disease process, but common external signs are pallor of the gills, haemorrhage on the fin base, redness and swelling. Internally, the fish is pale due to anaemia. V. salm is transmitted through the water from fish to fish.

Causative agent
Vibrio salmonicida is a Gram-negative, peritrichously flagellated, non-spore-forming, curved, rodshaped bacterium. The usual size is 1.5-1.8 µm by 1.0-3 µm. V. salm is a facultative, anaerobic, catalase and oxidase positive bacterium fermenting glucose with production of acid.
The surface layer antigen of V. salm is called the VS-PI antigen. This cell surface product is a single polypeptide, the monomeric form of which has an apparent molecular weight of 40 kD. There are also oligomeric forms with molecular weights in the range 300-700 kD. The antigen contains 6% carbohydrate and several isoelectric forms can be distinguished. Whether VS-P1 is related to the pathogenesis of V. salm is not known, but it is hypothesised that VS-P1 is released from the bacteria and binds specific antibodies, thus saving the bacteria from complement-mediated killing and phagocytosis. Whether plasmidal or chromosomal genes encode the virulence of V. salm or a combination of the two is not known. Some experiments indicate, however, that the virulence is completely or partly plasmid mediated.

Prevention and Control

General methods
Control of the described disease is best achieved by maintenance of water quality, good husbandry and low stocking densities. However, this is not always possible and, where outbreaks occur, treatment with antibiotics is the only option.  In areas where a disease is not endemic, it is possible to exclude the causative agents by a legislative policy such as (1) restrictions on importation/movement of live fish/eggs and (2) slaughter and disinfection in infected fish farms.  

Antibiotics
Treatment of established infections with antimicrobial compounds has been and will be extensively used for the control of many infectious diseases. However, its value is limited since clinically affected fish do not eat and therefore cannot be well treated. A successful treatment is dependent on a rapid diagnosis and immediate treatment.

Vaccination
Vaccination has proven to be an efficacious method in preventing many bacterial diseases, including furunculosis, vibriosis and coldwater vibriosis. Vaccines are preparations of inactivated antigens derived from pathogenic organisms, which will stimulate the immune system to increase the resistance to disease from subsequent infection by a pathogen.
The method of choice for prevention of the disease is therefore vaccination with inactivated whole cells or subunits and an appropriate adjuvant for enhancing the immune response.