Diseases of wild and farmed Finfish

Listonella (Vibrio) anguillarum causes a haemorrhagic septicaemia involving salmonids in salt and brackish waters, particularly during the summer months. The bacterium is a motile rod-shaped organism that stains Gram negative. L. anguillarum may occur among the normal gut microflora of healthy fish and clinical outbreaks triggered by poor water quality and rapid temperature changes.

External signs of disease include dark skin coloration, pale gills, swollen vent and haemorrhage near the base of the fins. Extensive liquefactive muscle necrosis and haemorrhage is common. Dermal or subdermal skin lesions can be hyperaemic and haemorrhage, and may be linked to ‘boil’ lesions in the muscle. These may rupture and release blood and bacteria to the surrounding water. 

Internally, the liver is generally swollen and some fish show petechiae. Colonies of the bacterium are also found throughout the digestive tract and in the gills. Bacteria can also be found at the back of the eye, followed by corneal lesions and ulceration. Splenomegaly is common and even rupture of the organ may occur, particularly in rainbow trout.

Bacteriology

Vibriosis is a disease of bacteria belonging to the genus Vibrio.  Listonella (Vibrio) anguillarum is most commonly associated with the disease and has been the most devastating in the marine aquaculture industry.  Clinical signs in fish include septicaemial haemorrhaging on the base of the fins, exopthalmia and corneal opacity.  Moribund fish are frequently anorexic with pale gills which reflects a severe anaemia. L. anguillarum occurs as the normal gut microflora of healthy fish and clinical outbreaks are triggered by poor water quality and rapid temperature changes.  Many different phenotypes and serotypes of the bacterium exist however O1, O2, O3 from the O serogroup are those responsible for causing mortalities in fish (Austin et al. 1995)  Vaccinations aid the prevention of Listonella anguillarum.  Vibriosis mainly occurs 15ºC - 21ºC.

Allivibrio salmonicida is the causative agent of coldwater vibriosis (or Hitra disease).

Impacts of the disease are most severe at low temperatures in winter.  Clinical signs include darkening, exopthalmus, a swollen vent and pinpoint haemorrhaging along the belly and bases of fins.   Multivalent vaccines, protecting against furunculosis, vibriosis and coldwater vibriosis are widely used and are effective however outbreaks do still occur and antibacterial treatments are still required.

Many other vibrio species have been associated with disease in fish and shellfish.  Vibrio woodanis, Vibrio splendidus, Vibrio aestuarianus are other members of the genus that have been associated with the disease in fish and shellfish.

Diagnosis

• Bacteriological culture and biochemical testing
• Histopathology
• Molecular confirmation