INTRODUCTION

Vibriosis is a bacterial disease of salt-water and migratory fish, and the severity of vibriosis has increased proportionately with the development and expansion of fish farming world-wide. It has a global distribution with epizootics on all continents and a wide range of fish. The disease causes significant losses in cultured Pacific salmon, Atlantic salmon and Rainbow trout and in cultured non-salmonids including eel, yellowtail, red sea bream and sea bass. The main species causing the disease is Vibrio anguillarum (Smith, 1988). 

Clinical signs of the disease are haemorrhage at the bases of the fins, around the vent and gills and inside the mouth.  Petechiae, necrotic lesions and diffuse haemorrhages can appear on the body surfaces. The intestine is often inflamed with petechiae present on the viscera and musculature. The intestine may be distended and filled with clear viscous fluid.

Vibriosis is a stress-related disease and is associated with a number of factors such as high water temperatures, rapid changes in water temperature or salinity, over-crowding or poor water quality. Outbreaks of vibriosis mainly occur in spring and autumn (Smith, 1988). 

ETIOLOGY

Vibrio (Listonella) anguillarum is characterised by a vibrionic or comma shape bacteria. It is a Gram-negative, fermentative, oxidase-positive, motile bacterium, which requires sodium ions for growth and is sensitive to the pteridine vibriostat 0/129 (Hjeltnes & Roberts, 1993).

The antigens are heat stable lipopolysaccharides in the cell wall. Various proteins in the outer membrane were also found to be antigenic (Chart & Trust, 1984).

Kodama et al., 1984 showed that extracellular products such as toxic materials, haemolysin, and protease, as well as other undefined materials, may contribute to the pathogenesis of fish vibriosis.

More than twenty different serovars of V. anguillarum (designated O1 to O23) have been described (Pedersen et al., 1999). Serovars O1 and O2 occur world-wide and are those most often found in connection with diseases in fish (Toranzo, 1997), particularly in salmonids and species of cod fish.