The Disease

Introduction
Vibriosis is one of the most prevalent fish diseases caused by bacteria belonging to the genus Vibrio. Vibriosis caused by Vibrio anguillarum has been particularly devastating in the marine culture of Salmonid and Perciformes fish.

Vibriosis occurs in cultured and wild marine fish in salt or brackish water, particularly in shallow waters during late summer. It was originally believed that scavenger fish feeding around the farms were the natural reservoir of V. anguillarum, and contact between fish seems to be an important factor for the spread of this pathogen. However, there is evidence that V. anguillarum is normally present in the intestinal microflora and food of cultured and wild healthy fish. The temperature and quality of the water, the virulence of the V. anguillarum strain and stress on the fish are important elements influencing the onset of disease outbreaks.

Etiology
The bacterium Vibrio anguillarum is a polarly flagellated, gram-negative, curved rod. The causative agent, of this Vibriosis disease: V. anguillarum, was first described in 1909 as the aetiological agent of the 'red pest of eels' in the Baltic Sea. An earlier report from the early 1800's, describing epizootics in migrating eels (Anguilla vulgaris) implicated a bacterium named Bacillus anguillarum. The pathology of the disease and the characteristics of the bacterium in these two reports suggested that the etiological agents were the same.

Vibriosis was not reported in North America until 1953, when V. anguillarum was isolated from chum salmon (Oncorhynchus keta).

Vibrioanguillarum belongs to one of the halophilic groups of Vibrios and survives at different salinities. Studies have shown that it is able to survive in sea water for more than 50 months.

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.