Introduction

Vibrosis 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 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 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.

Aetiology
Vibrio anguillarum

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. Before this report, described epizootics in migrating eels (Anguilla vulgaris) dating back to 1817 that 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).

Vibrio anguillarum 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.

Epidemiology

Outbreaks affecting close to 50 species of fresh and salt-water fishes have been reported in several countries in the Pacific, as well as the Atlantic coasts. The losses produced by this disease are so disastrous that vibriosis caused by V. anguillarum has been recognized as a major obstacle for salmonid marine culture.

Diagnosis
Clinical Diagnosis

The characteristic clinical signs of vibriosis include red spots on the ventral and lateral areas of the fish and swollen and dark skin lesions that ulcerate, releasing a blood exudate . There are also corneal lesions, characterized by an initial opacity, followed by ulceration and evulsion of the orbital contents. However, in acute and severe epizootics, the course of the infection is rapid, and most of the infected fish die without showing any clinical signs.