Fish Diseases and Management

5.1.4.1 Edwardsiella septicaemia

This is a mild to severe systemic disease of primarily warm water fishes in the U.S.A and Asia. Causative agent is Edwardsiella tarda, a typical enteric bacterium. It is a Gram negative motile rod. Normally, this disease occurs in the summer months with an optimum temperature of 30⁰ C. The source of E. tarda is presumably intestinal contents of carrier animals. In Taiwan and Japan, it is one of the most serious bacterial disease of cultured eels.

Control of this disease is by application of antibiotic medicated feed. Any approved drug can be used. Enteric septicemia of cat fish is caused by E. ictaluri. It is a highly contagious systemic disease. This disease is also called as the hole-in-the-head due to the presence of an open lesion appearing in the skull of some chronically ill fishes. E. ictaluri is a typical member of the Enterobacteriaceae; Gram negative, cytochrome oxidase-negative rod and weakly motile at 25⁰ C – 30⁰ C. The diseased fish hang listlessly at the surface with a head-up-tail down position, sometimes spinning rapidly increased, usually followed by death.

Externally petechial hemorrhage or inflammation occurs in the skin under the jaw on the opercula and belly. These lesions often become so numerous that the skin is bright red. Inflammation and haemorrhage also occurs at the base of all fins. In chronically infected fish, an open lesion will develop between the frontal bones of the skull, posterior to or between the eyes. Affected fish have pale gills, exophthalmia and often an enlarged abdomen. Internally, the body cavity may be filled with a cloudy, bloody or rarely a clear yellow fluid. The kidney and spleen are enlarged and spleen is dark red. Inflammation occurs in adipose tissue, peritoneum and intestine and the liver is either pale or riddled with congestion.

The only control is through the use of antibiotics incorporated into the feed. Oxytetracycline and a potentiated sulfonamide are the only drugs currently in use. Since the appetite of the fish in lost after some time the medicated feed should be given early in the epizootic.