VIBRIOSIS

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Causative Agent(s)

This is a bacterial disease condition caused by various species of Vibrio in fish. The most important species is Vibrio anguillarum. Some other species of Vibrio like Vibrio alginolyticus and Vibrio parahemolyticus also cause disease in freshwater fish. These organisms are Gram negative rods, motile, facultatively anaerobic, oxidase positive, catalase positive, and indole positive. V. parahemolyticus and V. alginolyticus grow well at 37°C in culture media like TCBS agar, 3% NaCl blood agar and TSA. These 2 species ferment glucoses but not lactose and both are citrate +ve. On TCBS agar, V. alginolyticus and V. parahemolyticus appear as yellow and green colonies, respectively. Catfishes that were experimentally infected with Vibrio alginolyticus, were shown to have marked hyperaemia of gastric mucosa (Oladele et al., 2019).

Clinical Presentation and Epidemiology

Vibrio anguillarum causes outbreaks more in marine/saltwater fishes than in freshwater. Transmission of infection is oral as organism naturally resides in fish intestine but could be influenced by stress to induce infection. Transmission could also occur through injury. Disease is septicaemic with presence of the pathogen in the blood, liver, spleen, and kidney of sick fish.

It is an acute, subacute, or chronic bacterial disease affecting both marine and freshwater fishes. It is characterized by large sores on the skin and muscles arising from rupture of boil- like lesions (like that of furunculosis). Egidius et al. (1981) described this as ‘Hitra disease” of fish.

The clinical signs and lesions observed include

erythema of base of fins, mouth, grooves of lower jaw, opercula and round the anus. Ulcerative dermatitis and myositis are common features resulting from rupture of boil-like lesions observed at early stage of the disease.

During post-mortem examination, petechial haemorrhages are seen on the peritoneum with hyperaemia of visceral organs. Muscles show petechiation and in some cases, large abscesses.

Diagnostic Consideration

Affected organs observed during necropsy are harvested (e.g., liver, kidney, spleen) and swabs from abscesses in the skeletal muscles are subjected to microbial analysis for isolation and identification of the pathogen Vibrio anguillarum. Several biochemical tests are conducted for confirmation. Organism grows well on Thiocitrate bile sucrose agar (TCBS) which is selective for Vibrio species. It also grows well on 5% blood agar.

Management and Treatment

Vibriosis is enzootic, but situation can be controlled. Stress should be avoided in fish and water quality should be monitored and maintained in good condition. Drugs like oxytetracycline (50 - 75 mg/kg body wt.) could be used for treatment via feed for 5 to 7 days.

In some parts of the world, there is a specific vaccine administered to fishes to prevent this condition.