HAEMORRHAGIC SEPTICAEMIA

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Causative Agent(s)

The causative organism is Aeromonas hydrophila, a Gram-negative rod, motile, oxidase positive, indole +ve, citrate +ve, catalase +ve, ferments sugars like glucose and sucrose and variably ferments lactose and grows well at 37°C and without NaCl in culture media. It grows well on Tryptone Soy Agar, Blood agar and on MacConkey agar with variable degree of lactose fermentation.

Clinical Presentation and Epidemiology

This disease condition, also referred to as motile Aeromonas septicaemia is among the most common bacterial disease observed in farmed fish (Austin and Austin, 2007). It affects all ages and mortality in catfish hatchery can be up to 100%, within 3-4 days from onset of clinical signs. Affected fishes are usually seen to be lethargic, anorexic and in severe cases may be seen hanging by the side of the pond or in lateral recumbency when seen on the floor of fish tanks.

Fishes often exhibit exophthalmia, erythema and ulceration of the skin and may have distended abdomen filled with ascitic fluid. The liver may be slightly greenish or have a light chocolate colour appearance with necrotic patches. The kidney often may be inflamed and friable. This disease is worldwide in occurrence probably due to the ubiquitous nature of the pathogen in water.

Diagnostic Consideration

Diseased organs like liver, kidneys and spleen are harvested from moribund fishes and fresh carcasses during post-mortem examination for isolation of the pathogen on 5% blood agar and MacConkey agar. The isolate is subjected to Gram staining and various biochemical tests for identification.

Management and Treatment

It is important to identify the source of outbreak and ensure control from such points. Control of this disease is not limited to the use of antibiotics. Antibiotics can be administered through feed or by bath. The use of antibiotics should be based on outcome of antibiotic susceptibility test. For example, Amoxicillin could be administered via feed at 40mg/kg body weight for 5 to 7 consecutive days.

There may be need to step up other biosecurity measures like routine disinfection, use of 0.5- micron filters in hatcheries, in combination with U.V. light, (especially where microbial load

 

from water source is very high or too numerous to count (TNTC)), ensuring that microbial analysis of fish feed is done, especially where outbreaks were noticed after a change in type of feed.

Another very important specie of Aeromonas that causes disease in a large host of fish species is Aeromonas salmonicida which causes boil-like lesions in a condition called Furunculosis. Beyond the use of antibiotics, this condition is currently well controlled by vaccination in some countries like Norway.