BACTERIAL GILL DISEASE

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Causative Agent(s)

This is a disease condition caused by exposure of fish to irritants that damage gill membrane, thus allowing opportunistic bacteria to invade the tissue. The primary cause are those environmental irritants, while the bacteria are secondary invaders. Most bacteria associated with this condition are Gram negative ones like Flavobacterium, Aeromonads or Pseudomonads. More common is Flavobacterium branchiophilium a yellow-pigmented, Gram-negative, nonmotile, rod-shaped bacterium.

Clinical Presentation and Epidemiology

The disease is multifactorial. One of the main predisposing factors of fish to this condition is ammonia in water as a result of fish metabolic activities. Affected fish at early stage, break the water surface to obtain mouthful of air. Grossly, the gills appear swollen and hyperaemic and protruding under the opercula. White to grey spots are observable on the affected gill. Infection may be unilateral or bilateral.

Diagnostic Consideration

Diagnosis is based on observed clinical signs, microscopic evaluation of affected gill. Epithelial hyperplasia, fusion of gill lamellae, and accumulations of long, thin Gram-negative bacteria may be seen during histopathologic examination. Isolation of the bacteria from diseased gills on Cytophaga agar (a selective culture medium) helps in giving definitive diagnosis.

Management and Treatment

Water quality must be improved by flushing out toxic irritants, then 1 hour bath exposure in 5mg KMnO4 /L solution per day in pond water and to be done for 3 days. This could be followed up with administration of broad-spectrum antibiotic like Amoxicillin or Oxytetracycline through feed for 5 to 7 consecutive days at 20mg/kg or 50mg/kg body weight respectively.