Flavobacterium columnare (previously named as Flexibacter columnaris)
Ayu (Plecoglossus altivelis), tilapia (Oreochromis niloticus), carp (Cyprinus sp.), channel catfish (Ictalurus punctatus), goldfish(Carassius auratus), rohu (Labeo rohita)
The first indication of infection is generally the appearance of a white spot on some part of the head, gills, fin or body. A zone with a distinct reddish tinge usually surrounds this. The lesions are circular as if spreading from a single focus towards all directions at the same rate. On the gills, the lesions are more necrotic. On the skin, they develop into hemorrhagic ulcers, with an overlying seroma of bacterial cell and necrotic tissue. Histologically, there is epidermal spongiosis, necrosis, and ulcerations.
EFFECTS ON HOST:
F. columnare is an opportunistic pathogen widely distributed in the water. Thedisease does not usually occur as spontaneous infection but results from inju-ries to the fish, or physical and nutritional deficiencies. Outbreaks are affected by factors such as temperature and stress. Dissolved cations such as sodium, potassium, calcium and magnesium enhance their infectivity. F. columnare at-tacks fish primarily through the gills or abraded epidermal areas. Once the pathogen is established, proteolytic enzymes break down the skin and muscle to open necrotic lesions. The bacterium appear systematically after extensive tissue necrosis. Gill lesions may cause respiratory difficulty and the fish eventu-ally dies. Body lesions are subject to secondary infection by other microorgan-isms. Fish that survive the infection may become carriers of the disease.
Columnaris disease can be presumptively diagnosed from disease signs on the skin and gills of the host and from squash preparations made from scrapings of the affected areas. In wet mount preparations of infected tissues, the bacteria show a slow gliding movement, gathering into characteristic column-like masses that give the disease its name. Microscopic examination of lesions shows the presence of long, slender, possibly filamentous, rod-shaped, gram-negative bacteria. The growth of F. columnare on solid media is usually charac-terized by yellow-green, flat and rough spreading colonies that adhere to the media. F. columnare can also be detected from fish and water using indirect fluorescent antibody technique (IFAT).
PREVENTION AND CONTROL:
• environmental manipulation like lowering water temperature
• addition of competitive bacteria like Citrobacter fecundii and Aeromonashydrophila
• copper sulfate dip at 40 mg/L for 20 min or 500 mg/L for 1 min
• oxolinic acid dip at 1 mg/L for 24 h
• sulphamerazine and oxytetracycline at 220 mg/kg/day for 10 days fol-lowed by 50 to 75 mg/kg/day for 10 days.
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