Bacterial cold water disease
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Bacterial cold water disease (BCWD) is a bacterial disease of salmonid fish. It is caused by Flavobacterium psychrophilum (previously classified in the genus Cytophaga), a gram-negative rod-shaped bacterium of the family Flavobacteriaceae. The disease typically occurs at temperatures below 13⁰C, and it can be seen in any area with water temperatures consistently below 15⁰C. Salmon are the most commonly affected species. This disease is not zoonotic.
BCWD may be referred to by a number of other names including cold water disease, peduncle disease, fit rot, tail rot and rainbow trout fry mortality syndrome.
Fish infected with typical BCWD have lesions on the skin and fins. Fins may appear dark, torn, split, ragged, frayed and may even be lost completely. Affected fish are often lethargic and stop feeding. Infection may spread systemically. Salmonid fish can also get a chronic form of BCWD following recovery from typical BCWD. It is characterised by erratic “corkscrew” swimming, blackened tails and spinal deformities.
A presumptive diagnosis can be made based on the history, clinical signs, pattern of mortality and water temperature, especially if there is a history of the disease in the area. The organism can be cultured for definitive diagnosis. Alternatively, histology should show periostitis, osteitis, meningitis and ganglioneuritis.
Quaternary ammonium compounds can be added to the water of infected adult fish and fry. Alternatively, the antibiotic oxytetracycline can be given to adults, fry and broodstock. To prevent the disease, it is necessary to ensure water is pathogen-free and that water hardening is completed effectively for eggs.
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