Cold Water Disease
Also Known As: Fin Rot — Peduncle Disease
Caused by: Flavobacterium psychrophilum previously known as: Cytophaga psychrophila — Flexibacter psychrophilus
Introduction
Flavobacterium psychrophilum is a gram negative rod shaped bacterial pathogen, known for causing Cold Water Disease (CWD) in salmonid fish. The disease typically occurs at temperatures below 13⁰, and is most serious and prevalent below 10⁰C.
This disease is not zoonotic and poses no public health risk.
Distribution
CWD is common in any area where water temperatures are consistently below 15⁰C. This includes North America, the UK, Europe and parts of Oceania.
Asymptomatic carrier fish and contaminated water provide reservoirs for disease. Transmission is mainly horizontal from these sources. Vertical transmission is also possible, via ovarian fluid, milt, egg surfaces and mucus.[1] Eggs can also be experimentally contaminated and infected.
Signalment
Most fishes worldwide are susceptible and the organism is ubiquitous.
Cultured salmon species are the most frequently affected: brook trout, rainbow trout, brown trout, lake trout, walleye, whitefish, carp, dace and suckers.
Clinical Signs
Fish infected with typical CWD have external skin lesions, often on the dorsum, lose their fins and may also have severe erosion of the caudal fin. Fins may appear dark, torn, split, ragged, frayed or have haemorrhagic red patches/spots visible. Affected fish are often lethargic and stop feeding.
Internal, systemic infection can also be a sequel of CWD.
In Rainbow Trout Fry Syndrome, acute disease is usual and mortality can approach 60% which is devastating in the commonly affected populations of tens of thousands of young fish. Shortly before death, fish are lethargic, have darkened skin, inappetance and demonstrate exopthalmos.
Salmonid fish can also acquire a chronic form of CWD characterised by erratic “corkscrew” swimming, blackened tails and spinal deformities.[2] This may follow recovery from typical CWD.
Diagnosis
Diagnosis is often presumed from history, clinical signs, pattern of mortality and water temperature etc, especially if the location has previously encountered CWD.
Cultures can be made from smears or samples of lesions and the organism cultured on reduced nutrient agar, e.g. cytophaga agar, forming yellow creamy non-adhered colonies in 2-4days.
Histologically, periostitis, osteitis, meningitis and ganglioneuritis may be observed. In chronic disease cases, bacterial cells can accumulate in the cranial and vertebral areas causing inflammation and cartilage necrosis which leads to the spinal deformities noted.
Treatment
Quaternary ammonium compounds can be delivered in a static bah or flow through system for infected adult fish and fry.
Terramycin can be added to feed as an alternative for adults, fry and broodstock. This can also be used preventatively. Some resistance is emerging however.
Control
Ensuring water is pathogen-free and that water hardening is completed effectively for eggs is imperative.
Prophylactic antibiotic use should be carefully considered before implementation due to emerging resistance.
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References
- ↑ Brown, L. L., Cox, W. T., Levine, R. P (1997) Evidence that the causal agent of bacterial cold-water disease Flavobacterium psychrophilum is transmitted within salmonid eggs. Diseases of Aquatic Organisms, 29(3):213-218
- ↑ Blazer, V., Stark, K., Starliper, C (1996) Unusual histologic manifestations of Flexibacter psychrophila in hatchery salmonids. In: 21st Annual Eastern Fish Health Workshop. Gloucester Point, Virginia, 5-6 September, 10
Animal Health & Production Compendium, Flavobacterium psychrophilum datasheet, accessed 10/07/2011 @ http://www.cabi.org/ahpc/
Animal Health & Production Compendium, Cold Water Disease datasheet, accessed 10/07/2011 @ http://www.cabi.org/ahpc/