Bacterial Coldwater Disease

Synonyms

  • Rainbow Trout Fry Syndrome (RTFS)
  • Bacterial fry anemia
  • Low temperature disease
  • Peduncle disease
  • Saddleback
  • Tail-rot
  • Systemic myxobacteriosis

Cause of the Disease

Bacterial Coldwater Disease (BCWD) is caused by Flavobacterium psychrophilum, also known as Flexibacter psychrophilus and Cytophaga psychrophila. There may be strain differences among isolates but their importance from a fish health management perspective has not been demonstrated. Because of problems with isolating

the causative agent, correctly diagnosing the disease and differentiating F. psychrophilum from other closely related and concurrent diseases, there is a lot of confusion and misinformation regarding BCWD.

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Host range, distribution, and Occurrence

BCWD is a serious disease of all species of salmonid fish. It is also known to cause disease in carp, tench, chub, and eel. It is known only to infect fish in fresh water. The disease does not appear to be host specific and may infect many aquatic vertebrates. BCWD is particularly virulent at water temperatures below 12 C. (54 F.). However, untrue to its name, the disease is known to cause significant losses at temperatures up to 16 C. (61 F.)

BCWD was first described in 1946 as a disease of trout and salmon in the United States. Today it is found in many areas of the world including North America, Europe, Scandinavia, Japan, Southeast Asia, Australia and Chile.

BCWD infects all life stages of fish from gametes and eyed eggs through to broodstock. The disease is most acute in eggs, fry and fingerlings and tends to be more chronic in larger fish. Mortality typically begins within 5 to 10 days following infection and peaks 20 to 60 days later, depending on water temperature and fish age. The disease is known to recur, particularly following stress or handling; such as during smoltification, grading, sorting, transport, vaccination, marking, etc. Total cumulative mortality is typically 5% to 20% but mortality of up to 90% has been noted. BCWD is often observed to precede or occur in association with other diseases such as IPN, IHN and VHS virus. mortality is typically 5% to 20% but mortality of up to 90% has been noted. BCWD is often observed to precede or occur in association with other diseases such as IPN, IHN and VHS virus.

Clinical Signs, Diagnosis and Confirmed Identification

The clinical signs of BCWD vary with fish size and water temperature. In eggs and sac fry, the disease can cause softening of the shell, premature hatch, and rupture of the yolk sac. The most common form of BCWD is a sub-acute to acute infection in fry and fingerlings. Infected fish typically stop feeding, become lethargic and rest near the bottom, wall or outlet screens. Diseased fish often have a distended abdomen filled with fluid and bilateral exophthalmia, a condition in which both eyes protrude from their sockets. Due to severe anemia, the gills, kidney and spleen are often pale and at times almost white. Dead fish

typically accumulate on the bottom and screens with their gill covers flared, their bodies bent in a comma shape and the skin color darkened towards the tail. When the brain becomes infected, particularly in fry, there may also be a soft hemorrhagic bulge on the top of the head and affected fish will swim erratically or in a spiral motion.

In larger fish and at water temperatures above 12 C., BCWD tends be more chronic. It often begins as “finrot”, particularly on either the dorsal, peduncle or caudal fins. As the infection progresses to the base of the fin, strong proteolytic enzymes erode the skin and muscle until even the backbone becomes exposed.

The most chronic form of the BCWD occurs in larger fish or in fish that have survived an earlier outbreak. In such cases, F. psychrophilum tends to infect only the brain or a short portion of the spine. When the brain is infected, the fish lose equilibrium but may not die. When F. psychrophilum infects the spine, some hemorrhage may be noted but the fish

otherwise appear normal and survive to harvest. The infected portion of the spine stops growing while the associated muscle around it continues to grow. Months later, this can result in a grossly deformed fish. Such fish often have to be culled or discarded and can result in significant economic loss. It should be noted that there are various causes of spinal deformities in fish, but BCWD should be considered whenever this is a problem.

BCWD can be difficult to detect and differentially diagnose. Whenever BCWD is suspected, typical diseased fish should be submitted to a qualified laboratory. Presumptive diagnosis is made on the basis of the gross clinical signs together with the observation of typical bacteria in tissue smears. Characteristic long thin (0.3-0.7 X 2.0-7.0 um) filamentous, Gram-negative, non-flagellated bacteria are readily seen in the spleen, kidney, blood, brain or tissues taken from the edge of necrotic lesions. Presumptive diagnosis on the basis of stained smears can often be made within a few minutes, even in the field, and can be justification for beginning treatment.

For clinical isolation and confirmed identification, the same tissues should be streaked for single colony isolation on Tryptose Yeast Extract Agar supplemented with 5% fetal calf serum and incubated for 7 to 10 days at 15 to 20 C. Typical colonies are small (0.5 to 2.0 mm dia.), mucoid, smooth, round, convex, and entire and are easily recognized by their distinct yellow to orange color. The identity of the isolate should be confirmed by plate agglutination with polyvalent antisera or other validated confirmatory method.

Reservoirs of Infection and Transmission

F. psychrophilum is carried by clinically diseased fish as well as asymptomatic survivors that shed the bacteria throughout their lives. BCWD has also been found in wild fish and amphibians, which may also serve as reservoirs of infection. The disease easily spreads from fish to fish through the water. The primary site of infection is the gills and fins,

particularly when these tissues have been previously damaged by other infectious diseases or environmental conditions. F. psychrophilum can also be transmitted by infected broodstock. Because the bacterium can occur inside the egg, water-hardening eggs in an iodophor disinfectant does not eliminate the risk of vertical transmission.

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Prevention, Treatment and Control

If BCWD has never been found on your farm, every precaution should be taken to prevent its introduction. The best way to reduce risk of infection is by only restocking your farm with eyed eggs obtained from a reputable supplier like Troutlodge. It is strongly recommended that eggs, fry and fingerlings be reared only on closed water supplies such as wells (bore holes) or covered springs which are free of resident fish or amphibians that can carry F. psychrophilum.

There is no evidence of natural resistance to BCWD in any particular strain of fish and no attempts have been made to breed for disease resistance. There are currently no licensed vaccines available for the prevention of F. psychrophilum. Experimental vaccines are being tested and preliminary results are promising, but effective commercial products are at least a few years away.

F. psychrophilum infects fish externally, internally and even intracellularly. This means that chemicals or antibiotics added to the water may be of limited value in treating BCWD disease. For effective control, external treatments usually have to be combined with an antibiotic in the feed. It is recommended that you first consult with your veterinarian or other fish health professional for advise in developing an effective fish health management strategy and control plan consistent with the laws and regulations in your area.

Since BCWD often recurs at the same stage in the production, it may be advisable to feed a medicated feed immediately prior to the anticipated outbreak of the disease. One of the most common treatments for BCWD is a combination of one-hour flush treatments using copper sulfate, Chloramine- T™ or a quaternary ammonium disinfectant such a Hyamine while feeding a diet medicated with Terramycin™ to deliver 150 mg active ingredient per kilogram of fish per day for 10 to 14 days. In addition to Terramycin™, Romet™, oxolinic acid,

Sarafin™, Florfenicol™ and amoxycillin have all been used with varying degrees of success. However, since F. psychrophilum seems to develop resistance to antibiotics quickly, the relative antibiotic sensitivity of an isolate should be determined, particularly if the disease is not responding to treatment.

Regulatory Status & Public Health Importance

BCWD is not generally considered to be a disease of concern by regulatory authorities and F. psychrophilum is not listed by the O.I.E. Currently, there are no accepted or validated protocols for surveillance, inspection or certification of fish, eyed eggs or gametes. However, Troutlodge periodically tests their stocks for F. psychrophilum as part of its routine fish health management program.

F. psychrophilum poses no known health risk to humans nor any animals other than freshwater fish and possibly amphibians. There are no known restrictions on processing or selling infected fish and no reason for concern from a public health perspective.

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References

Cipriano, R.C. et at. 1996. Epizootiological study of bacterial cold-water disease in pacific salmon and further characterization of the etiologic agent, Flexibacter psychrophila. Journal of Aquatic Animal Health 8:28-36.

Thoesen, J. (Ed). 1994. Suggested procedures for the detection and identification of certain finfish and shellfish pathogens. 4th ed., Fish Health Section, American Fisheries Society, Bethesda, MD, US