Channel Catfish Virus

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Also Known As: CCV — Channel Catfish Viral Disease — CCVD

Introduction

Channel Catfish - the primary species affected by CCVD. Wikimedia Commons

Channel Catfish Viral Disease (CCVD) is caused by a herpesvirus and causes significant economic losses in channel catfish farming. High mortalities in fry and young fish are the biggest direct loss.

Distribution

CCVD is endemic in the USA, where at least two catfish farms have been closed partially because of the virus. Imported fish may have also introduced the disease to Honduras and Russia. CCVD is also present in the UK.

Reservoirs of disease are clinically affected fish and recovered covert carriers – see below for more details.

Transmission is both horizontal and vertical. Horizontal transmission can be direct or via water.

Signalment

Channel catfish and the closely related blue catfish are the only fish known to be susceptible to CCVD. Natural outbreaks are seen almost exclusively in cultivated channel catfish. The virus has not been reported from wild channel catfish or in any other wild catfish.

Disease occurs almost exclusively in fish less than a year old, and is most common in those under 4 months of age. Natural clinical disease is generally confined to fish weighing less than 10g.

The critical environmental factor is water temperature, with mortality highest above 27⁰C and zero when below 18⁰C. Thus, disease occurs during warm weather in May to September.

Clinical Signs

Affected fish have marked ascites giving them a pot-bellied appearance, show exopthalmos and have multiple haemorrhages of the fins and musculature. The vent may be swollen and protrusive and the gills pale. Opercular movements are often increased. Affected fish may seem to be off balance, swimming erratically or close to the surface and eventually sinking to the bottom. Up to 50% of moribund fish may hang head upwards at the surface.

Fish that survive CCVD have strong protective immunity for the remainder of their lives but remain in a covert latent carrier state which is undetectable antigenically and difficult at best. This is a significant source of disease for vulnerable fish. It is unclear how these fish shed the virus.

Diagnosis

Diagnosis of CCVD is generally based on isolation of CCV in cell cultures from infected fish followed with its immunological and/or acid-based identification such as using PCR.

Increased fry or fingerling mortality in catfish in warm weather, especially after stress, should raise suspicion of CCVD.

CCV can be detected in water containing infected fish and organs of diseased fish, particularly the kidneys. All viscera should be taken from fish of 4-6cm, whole fish if smaller and kidney, spleen and encephalon if larger than 6cm. All should be transported cooled or on ice but not frozen. The virus can be isolated in cell culture and identified by virus neutralisation, fluorescent antibody testing (FAT), ELISA and PCR. FAT and ELISA are best for diagnosis of clinically infected fish while carrier fish can only be detected by neutralisation or PCR. The formation of syncytia in ovary cell cultures is also specific for CCV.

At necropsy, the peritoneal cavity is hyperaemic and a serosanguinous fluid often present. The liver and kidneys may be pale and petechiated. Yellow mucoid material is found within the digestive tract.

Histologically, extensive tubular and interstitial tissue necrosis is evident in the kidneys of infected fish. Haematopoeitic tissues show lymphoid aggregation, oedema, necrosis and macrophage accumulation. Similar congestion, oedema, macrophage infiltration and focal haemorrhage is seen in the liver, spleen, gastrointestinal mucosa and infrequently, heart. Occasional eosinophilic inclusions may be seen in hepatocytes.

Treatment

No treatment is available for fish once infected.

Control

Maintaining low stocking density and avoiding stressful handling of young fish will help to reduce prevalence of infection.

Control policies and hygiene practices are also often implemented n the catfish industry. The virus is sensitive to acidic pH, heat and UV light and is inactivated immediately by pond mud as soil sediments rapidly absorb the virus. It is also unstable in sea water.

Fry should be reared and eggs incubated in a separate facility from any potential carrier population to maintain CCV-free status.

No vaccination is available at present. Juvenile catfish can however be passively immunised by injection of antiserum from adults.

Channel Catfish Virus Learning Resources
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References

Sano, M., Naki, T. and Fijan, N. (2011) Viral Diseases and Agents of Warmwater Fish. In: Fish Diseases and Disorders, Volume 3: Viral, Bacterial and Fungal Infections, 2nd edition (eds. Woo, P.T.K. and Bruno, D.W.), CABI, Wallingford, UK. pp 166-244.

This article was originally sourced from The Animal Health & Production Compendium (AHPC) published online by CABI during the OVAL Project. The datasheet was accessed on 11 July 2011.

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