Channel catfish virus disease (CCVD)
The most serious virus disease observed in channel catfish in culture facilities in the USA is caused by a herpes virus and occurs in fry and fingerlings less than four months old. It has been shown to be infective also in blue catfish (Ictalurus punctatus). The virus can be transmitted to fry and fingerlings through water in culture facilities. It is generally believed that channel catfish brood stock are carriers of the disease and they transmit the disease throughreproductive cells and/or fluids associated with reproduction. The virus has, however, not so far been detected in alleged carrier adults.
The disease occurs about 24 hours after infection, when water temperature ranges between 25 and 30°C. Affected fish may swim erratically or hang vertically in the water column with the head uppermost. The lesions begin at the posterior part of the kidney with an increasing number of lymphoid cells and proximal renal tubular necrosis. Focal necrotic lesions also develop in the liver, spleen and the digestive tract.The spleen is generally very dark and enlarged. Oedema and necrosis of the digestive tract result in massive sloughing of the intestinal lumen. Distension of the abdomen, exopthalmia and anaemia may also occur (fig. 9.5). Haemorrhages can be found in the muscles, gills, skin and fin bases. The disease is frequently associated with a secondary bacterial infection of Aeromonas hydrophila or Flex-ibacter columnaris. The virus retains infectivityin pond water for about two days at 25°C. The incubation period is about 32–72 hours at 30°C.
Since many of the symptoms of the disease are similar to other viral and bacterial diseases, the diagnosis has to be confirmed by isolation and identification of the virus. There is no known cure for the disease and no means of immunization has been developed, even though it has been observed that some individuals which survive the disease acquire a high level of immunity.The best control measures are pro-phylactic, including segregation from infected stocks and the use of uncontaminated water supplies. When disease occurs, infected stock should be removed and destroyed and the facilities thoroughly disinfected with a suitable disinfectant such as chlorine.
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