Viral Disease in Fishes

• Viruses are transmitted from one host to the other through a structure called “virion”.
• Viruses are classified mainly based on external structure, shape, size, capsid structure, RNA and DNA nucleic acids.
• Viruses cause disease by weakening the host tissue or by forming tumors in the host tissues.
• There is no treatment for viral diseases, only prophylactic measures have to be taken up.

     a. Lymphocystis:

• Woodcock (1904) identified this disease in fishes.
• Marine, freshwater and aquarium fishes are susceptible to this disease.
• Tumor formation is the important character of this viral disease.
• The external lesions are raised and made up of the growing of granular, nodular tissue which is composed of many greatly enlarged host cells.
• Matured lesions may become slightly hemorrhagic. Within 6-15 days of infection the tumors grow to 50 thousand times.
• The only control measure is prevention.

     b. Viral Hemorrhagic Septicaemia (VHS):

• This disease is caused by an unequal shaped fish virus with RNA.
• This disease occurs in salmon fishes. Transmission of the disease occurs through the water by a flagellates.
• The symptoms are kidney swelling, reduced appetite, obvious distress, erratic spiral swimming, multiple haemorrhages in skeletal muscles, change in body colour, reddish fins.
• The only control measure is prevention.

     c. Infectious Pancreatic Necrosis (IPN):

• This disease is found in trouts.
• This disease causing high mortality of fry, fingerlings and occasionally larger fishes.
• The symptoms are darkening distension, haemorrhages in ventral areas including bases of fins. There is pronounced pancreatic necrosis.
•  200 ppm of chlorine is effective for treatment.

     d. Infective Haemopoitic Necrosis (IHN):

• IHN was observed for the first time in trouts in British Columbia (Canada) in 1967.
• Necrosis is observed in the haemopoitic tissue of kidney in infected fish.
• This disease occurs more in fry and fingerlings and occasionally in adults.
• The symptoms are pale gills, reddish fins, black colouration of the body, abdomen swelling and huge mortality.
• The symptoms are clear in 12-45 days after the entry of virus into the host body.

     e. Chinook disease:

• A small size virus is responsible for this disease in Chinook salmon (Oncorhynchus tshawytscha) fingerlings.
• The symptoms are exophthalmus, distended abdomen, a dull red area on the dorsal surface anterior to dorsal fin.
• The liver, spleen, kidney, gills and heart are pale.
• The disease is transmitted by the egg from the carrier female.
• No treatment.

     f. Channel Cat Fish Virus Disease:

• This disease occurs in fingerling of cat fish (Ictalurus punctatus).
• The symptoms are that the fish show abnormal swimming and rotating, haemorrhagic areas on fins and abdomen, fluid accumulation in abdomen and pale gills.
• There is no treatment for this disease. Destruction of infected fish may prevent spread of the disease