Fish Diseases and Management

5.3.6 Viral diseases of Fish

DNA viral diseases

Koi carp herpesvirus : KHV causes mass mortality in common carp and koi in many countries throughout the world. Based on its pathogenic effect in fish, it has also been termed carp interstitial nephritis and gill necrosis virus (CNGV). Infected fish exhibit lethargy, separate from the shoal and gather at the water inlet or sides of a pond and gasp at the surface of the water. They may also show hyperactivity, loss of equilibrium and disorientation. Morphologically, pale discolouration or reddening of the skin, with rough texture, focal or total loss of epidermis, over - or under - production of mucus on the skin and gills, enophthalmia (sunken eyes) and haemorrhages on the skin and base of the fins and fin erosion are associated with this infection. There are currently no widely applied control methods for KHV. Artificially elevated water temperatures as a means to limit KHV infections and to induce anti-viral immunity are currently practiced for control.

Iridovirus : Iridoviruses have been associated with severe disease and economic loss in farmed food fish and ornamental fish, with mortality often reported to reach 50% or more. Iridoviruses are reported from a number of ornamental fishes such as dwarf gourami Colisa lalia, orange chromide cichlid Etroplus maculatus, African lampleye Aplocheilichthys normani and other marine fishes. Diseased fishes display distinct histopathological signs of iridovirus infection such as systemic appearance of inclusion body-bearing cells, and necrosis of splenocytes and hematopoietic cells.

Epizootic Haematopoietic Necrosis (EHN) : is caused by a double-stranded DNA, non-enveloped iridovirus. Moribund fish have loss of equilibrium, flared opercula and may be dark in colour. Fish may have swollen of kidney, liver or spleen. There may be focal white to yellow lesions in liver corresponding to areas of necrosis. Acute focal, multifocal or locally extensive coagulative or liquefactive necrosis of liver, haematopoietic kidney and spleen are common. Necrotic lesions may also be seen in heart, pancreas, gastrointestinal tract, gill and pseudobranch. Basophilic intracytoplasmic inclusion bodies are noticed in necrotic areas in liver and kidney. The virus is found to infect perches and rainbow trout and is reported from Australia and Europe.

Lymphocystis disease : Is caused by iridoviruses in a broad range of fish species. The disease is characterised by the development of macroscopically visible pearl-like or wart-like nodules primarily on body surface but also on internal organs. The infection is chronic, rarely fatal and in most cases is self limiting. However, the infection reduces consumer acceptance of the fish. The disease is diagnosed by the appearance of the nodules but can sometimes be confused with other infections such as "white spot" caused by Ichthyophthirius multifilis.