Most common fish viral diseases

- Viral Encephalitis of Tilapia Larvae

Definition:

It is an acute highly contagious viral disease affecting tilapia larvae characterized clinically by high mortalities, erratic swimming behavior, and exophthalmia and eye cataract.

Etiology:  

Viral encephalitis of tilapia larvae,  Alpha-herpesvirus, icosahedral hexagonal, double strand (dsDNA) virus (Shlapobersky et al., 2010). It primarily attacks the central nervous system, causing extremely high mortality rates among larvae in hatcheries

Mode of Transmission:

It is transmitted both horizontally through cohabitation with infected fish and vertically from the spawner to offspring by viral particles shed during spawning (Machimbirike et al., 2019).

Susceptible host: Tilapia larvae and fry

Clinical Signs:

 Abnormal swimming behavior "whirling syndrome", dark pigmentation, skin erosion, discoloration, and loss of scales, exophthalmia, abdominal distension, gill paleness and loss of appetite. High mortality rates, sometimes exceeding 90%.

PM Lesions:

Paleness of liver and kidney, enlarged spleen and gall bladder. Mild abdominal distention with ascitic fluid, and exophthalmia.  Intestinal tract empty or filled with fluid accumulation.

Histopathological lesions

Foci of gliosis and perivascular cuffing of lymphocytes in the brain cortex; severe necrosis and vacuolation in brain tissue ‘encephalon’, and specific inclusion bodies within the affected neuron. Brain hemorrhage also observed

Syncytial cells formation (fused multinucleated cells), inclusion bodies and increased melanomacrophages centers in Liver

Necrosis, lymphoid depletion and increased melanomacrophages centers of spleen.

severe necrosis in renal tissues.