A fish nodavirus associated with mass mortality in hatchery-reared Asian Sea bass, Lates calcarifer

Virus responsible for viral nervous necrosis (VNN) was isolated from infected Asian sea bass (Lates calcarifer) larvae during the massive outbreak in sea bass hatcheries located in Chennai and Nagapattinam of Tamilnadu, India. The infected larvae showed anorexia, pale-grey pigmentation of body, loss of equilibrium and corkscrew-like or whirling swimming behavior before to death. The causative organism was isolated using SISK cell line. The typical cytopathic effect of nodavirus characterized by vacuolation was observed in SISK cell line 2 days after inoculating with the filtrate of diseased sea bass larvae. The recovered virus from cell line exhibited 8 Log TCID50 mL− 1 when titrated in the presence of SISK cell line. Electron microscopic observation revealed the vacuolation and aggregation of numerous virus particles in the cytoplasm of nodavirus-infected SISK cells. Histological investigations reveal vacuolations in brain and retina of naturally and experimentally infected sea bass juveniles. Fish larvae challenged by immersion exhibited abnormal swimming pattern and dark coloration 2 days post infection (p.i.) and 100% mortality was observed at 4 days p.i. Intramuscularly challenged juveniles developed dark coloration and abnormal behavior 10 days p.i. and all animals died at 15 days p.i. All naturally and experimentally infected sea bass larvae or juveniles showed positive for piscine nodavirus by RT-PCR.