Development of control methods of factors suppressing sustainable production of aquaculture species: Isolation of a piscine nodavirus from hatchery-reared sea bass Lates calcarifer in the Philippines

The etiological study on mass mortality of larval sea bass Lates calcarifer cultured in the Philippines, demonstrated that the mortality was caused by viral nervous necrosis (VNN). Affected sea bass exhibited abnormal swimming behaviors and pale-gray coloration of the body. Pathological changes of affected fish showed marked vacuolation and necrosis in the retina and brain. The SSN-1 cell line inoculated with the filtrate of tissue material from affected fish demonstrated extensive cytopathic effects. Piscine nodavirus (betanodavirus), the causative agent of VNN, was detected by reverse transcription-polymerase chain reaction. Electron microscopic examinations revealed that spherical and non-enveloped viral particle 22-28 nm in diameter were detected in the cytoplasm of brain and retina of affected fish and in the cytoplasm of degenerated SSN-1 cells after CPE appeared. These results indicate that mass mortality of sea bass larvae in the Philippines was caused by piscine nodavirus.