Abstract
Viral nervous necrosis (VNN) of sevenband grouper Epinephelus septemfasciatus at grow-out stages, which was characterized by upside down swimming or floating behavior, has been spreading in Japan. The present study describes pathogenicity of a nodavirus found in diseased sevenband grouper. When young sevenband grouper or juvenile redspotted grouper E. akaara were challenged by an intramuscular injection with the filtered homogenate of infected organs (brain and eye), abnormal swimming behavior and mortality were produced, but some affected fish recovered from the disease after exhibiting the characterized behavior. Necrosis and vacuolation of the brain and retinal tissues, characteristic to VNN, were produced in the dead and abnormally-swimming fish and the viral antigens were detected in the degenerated nerve cells by FAT with an anti-SJNNV (striped jack nervous necrosis virus) serum. The infection experiment with redspotted grouper also indicated that rearing water temperature (16-28°C ) influenced development of the disease : higher mortality and earlier appearance of the disease signs were observed at higher water temperatures.
