Histopathological examination and electron microscopy observation of viral nervous necrosis in red-spotted grouper, Epinephelus akaara

A specific band of Betanodavirus RNA2 was detected from red-spotted grouper（ Epinephelus akaara） larvae by reversetranscription polymerase chain reaction （ RT-PCR）. Under light microscopy, the brain, retina, and spinal cord of affected fish showed conspicuous vacuolation. In the brains, vacuoles were mainly apparent in the telencephalon, the diencephalon and the cerebellum. Affected cells were pyknosis accompanying marked shrinkage and basophilia. Inclusion bodies were usually round and different in size. Transmission electron microscopy revealed affected cells containing dense bodies filled with numerous viral particles in the cytoplasm. Virions were 25 - 28 nm in diameter, icosahedral and non-enveloped. Virions were distributed randomly in the cytoplasm or arranged in crystalline arrays in the dense bodies. The sizes of dense bodies were different. Occasionally, the unit membrane surrounding the larger bodies had ruptured, allowing the viral particles to escape into the cytoplasm.