Isolation and identification of Betanodavirus causing mass mortalities in golden grey mullet (Liza auratus) in the Caspian Sea

Outbreaks causing mass mortalities have been observed in the golden grey mullet (Liza auratus) population in southern coastal area of the Caspian Sea since 2002. The clinical signs include neurological abnormalities, such as erratic swimming behavior, spiral swimming, belly-up at rest and over inflation of the swim bladder. Three hundred and twelve samples of moribund fish with sizes that ranged from 15-26 cm showing abdominal enlargement were collected from 2006-2008. No pathogenic bacteria and parasite were found. The observed histopathological changes in 29 fish were necrosis and vacuolation of the brain, optic nerve and retina. Cytopathological effects of nodavirus characterized by vacuolation were observed in the SSN-1 cell line at 25?C, 5 d after inoculation with the filtered supernatant of the brain and eye of four affected fish. The recovered virus from cell line exhibited 104 TCID50 per mL when titrated. Indirect immunoflorescent antibody test showed nodavirus antigens in the retina and positive-CPE SSN-1 cells. Also, a RT-PCR product of approximately 289 bp was amplified from the brain and retina of the 23 samples, including the SSN-1 positive samples. This is the first report of the isolation and characterization of nodavirus from golden grey mullet from the Caspian Sea and, therefore, possible infections in other valuable species of the Caspian Sea warrant further studies.