Viral erythrocytic infection in sea bass: virus purification and confirmative diagnosis.

The viral erythrocytic infection (VEI) of sea bass (Dicentrarchus labrax) is evidenced by the presence of inclusion bodies in the erythrocytes and erythroblasts. Virus-like particles (VEIV) about 135-150 nm in diameter were observed in erythroblasts of affected fish. No culturable virus could be recovered from the organs and blood of VEI-affected fish, after inoculation of different fish cell lines. Histochemical staining revealed the presence of RNA but not of DNA in the inclusion bodies. Specific, rabbit hyperimmune sera were prepared after extraction and purification by gel chromatography of viral material from pelleted blood cells of infected fish, and evaluated by immunocytochemical assays. Electron microscopic observations revealed the presence of enveloped particles of about 125-150 nm in the antigenic preparation. The antisera specifically reacted to inclusion bodies, cellular membranes and nuclei of VEI-affected erythrocytes in immunofluorescent and peroxidase-antiperoxidase assays. Immunoelectron microscopy in ultrathin sections of head kidney samples from VEI-affected fish showed recognition of inclusion bodies and virus particles. Confirmative diagnostic procedures could be established through the use of specific anti-VEIV sera, which enabled differentiation between similar syndromes described in other fish species.