Experimental vertical transmission of nodavirus from broodfish to eggs and larvae of the sea bass, Dicentrarchus labrax (L.)

Adult broodstock sea bass, Dicentrarchus labrax (L.), were intramuscularly inoculated with live Sb-Atl nodavirus strain and maintained at two different water temperatures (13 and 20 degreesC) until spawning. Each spawn was then incubated separately in UV treated water for 3 days and newly hatched larvae reared for 22 days at 13-15 degreesC. The inoculated broodfish developed a specific immune response following inoculation with the live virus and specific antibodies were detected in the sera of all the inoculated fish whatever the temperature used (13 and 20 degreesC). Specific nodavirus antigen was also detected by enzyme-linked immunosorbent assay (ELISA) and nested reverse transcription-polymerase chain reaction (RT-PCR) in the brain and the liver of these fish. Vertical transmission of the nodavirus disease was demonstrated following inoculation of the broodfish with the live virus strain, as typical lesions associated with the nodavirus antigen were detected by immunohistochemistry in the brain of larvae at days 14-22 following hatching. The eggs from seven inoculated females that spawned during the experiment were tested for nodavirus by antigen ELISA, RT-PCR and nested RT-PCR. Fertilized and non-fertilized eggs, as well as developing larvae, were found nodavirus positive using these techniques. However, the nested RT-PCR was the only technique that could detect the nodavirus in ovarian biopsies before spawning. Therefore, the selection of nodavirus-free sea bass broodstock cannot be achieved by ELISA or RT-PCR testing of ovarian biopsies. Alternative methods such as serum nodavirus antibody detection of broodstock and/or nested RT-PCR testing of eggs before spawning should be used.