Surveillance for nervous necrosis virus-specific antibodies in barramundi Lates calcarifer in Australian hatcheries

We conducted single point-in-time and repeated cross-sectional studies of the prevalence of antibodies against nervous necrosis virus (NNV) in populations of adult barramundi Lates calcarifer in Australia. Serum samples collected between 2002 and 2012 were analyzed with indirect ELISA (n = 468). Most of the samples were sourced from broodstock with unknown exposure history, and these were compared with reference populations with confirmed history of exposure to NNV. Non-lethally collected gonad fluid samples from economically valuable barramundi broodstock (n = 164) were tested for the presence of NNV using RT-quantitative PCR at the time of blood sampling to compare infectivity with serostatus, but no virus was detected. NNV-specific immunoreactivity in broodstock was significantly lower than that for immunized and persistently infected populations. Seroprevalence increased over time in broodstock sampled longitudinally, probably reflecting repeated exposure to NNV in a region where the virus was endemic. The seroprevalence for the broodstock was 23.8% over the entire sample period while a cross-sectional survey conducted in 2012 found a seroprevalence of 34.5% with no significant difference between populations based on the geographic region or the history of occurrence of viral nervous necrosis (VNN) disease in the progeny in the respective hatcheries. Although serological surveillance was useful for studying the history of exposure of barramundi to NNV, the lack of association between serostatus in broodstock and the subsequent occurrence of VNN disease in their progeny indicates that ELISA tests for anti-NNV antibodies are not suitable for the purpose of preventing vertical transmission of NNV in barramundi.