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

Abstract

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.