Age dependency of nervous necrosis virus infection in barramundi Lates calcarifer (Bloch)

Age-dependent susceptibility to nervous necrosis virus (NNV) was demonstrated for barramundi (Lates calcarifer). The experiment used juvenile barramundi produced from a single spawning that were challenged consecutively by immersion with a redspotted grouper nervous necrosis virus (RGNNV) isolate. The dose and environmental conditions (35ppt salinity and 30 degrees C) were constant. Fish and water were sampled longitudinally for histopathology and RT-qPCR analysis to examine the evolution of the disease, virus replication, immune response and release of virus into water. Viral nervous necrosis (VNN) disease occurred in barramundi challenged at 3 and 4weeks of age while fish challenged at 5, 7 and 9weeks of age developed subclinical infection. Replication of NNV occurred faster and the concentration of virus reached higher concentrations in the younger fish with clinical disease. Virus isolation and qPCR tests indicated that infectious NNV was released from carcasses into water when fish were affected with clinical disease but not when NNV infection was subclinical. Based on these observations, we consider that carcasses from clinically infected fish have a potentially important role in the horizontal transmission of NNV, and barramundi juveniles should be protected from exposure to NNV until they are 5weeks of age and reach the disease resistance threshold.