Recurrent outbreaks of viral nervous necrosis in intensively cultured barramundi (Lates calcarifer) due to horizontal transmission of betanodavirus and recommendations for disease control

Betanodavirus is a significant constraint to marine finfish aquaculture worldwide and the development of control strategies will depend on accurate data about mechanisms of infection. A repeated cross-sectional survey for betanodavirus infection using a real-time reverse transcriptase-polymerase chain reaction assay and virus isolation was conducted in barramundi (totes calcarifer) at a hatchery in Australia to determine whether transmission occurred vertically from the broodstock or horizontally from the environment. Six consecutive production batches were studied in 2007-2008. Subclinical infection was detected in Batch 2007-1 with onset of infection after 26 d of age, reaching a true prevalence>90.9% (lower 95% probability limit) by 40 d. Two days later an outbreak of VNN with 100% mortality occurred in 12 d old larvae in Batch 2007-2, due to infection with an identical betanodavirus, which was not detected in this batch 7 d previously. A point source epidemic initiated by horizontal transmission from the environment (probably seawater) to Batch 1 then Batch 2 was confirmed. Betanodavirus was not endemic in the hatchery and was probably not vertically transmitted because: (i) infection was not detected in the eggs or larvae of any batch; (ii) infection was not detected in 4 out of 6 batches; (iii) infection was not identified in 20 broodstock; iv) freedom from infection of life history stages was determined with a very high degree of confidence; v) similar data were derived from archival samples from a batch in 2005. The data suggested that outbreaks were initiated because of vulnerabilities in the water supply and spread because biosecurity measures failed. The age of fish appeared to be a major risk factor for susceptibility to disease. (C) 2011 Elsevier B.V. All rights reserved.