Longitudinal field survey during deployment of an emergency autogenous vaccine against Betanodavirus in farmed giant grouper (Epinephelus lanceolatus): Multiple factors contribute to outbreaks and survival

Abstract

Betanodavirus or nervous necrosis virus (NNV) continues to pose a formidable threat to aquaculture worldwide, with a widening host range and outbreaks incurring significant mortalities in both hatchery and grow-out phases. In Australia, several fish species with high susceptibility to the Red Grouper Nervous Necrosis Virus (RGNNV) genotype are intensively farmed, including barramundi (Lates calcarifer) and giant grouper (Epinephelus lanceolatus). In response to a major outbreak of viral encephalopathy and retinopathy (VER) in grow-out of E. lanceolatus on a farm in Southeast Queensland, we developed an autogenous protein subunit vaccine by cloning the nucleocapsid protein gene sequence into established platform technology already in use for veterinary vaccines in Australia. The experimental vaccine was deployed under a research use permit as a cold-formalin inactivated lysate of E.coli over-expressing the capsid protein, emulsified in the oil adjuvant Montanide ISA763A VG and delivered by intraperitoneal injection. We report longitudinal farm data over three grow-out batches of grouper before and during vaccine deployment. Monthly sampling was conducted for pathology, viral detection and serum antibody response post-deployment. We found no adverse reaction to the vaccine, with melanised encapsulated vaccine emulsion confined to the mesenteric fat, which dispersed over time. An outbreak of disease due to NNV occurred during deployment but was rapidly brought under control. Coupled with specific antibody response detected in vaccinated fish during grow-out, the vaccine appeared to be useful in managing VER on the farm. Water quality data, pathology and the dynamics of infection and mortalities, indicate the critical role environmental and dietary factors play in outbreaks of Betanodavirus. The study is illustrative of the multifactorial nature of disease outbreaks in complex commercial farm operations, the value of veterinary oversight and the usefulness of relatively simple and swiftly deployed autogenous vaccine platforms.