An oral nervous necrosis virus vaccine using Vibrio anguillarum as an expression host provides early protection

Oral vaccination is the most desirable immunization method by which to prevent fish disease occurring at the early larval stage. We developed an Escherichia coli-based oral nervous necrosis virus (NNV) subunit vaccine and demonstrated its effectiveness in orange spotted grouper (Epinephelus coioides) fry. However, the incubation period was determined to be over 2. weeks. To enhance the efficacy and shorten the incubation period, a safe and digestible biological adjuvant should be found. Vibrio anguillarum, a common marine bacterium with immune-stimulatory capability was selected as an expression host. An expression vector containing its heat shock protein 60 gene (groE) as an inducible promoter was constructed to express recombinant NNV coat protein as an antigen. The recombinant V. anguillarum was inactivated and encapsulated in Artemia nauplii to formulate a V. anguillarum-based oral vaccine. Compared to the E. coli-based oral vaccine, this novel oral vaccine not only elicited a higher expression of Mx (encoding for an anti-viral protein) in the brain and viscera but also a higher specific antibody titer against NNV in the first week after inoculation. Meanwhile, a higher survival rate compared to an equal dosage of an E. coli-based oral vaccine was also noticed in two individual challenge trials, indicating that an antigen expressed in an V. anguillarum-based oral vaccine could enhance efficacy in a shorter incubation period and could reduce the risk of NNV infection.