Evaluating a Novel Oral Vaccine Delivery Platform in Rainbow Trout Oncorhynchus mykiss

1Evan M. Jones, 1Jie Ma, 2Matt Hawkyard, 3Mary R. Arkoosh, 3Joseph P. Dietrich, 3Cameron M. Schuster, 1Luke P. Oliver, 1Veronica L. Myrsell, and 1Kenneth D. Cain

1 Department of Fish and Wildlife Sciences, University of Idaho, Moscow, ID 83844;  2 Aquaculture Research Institute, University of Maine, Orono, ME 04469; 3 NOAA Northwest Fisheries Science Center, Newport Research Station, Newport, OR 97365

ABSTRACT

As aquaculture continues is pace of the fastest growing food sector, farmers require modern tools to deal with inefficiencies of production. A major cost to growers is loss caused by disease, this combined with new legislation meant to curb overuse of antibiotics has refocused efforts to preventative management of diseases. Vaccines are a major component of disease management, but small farms may not be able to afford specialized equipment required to implement vaccination programs. Oral vaccination can significantly reduce costs of these programs while allowing for low stress vaccine delivery. Oral vaccines can also easily be administered to small fish, expanding protection in more disease susceptible life stages. This study evaluates the ability of a novel alginate-based oral vaccine particle to stimulate immunity and provide protection against the causative agent of furunculosis, Aeromonas salmonicida.  A formalin-killed, whole cell A. salmonicida vaccine was produced and used to vaccinate ~ 2.5 g juvenile rainbow trout (Oncorhynchus mykiss) via four routes: intraperitoneal injection (IP), anal intubation (AL), immersion (BA), and oral vaccination by alginate-particle ingestion (OV). Control groups included a particle containing no vaccine (CP) and a sterile PBS intraperitoneal injection (PB). All treatment groups received a booster 2 weeks after their first dose. Two trials were performed, with some differences in vaccine formulations. Specific antibodies in serum were measured at 0, 2, 4, 6, 8, and 13 weeks post vaccination (wpv). Fish were then challenged in triplicate tanks for each treatment by a 24 hour static immersion with 1×10^7  cfu/mL of A. salmonicida. Fish were challenged at 13 and 4 wpv for trials 1 and 2, respectively.  In trial 1, log antibody titers were significantly elevated in the OV group (1.57) relative to both CP and PB groups (0,0) at 4 wpv, before declining. For trial 2, the OV group had a slight, but not statistically significant, elevation of antibody titers at 6 wpv (0.86) before declining to baseline levels by 8 wpv. The IP group had the highest titers for all timepoints throughout both trials, ranging from 1.89 to 4.16. This was followed by the BA and AL groups, whose titers throughout the trial ranged from 0.34 to 2.06. For the pathogen challenge in trial 1, survival in all vaccination treatment groups was significantly elevated relative to the PB group (17.8%). Interestingly, the CP group survival (64.4%) was also significantly higher than the PB group. In trial 2, the OV group (68.3%) had significantly higher survival relative to the PB group (36.7%), but not CP fish (51.7%). The IP group, which incorporated an adjuvant in trial 2, had significantly higher survival (96.7%) relative to all other treatments including AL (63.3% survival) and BA (65.0% survival).  The oral vaccine particle successfully stimulated an antibody response and both groups fed the alginate-particle showed resistance to A. salmonicida in a pathogen challenge. The anal vaccination demonstrated the ability of killed vaccines to stimulate gut associated lymphoid tissue (GALT) which can provide protection against pathogens. Protection was also associated with the control alginate particle; the mechanisms are unclear but may be associated with an adjuvant effect of the alginate. Importantly, the oral vaccine stimulated a specific antibody response, which provides evidence for successful delivery of oral vaccines using this alginate-particle platform. However, adjustments to particle formulation, dose, and delivery strategy requires further research.