The injection route is the most common method used for the administration of commercial vaccines but it presents some drawbacks; it is time consuming, requires qualified personnel and may induce local reactions at the injection site. Mucosal vaccination is a good alternative and allows to develop local immunity in order to protect against infectious diseases. Oral administration of vaccines is particularly suitable for protection of breedings with high density of animals like in fish farming. The main challenge linked with this route of administration is to preserve the integrity of the antigen from the harsh environment created by the digestive system to reach gut associated lymphoid tissue (GALT) in order to induce a potent local immune response. This issue could be gotten around by using an adapted adjuvant.
The Montanide™ GR 01 (GRA) adjuvant tested here is able to provide a matrix protecting the antigen from gastric-route. A first study in tilapia was performed on a Streptococcus agalactiae vaccine formulated with GRA adjuvant and mixed at 2% or 20% with feeding pellets. The tilapia were orally vaccinated under 2 periods one week apart as follows : first, during 4 consecutive days, then during 3 other days. Each vaccine group was constituted with 3 replicates of 40 tilapia. At D49, the tilapia were challenged with S. agalactiae serotype III by intraperitoneal route. The protective rate of GRA groups has reached 91.1% and 88.9% for vaccines formulated at 2% and 20% respectively while it was only 73.3% in the control group (vaccination with non-formulated antigen).
A second study was conducted at a larger scale on 9000 fishes in farming standard conditions to evaluate vaccine potential. The tilapia were orally vaccinated with Streptococcus agalactiae vaccine, whether with antigen alone or formulated with the Montanide™ GR 01 adjuvant and compared to an unvaccinated control group, under two 5 day-periods, 5 days apart for the priming. Oral boost was then given during a 5 day-period at day 95. Immune response of fish was followed by measuring specific IgM antibody levels at different time points (up to day 128). The use of the vaccine did not affect survival rate and appeared to improve economic performance of this fish farming. The group receiving the Montanide™ GR 01 based vaccine also showed significantly higher specific antibody titer than the non vaccinated control group from day 44.
Taken together, these results show that the Montanide™ GR 01 is well adapted for the oral administration of safe and efficient vaccines in tilapia.