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 protective local immunity in order to struggle 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 Gastro-Resistant Adjuvant (GRA) tested here, is an oily adjuvant containing a gastro-resistant matrix to protect antigen from gastric degradation. Indeed, an in vitro experiment showed GRA formulated with BSA was able to retain the antigen in acidic media and release it under neutral pH conditions. Then, a study in tilapia was performed on a Streptococcus agalactiae vaccine formulated with GRA and mixed at 2% or 20% with feeding pellets. The tilapia have been 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).
Taken together, these results show that GRA is well adapted for the oral administration of vaccines in tilapia with a good palatability, safety and efficacy. However, more data is needed especially in a large-scale trial and also for cold water fishes to confirm these first observations before considering other animal species.