Optimisation of novel mucosal vaccines to prevent bacterial diseases of Tilapia (Oreochromis niloticus)

Summary

The majority of fish vaccines are multivalent oil-adjuvanted and given by injection. While they provide good protection against bacterial and viral pathogens, they have some drawbacks such as side effects and can only be delivered by injection. Mucosal vaccination of fish - by immersion and oral (in-feed) routes - has many benefits over injection: cost of application, mass vaccination of juveniles, less handling stress leads to less side effects and infections. At present, a limited number of mucosal vaccines are available for fish, primarily due to them being less effective than injection vaccines with shorter duration of immunity. The benefits of application of vaccine to fish by immersion or oral routes are numerous; ease of application being the foremost especially in less developed countries where expensive vaccination machines are not feasible for farmers. Many fish farmers in LMIC countries would benefit from immersion and oral vaccines that can protect their stocks for the duration of production. Development and optimisation of mucosal vaccines is essential to provide protection of aquaculture species worldwide and contribute to reducing the use of antibiotics and thereby antibiotic resistance. This project will modify an existing monovalent vaccine for a bacterial disease of tilapia, Francisella noatunensis subsp. orientalis (Fno), as a proof of concept to demonstrate the benefits of mucosal vaccination. The technology has the potential to be expanded to encompass other bacterial pathogens of fish in future polyvalent vaccines. Training and dissemination of the resulting technology to LMIC’s will be through a workshop in Vietnam.

Project outcomes

Although tilapia is a well-established cultured species globally, there are few effective commercial vaccines available or used in this species. Tilapia is one of the most important species farmed in developing countries and the majority of diseases in tilapia are bacterial, with treatment by antibiotics common. The benefits of application of vaccine to fish by immersion or oral routes are numerous; ease of application being the foremost especially in less developed countries where expensive vaccination machines are not feasible for farmers. Many fish farmers in LMIC countries would benefit from immersion and oral vaccines that can protect their stocks for the duration of production. Development and optimisation of mucosal vaccines is essential to provide protection of aquaculture species worldwide and contribute to reducing the use of antibiotics and thereby antibiotic resistance. 

The current work was performed to optimise the use of mucosal vaccines for tilapia by adapting an existing bacterial vaccine against Francisella noatunensis subsp. orientalis (Fno) as a proof of concept. The vaccine has been shown to provide excellent protection by injection (RPS 82%) as part of a previous study. The current study aimed to optimise the formulation and dose of the Fno vaccine and mucosal adjuvants for oral and immersion delivery.  A combination of vaccination routes and booster were tested to optimise vaccine efficacy: immersion/immersion and immersion/oral regimes and also by oral gavage. Uptake of vaccine in tissues and specific antibody responses in serum and mucus were measured.

For efficacy testing of the vaccine, duplicate groups of tilapia were vaccinated by different combinations of routes, for a total of 65 days with a booster at 15 days. Blood and mucus samples were taken for measurement of antibody responses. Efficacy of the vaccines was tested by immersion challenge with a virulent isolate of Fno at 65 days post-vaccination. Interestingly, the uptake study to optimise doses found specific antibody levels were significantly elevated in serum and mucus of fish given the adjuvanted vaccine by gavage. However, significant protection was not induced by any of the vaccine route combinations following challenge. The group vaccinated by immersion with an oral booster had the highest protection with a relative percentage survival of 28%. 

Oral vaccination of fish has been problematic due to the need for production of large amounts of antigen, the need to protect the antigen from the harsh environment of the stomach and the lack of knowledge as to immune induction on the one hand and induction of tolerance on the other. The lack of efficacy of oral and immersion vaccines for fish is also attributed to the lack of mucosal adjuvants. 

Adjuvants are essential components of the highly protective injectable vaccines available in aquaculture, for example in salmonid species. Therefore, the induction of high levels of specific antibodies against a bacterial pathogen in tilapia as demonstrated by the oral vaccine in conjunction with a novel oral adjuvant (by gavage) in this project is very promising. Further studies are needed to optimise delivery of the oral vaccine in a manner feasible to use on farms.