Abstract
Immersion and intraperitoneal injection are the two most common methods used for the vaccination of fish. Because both methods require that fish are handled and thereby stressed, oral administration of vaccines as feed supplements is desirable. In addition, in terms of revaccination (boosting) of adult fish held in net pens, oral administration of vaccines is probably the only feasible method to obtain proper protection against diseases over long periods of time. Oral vaccination is considered a suitable method for mass immunization of large and stress-sensitive fish populations. Moreover, oral vaccines may preferably induce mucosal immunity, which is especially important to fish. Experimental oral vaccine formulations include both non-encapsulated and encapsulated antigens, viruses and bacteria. To develop an effective oral vaccine, the desired antigens must be protected against the harsh environments in the stomach and gut so they can remain intact when they reach the lower gut/intestine where they normally are absorbed and transported to immune cells. The most commonly used encapsulation method is the use of alginate microspheres that can effectively deliver vaccines to the intestine without degradation. Other encapsulation methods include chitosan encapsulation, poly D,L-lactide-co-glycolic acid and liposome encapsulation. Only a few commercial oral vaccines are available on the market, including those against infectious pancreatic necrosis virus (IPNV), Spring viremia carp virus (SVCV), infectious salmon anaemia virus (ISAV) and Piscirickettsia salmonis. This review highlights recent developments of oral vaccination in teleost fish.
Highlights
It is suggested that oral immunization evokes a mucosal response, facilitated by the production and secretion of mucosal immunoglobulins (Igs)—where IgT (IgZ in zebrafish (Danio rerio)) is central [1,2,3]
There are numerous model vaccines that have evoked intestinal immune responses, where Ig response seems to be a major component for protection
The composition of intestinal microbiota may be changed during oral administration of antigens/plasmids, especially when these are entrapped in vaccine carriers
Summary
It is suggested that oral immunization evokes a mucosal response, facilitated by the production and secretion of mucosal immunoglobulins (Igs)—where IgT (IgZ in zebrafish (Danio rerio)) is central [1,2,3]. Oral vaccination is an attractive modality to immunize fish for increased protection against pathogens. A significant antibody level against OmpK was elicited and the challenge of vaccinated fish revealed protection against the pathogen. Immunized European sea bass (Dicentrarchus labrax) conferred protection against V. anguillarum challenge throughout a short time period. The ompK gene transcripts were found in the mid-intestine, liver, kidney and muscle after administration, and the expression evoked an immune response that protected fish against infection (RPS of 72.3%). Do vale Pereira et al (2015) determined the efficacy of different inactivated A. hydrophila vaccines administered intraperitoneally with or without an oral booster [13] Fish treated both intraperitoneally and orally boosted with bacterin + toxoid (inactivated extracellular products) showed the lowest cumulative mortality (10%) when intraperitoneally challenged with A. hydrophila. The high antigen dose resulted in higher antibody levels than the low antigen dose after oral vaccination and corresponded with the relative percent survival (RPS)
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