Recent development in oral vaccine delivery systems

Oral delivery of vaccines is the least effective method of vaccinating fish. However, anal intubation of antigens is very effective in inducing high serum antibody levels and protection against vibriosis and enteric redmouth. It is generally considered that degradation of antigen in the stomach and anterior intestine prevents immunostimulation taking place and that if antigens could be protected during passage through the foregut effective immunization could be achieved. Recent attempts to protect antigens have included the concomitant administration of antacids and antiproteases. Protective coating procedures have included encapsulation of particulated antigen with methacrylic acrylic acid polymers which resist acid but dissolve in the high pH of the intestine. Incorporation of antigen into microparticles of poly lactide co-glycolide polymers protects against enzyme degradation and allows increased uptake of intact antigen. Certain substances can enhance uptake of antigen by the intestine of fish and have adjuvant activity, such as cholera toxin beta-subunit and Quil-A saponin. Overcoming problems of palatability of oral vaccines delivered to fish fry has been achieved by incorporating vaccine into live food such as Artemia. However, delivery of antigen to very young fish, before they are fully immunocompetent can induce immune suppression. Many of these methods have improved the level of antigen uptake by the intestine and the antibody response in fish and the prospects for improving the efficacy of oral vaccination appear optimistic.