Immunization of rabbits against a bacterial pathogen with an alginate microparticle vaccine

Pasteurella multocida is an important bacterial pathogen of domestic rabbits. To evaluate the ability of a thiocyanate extract (PTE) of P. multocida to stimulate an immune response and protect against infection with P. multocida, rabbits were immunized subcutaneously or intranasally on Days 7, 21 and 35. Cholera toxin, a potent mucosal adjuvant, was included in one treatment group. Rabbits immunized subcutaneously (SC) or intranasally (IN) had significant increases in serum anti-PTE IgG but not IgA. In contrast, only rabbits immunized IN with PTE developed significant titers of nasal lavage anti-PTE IgA and cholera toxin significantly enhanced this response. In a second study rabbits were immunized via the drinking water with PTE incorporated into alginate microparticles on Days 7, 14 and 21. Mild increases in serum IgG were noted in rabbits immunized with PTE in microparticles, with or without cholera toxin, and this increase was significant (P≤0.05) on Day 21 for rabbits receiving PTE and cholera toxin. Nasal lavage anti-PTE IgA was significantly (P≤0.05) increased in rabbits immunized orally with PTE, with or without cholera toxin, in microparticles. This effect was not enhanced by cholera toxin. Rabbits orally immunized with PTE in microparticles had significantly fewer colony forming units of homologous P. multocida recovered from the lungs and nasopharynx following an intranasal challenge. These results demonstrate that PTE incorporated into alginate microparticles and administered orally is immunogenic and confers protective immunity.