ORAL IMMUNIZATION OF RABBITS AGAINST PASTEURELLA-MULTOCIDA WITH AN ALGINATE MICROSPHERE DELIVERY SYSTEM

Oral delivery of microencapsulated antigens is a potential means to va ccinate rabbits against Pasteurella multocida, a common bacterial path ogen. Groups of five rabbits were dosed orally on days 0, 7, and 14 wi th alginate microspheres prepared to contain no added protein, 5 mg of a potassium thiocyanate extract of P. multocida (PTE), or 5 mg of PTE with 200 mu g of cholera toxin (CT). In addition, groups were dosed o rally with 5 mg of soluble PTE with or without 200 mu g CT, intranasal ly (IN) with 1 mg of soluble PTE, or with saline. Serum and nasal lava ge samples collected prior to initial immunization and 10, 16, and 21 days later were assayed by ELISA for anti-PTE IgG and IgA. Strong nasa l lavage IgA and serum IgG activities were found in samples From rabbi ts immunized with PTE IN or orally when incorporated into microspheres . Addition of CT did not significantly enhance either response. To exa mine the development of protective immunity, groups were similarly imm unized and challenge-exposed IN on day 16 with 10(6) CFU of P. multoci da. One week later, rabbits were euthanized, and specimens from the lu ngs, nasopharynx, liver, and inner ear were cultured for P multocida. Less severe infections of the lung and nasopharynx developed in rabbit s immunized with PTE IN or orally in microspheres, with or without add ed CT. In addition, culture of liver and tympanic bullae samples from these rabbits yielded growth of P. multocida less frequently compared to other P multocida-challenged rabbits. Coadministration of CT and PT E did not significantly improve protective immunity to challenge.