Infection of animals and humans with Salmonella is a consequence of oral consumption of food or fluids contaminated with Salmonella. Once in the intestine, Salmonella usually attach to, invade, and proliferate in enterocytes or the cells of the gut associated lymphoid tissue (GALT). The latter route of infection can lead to disease or to an asymptomatic carrier state or stimulate the induction of mucosal, systemic and cellular immune responses. Infection of animals with virulent invasive Salmonella can result in suppression of the immune responses which in turn can facilitate the establishment o of a carrier state. It is possible to attenuate Salmonella by introducing mutations that (i) confer auxotrophy, (ii) interfere with sugar metabolism and LPS biosynthesis or (iii) affect some global means of regulating genes needed for the full display of virulence. Oral immunization of animals such as mice and chickens with avirulent Salmonella strains usually is not associated with suppression but rather with stimulation of mucosal, systemic and cellular immune responses. Vaccination by injection of killed vaccines or bacterins does not lead to the induction of either mucosal or cellular immune responses, and humoral immunity may be relatively short lived. Thus, killed vaccines are inferior to orally administered live avirulent Salmonella vaccines which induce a long-lasting protective immunity. In this manuscript we discuss desirable attributes of a safe, efficacious live attenuated Salmonella vaccine, describe attenuated Salmonella mutants so far isolated and their properties and present information on the evaluation of a live attenuated Salmonella oral vaccine for poultry. These topics are discussed in terms of the long-term objective of diminishing infection, colonization and establishment of a carrier state in agriculturally important animals with the consequent reduction in the likelihood for transmission of Salmonella through the food chain to humans.
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