Application of the DIVA principle to Salmonella Typhimurium vaccines in pigs avoids interference with serosurveillance programmes

Salmonellosis is one of the most important bacterial zoonotic diseases in humans and Salmonella infections are often linked with the consumption of contaminated pork. In order to reduce Salmonella Typhimurium infections in humans, minimization of the Salmonella intake into the food chain is important. Vaccination has been proposed to control Salmonella infections in pigs. However, pigs vaccinated with the current vaccines cannot be discriminated from infected pigs with the lipopolysaccharide (LPS) -based serological tests used in European serosurveillance programmes. We therefore examined which LPS encoding genes of Salmonella Typhimurium can be deleted to allow differentiation of infected and vaccinated pigs, without affecting the vaccine strain’s protective capacity. For this purpose, deletion mutants in Salmonella strain 112910a, used as vaccine strain, were constructed in the LPS encoding genes: ∆rfbA, ∆rfaL, ∆rfaJ, ∆rfaI, ∆rfaG and ∆rfaF. Inoculation of BALB/c mice with the parent strain, ∆rfaL, ∆rfbA or ∆rfaJ strains but not the ∆rfaG, ∆rfaF or ∆rfaI strains protected significantly against subsequent infection with the virulent Salmonella Typhimurium strain NCTC12023. Immunization of piglets with the ∆rfaJ or ∆rfaL mutants resulted in the induction of a serological response lacking detectable antibodies against LPS. This allowed a differentiation between sera from pigs immunized with the ∆rfaJ or ∆rfaL strains and sera from pigs infected with their isogenic wild type strain.