The future of Salmonella vaccines in a geographically diverse and changing epidemiological environment

Most important global Salmonella serotypes that cause food poisoning in humans are Enteritidis, derived mainly from eggs and egg-derived products, and Typhimurium, derived from porcine (and poultry) meat. Salmonella vaccination programs in laying hens, using inactivated and live vaccines, have been shown to be efficient in reducing egg contamination, while vaccination of broilers and pigs is still not commonly done, although there are some vaccines marketed and used in some regions. Apart from Enteritidis and Typhimurium, different serotypes can be of local and regional importance. Novel trends are the global emergence of Salmonella serogroup C strains in poultry, including multi-resistant clonal strains worldwide (e.g. Salmonella Infantis clones). Vaccines for poultry that are commercially available can be inactivated and live vaccines, the latter mostly based on spontaneous mutations, and mainly derived from Enteritidis and Typhimurium strains. Vaccines for the host-specific serotypes Gallinarum and Choleraesuis, that cause systemic infections in poultry and pigs, respectively, also are in use. In poultry, inactivated vaccines have been used for parent and layer flocks, and live vaccines mainly for layer flocks. Live vaccines are stimulating cell-mediated and humoral responses, and mucosal innate responses, as they mimic a natural infection, while inactivated vaccines mostly result in antibody responses. Both have been shown to be able to confer (at least partial) protection. Egg contamination in layers is well under control using vaccines. The most important challenges for Salmonella vaccines are the production of efficient vaccines for pigs and broilers, and the constant changes in serotype distribution, and thus the development of cross-protecting vaccines against a range of serotypes in broilers. For broilers, the difficulty is the build-up of active immunity in the short life span of the bird. An early bacteriological colonization-inhibition effect conferred by live vaccines has been described in the intestine but this is mainly efficient within the same serotype, so there is a lack of cross-protection using this method. For pigs, lymph node colonization seems difficult to control using vaccines. In addition, often serology is used for monitoring of Salmonella in pigs, and there can be interference with monitoring when piglets are vaccinated. Although there are no safety issues with current vaccines, the detailed knowledge on the molecular pathogenesis of Salmonella infections should result in attenuated and highly characterized deletion mutant vaccines, and add markers to differentiate vaccine and field strains and serological responses against vaccine and field strains. Regulatory aspects that are of importance in this regard are the faster acceptance of live attenuated mutant strains for emerging serotypes that contain identical gene deletions as one that are already marketed for other serotypes. Other challenges are early protection as chicks are highly susceptible during the post-hatch period, and extension of the duration of protection, considering the trend to extend the productive cycle in layers.
 
Keywords: Salmonella, poultry, vaccinaton, Enteritidis, Typhimurium.

Abstract presented at the 3rd International Symposium on Alternatives to Antibiotics 2019.