Salmonellae infection normally manifests no clinical symptoms and natural resistance levels rise in chickens aged 3 weeks or older, but the infection may also, albeit less frequently, result in enteritis septicemia, and other pathologies especially in young birds. Salmonellae are readily and frequently transferred from birds to birds and from poultry to humans, especially by indirect but also by direct means.
Food poisoning caused by Salmonella enterica serovar enteritidis (SE) became a major public health problem in the middle of the 1980s, and several years were required to identify that the main causative food material was chicken eggs. SE live vaccines have been recognized as an important tool to reduce SE-contaminated chicken egg production on layer farms and to control infection of broiler breeder flocks.