While measures to control carcass contamination with Salmonella at the processing plant have been implemented with some success, on-farm interventions that reduce Salmonella prevalence in meat birds entering the processing plant have not translated well on a commercial scale. We determined the impact of Salmonella vaccination on commercial poultry operations by monitoring four vaccinated and four nonvaccinated breeder (parental) chicken flocks and comparing Salmonella prevalences in these flocks and their broiler, meat bird progeny. For one poultry company, their young breeders were vaccinated by using a live-attenuated Salmonella enterica serovar Typhimurium vaccine (Megan VAC-1) followed by a killed Salmonella bacterin consisting of S. enterica serovar Berta and S. enterica serovar Kentucky. The other participating poultry company did not vaccinate their breeders or broilers. The analysis revealed that vaccinated hens had a lower prevalence of Salmonella in the ceca (38.3% versus 64.2%; P < 0.001) and the reproductive tracts (14.22% versus 51.7%; P < 0.001). We also observed a lower Salmonella prevalence in broiler chicks (18.1% versus 33.5%; P < 0.001), acquired from vaccinated breeders, when placed at the broiler farms contracted with the poultry company. Broiler chicken farms populated with chicks from vaccinated breeders also tended to have fewer environmental samples containing Salmonella (14.4% versus 30.1%; P < 0.001). There was a lower Salmonella prevalence in broilers entering the processing plants (23.4% versus 33.5%; P < 0.001) for the poultry company that utilized this Salmonella vaccination program for its breeders. Investigation of other company-associated factors did not indicate that the difference between companies could be attributed to measures other than the vaccination program.
e Salmonella enterica serovar Kentucky has become the most frequently isolated serovar from poultry in the United States over the past decade. Despite its prevalence in poultry, it causes few human illnesses in the United States. The dominance of S. Kentucky in poultry does not appear to be due to single introduction of a clonal strain, and its reduced virulence appears to correlate with the absence of virulence genes grvA, sseI, sopE, and sodC1. S. Kentucky's prevalence in poultry is possibly attributable to its metabolic adaptation to the chicken cecum. While there were no difference in the growth rate of S. Kentucky and S. Typhimurium grown microaerophilically in cecal contents, S. Kentucky persisted longer when chickens were coinfected with S. Typhimurium. The in vivo advantage that S. Kentucky has over S. Typhimurium appears to be due to differential regulation of core Salmonella genes via the stationary-phase sigma factor rpoS. Microarray analysis of Salmonella grown in cecal contents in vitro identified several metabolic genes and motility and adherence genes that are differentially activated in S. Kentucky. The contributions of four of these operons (mgl, prp, nar, and csg) to Salmonella colonization in chickens were assessed. Deletion of mgl and csg reduced S. Kentucky persistence in competition studies in chickens infected with wild-type or mutant strains. Subtle mutations affecting differential regulation of core Salmonella genes appear to be important in Salmonella's adaptation to its animal host and especially for S. Kentucky's emergence as the dominant serovar in poultry.A ccording to the Centers for Disease Control and Prevention, an estimated 1 million cases of human disease due to salmonellosis occur annually in the United States (1). Salmonella outbreaks are often associated with the consumption of contaminated meat and eggs (2). While more than 2,000 Salmonella serovars have been identified, 20 Salmonella serovars account for 73% of salmonellosis cases in the United States (3). Salmonella enterica Enteritidis and S. enterica serovar Typhimurium are the top two serovars associated with outbreaks in the United States (3) and are responsible for 50% of human infections worldwide (4). The serovar S. Kentucky is rarely associated with human illnesses in the United States (0.18% of cases) (3); however, it is the most prevalent serovar in poultry in the United States (5, 6).The emergence of S. Kentucky as the dominant serovar in poultry is similar to the emergence of S. Enteritidis in the 1980s (7), except that S. Enteritidis quickly became the leading cause of human salmonellosis in many countries (8), including the United States (9). There has been extensive research on S. Enteritidis, focused especially on identifying factors that contribute to colonization and egg transmission (10). While S. enterica subspecies I serovars share many metabolic and virulence pathways, forming a Salmonella core genome (11,12), there are differences with regard to the distribution of mobile genetic elements (prophage...
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