and PCR Screening for a Live Attenuated Typhimurium Vaccine Strain.

The application of live attenuated Typhimurium vaccines has significantly helped control in poultry products. Because the U.S. Department of Agriculture-Food Safety Inspection Service (USDA-FSIS) scores all as positive, regardless of serovar, attenuated vaccine strains that are identified at processing contribute negatively toward performance standards. This study was designed to determine the incidence of a live attenuated serovar Typhimurium vaccine identified in broiler products by FSIS and to develop a PCR assay for screening of isolates. Typhimurium short-read sequences from broiler samples uploaded to the National Center for Biotechnology Information (NCBI) Pathogen Detection database by the USDA-FSIS from 2016 to 2022 were downloaded and assembled. These were analyzed using the Basic Local Alignment Search Tool (BLAST) with a sequence unique to field strains, followed by a sequence unique to the vaccine strain. The PCR assays were developed against field and vaccine strains by targeting transposition events in the and genes and validated by screening serovar Typhimurium isolates. Between 2016 and 2022, 1708 Typhimurium isolates of chicken origin were found in the NCBI Pathogen Detection database, corresponding to 7.99% of all identified. Of these, 104 (5.97%) were identified as the vaccine strain. The PCR assay differentiated field strains from the vaccine strain when applied to isolates and was also able to detect the vaccine strain from DNA isolated from mixed serovar overnight enrichment cultures. Live attenuated vaccines are a critical preharvest tool for control and are widely used in industry. With forthcoming regulations that will likely focus on Typhimurium, along with other serovars, there is a need to distinguish between isolates belonging to the vaccine strain and those that are responsible for causing human illness.