and PCR Screening for a Live AttenuatedTyphimurium Vaccine Strain.

Abstract

The application of live attenuatedTyphimurium vaccines has significantly helped controlin poultry products. Because the U.S. Department of Agriculture-Food Safety Inspection Service (USDA-FSIS) scores allas positive, regardless of serovar, attenuated vaccine strains that are identified at processing contribute negatively towardperformance standards. This study was designed to determine the incidence of a live attenuatedserovar 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 theandgenes and validated by screeningserovar Typhimurium isolates. Between 2016 and 2022, 1708Typhimurium isolates of chicken origin were found in the NCBI Pathogen Detection database, corresponding to 7.99% of allidentified. 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 overnightenrichment cultures. Live attenuatedvaccines are a critical preharvest tool forcontrol and are widely used in industry. With forthcoming regulations that will likely focus onTyphimurium, 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.