Emergence of Serovar 4,[5],12:i:- As the Primary Serovar Identified from Swine Clinical Samples and Development of a Multiplex Real-time PCR for Improved Serovar-level Identification

Rapid identification of the infecting serovar from porcine diagnostic samples is vital to allow implementation of appropriate on-farm treatment and management decisions. Although identification at the serogroup level can be rapidly achieved at most veterinary diagnostic laboratories, final serovar identification often takes several weeks because of the limited number of reference laboratories performing the complex task of serotyping. serogroup B, currently the dominant serogroup identified from swine clinical samples in the United States, contains serovars that vary from highly pathogenic to minimally pathogenic in swine. We determined the frequency of detection of individual group B serovars at the Iowa State Veterinary Diagnostic Laboratory from 2008 to 2017, and validated a multiplex real-time PCR (rtPCR) to distinguish pathogenic serogroup B serovars from those of lesser pathogenicity. Our results indicate that, since 2014, ssp. serovar 4,[5],12:i:- has been the dominant serovar identified from swine clinical samples at the ISU-VDL, with Typhimurium now the second most common serovar identified. We developed a rtPCR to allow rapid differentiation of samples containing 4,[5],12:i:- and Typhimurium from samples containing serovars believed to be of less pathogenicity, such as Agona and Derby. When combined with enrichment culture, this rtPCR has the ability to significantly improve the time to final serovar identification of the 2 most commonly identified pathogenic serovars in swine, and allows rapid implementation of serovar-specific intervention strategies.