SHIC-Funded Research on S. zooepidemicus Provides New Information and Resources

During 2019, outbreaks of S. zoo septicemia were reported in Ohio and Tennessee with mortality up to 50%. Then, in January 2021, a sow herd in Indiana experienced abnormally high mortalities related to S. zoo. The emergence of S. zoo associated with mortalities at a level not previously seen in the US prompted the Swine Health Information Center (SHIC) to fund a project conducted at Iowa State University (ISU) and the National Veterinary Services Laboratory to meet the urgent need for information. The result was the first study to experimentally infect and reproduce the disease in weaned pigs with a hypervirulent swine S. zoo strain. Furthermore, pathogenicity differences between genetically different swine strains were described. And a newly developed multiplex PCR provides an accurate and timely assay for detecting and monitoring S. zoo in swine herds.

In the SHIC-funded S. zoo study of the 2019 outbreak, researchers characterized eight isolates associated with high mortality from Ohio and Tennessee by performing whole-genome sequencing on these eight isolates along with another outbreak-unrelated swine isolate from Arizona and 15 S. zoo isolates from other animal species. Phylogenetic analysis showed that the eight 2019 outbreak isolates were clustered together, but genetically distant to the outbreak-unrelated Arizona swine isolate and the isolates from the other animal species. Three full-length complete genome sequences were further assembled, and comparative genomic analyses identified several genomic islands and virulence genes specifically in the outbreak isolates. 

The recently completed S. zoo challenge model work evaluated the disease process and diagnostics, which provided meaningful information about the pathogenicity and epidemiology of S. zoo in pigs by challenging one group of conventional six-week-old pigs with hypervirulent S. zoo strains and another group with a genetically different, less virulent S. zoo strain. Additionally, a multiplex PCR assay was designed for S. zoo identification and prediction of virulence in swine isolates.

Then in January 2021, two-year-old adult sows from a production system in Indiana experienced increased death loss. Cyanotic ears, abortion, and uterine discharge was reported and a laboratory diagnosis of S. zoo septicemia was given. With SHIC support provided for further diagnostics these three Indiana isolates were found to be genetically distant and independent to the Ohio and Tennessee isolates, warranting further studies to determine the virulence and to understand the underlying virulence mechanisms.

Findings from these projects provide significant and timely insights for a better understanding of the epidemiology and virulence of S. zoo isolates, consistent with the SHIC mission. In addition, identification of specific virulence genes and genomic islands and the development of molecular diagnostic tool may provide the basis for future investigation of virulence mechanisms and control measures.

As the world deals with the COVID-19 pandemic, SHIC continues to focus efforts on prevention, preparedness, and response to novel and emerging swine disease for the benefit of US swine health. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. SHIC is funded by America’s pork producers to fulfill its mission to protect and enhance the health of the US swine herd. For more information, visit or contact Dr. Sundberg at [email protected].