Development of an experimental challenge model for Streptococcus equi subsp. zooepidemicus in nursery pigs

Streptococcus equi subsp. zooepidemicus (SEZ) is an opportunistic pathogen in swine. However, cases of high mortality strongly associated with a particular strain of SEZ (ST194) in North America during 2019 warranted further investigation. The objective of this study was to establish a model of SEZ in pigs and to compare the pathogenicity of three SEZ strains. In the first phase, pigs challenged with a high dose (10 8 CFU/mL) of SEZ strain TN developed severe systemic disease with 100% mortality, while only half of the pigs challenged with the low dose (10 5 CFU/mL) succumbed to the disease. In the second phase, pigs were challenged with 3 SEZ strains (TN-74097, ATCC 35246, and AZ-45470). Mortality was observed mainly in pigs challenged with TN and ATCC strains, whereas pigs challenged with the AZ strain showed none. A qRT-PCR was developed to detect both SEZ and S. equi subsp. equi as well as to differentiate the highly pathogenic ST194 SEZ from other SEZ. This study experimentally reproduced severe systemic SEZ disease in nursery pigs. Additionally, differences in pathogenicity between distinct SEZ strains were observed. This experimental model will enable further investigation into the pathogenesis of SEZ in swine and the development of methods to control this emerging pathogen.