Replication of Streptococcus equi subspecies zooepidemicus disease in swine

Authors: Hau, Samantha; Brockmeier, Susan; LANTZ, KRISTINA - Animal And Plant Health Inspection Service (APHIS); STUART, KEIRA - Animal And Plant Health Inspection Service (APHIS); SITTHICHAROENCHAI, PAN - Iowa State University; MACEDO, NUBIA - Iowa State University; DERSCHEID, RACHEL - Iowa State University; BURROUGH, ERIC - Iowa State University; ROBBE-AUSTERMAN, SUELEE - Animal And Plant Health Inspection Service (APHIS)

Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2021
Publication Date: 1/1/2022
Citation: Hau, S.J., Brockmeier, S., Lantz, K., Stuart, K.L., Sitthicharoenchai, P., Macedo, N., Derscheid, R.J., Burrough, E.R., Robbe-Austerman, S. 2022. Replication of Streptococcus equi subspecies zooepidemicus disease in swine. Veterinary Microbiology. 264. https://doi.org/10.1016/j.vetmic.2021.109271.
DOI: https://doi.org/10.1016/j.vetmic.2021.109271

Interpretive Summary: Streptococcus equi subspecies zooepidemicus (SEZ) is a bacterium that colonizes horses and can cause infections in mammals, including humans. SEZ has not been a common cause of infection in U.S. pigs; however, severe outbreaks with SEZ have been reported in China and Indonesia previously. In 2019, infection with SEZ caused multiple events of high death loss in sows in North America. Currently, the source and mechanism of introduction of the outbreak strain of SEZ in North America are unknown. This highly virulent strain of SEZ is a concern for the swine industry. Infection of pigs with no prior SEZ exposure results in death losses reaching 30-50% of animals within days of infection. To develop control strategies for SEZ, we need to better characterize SEZ infection in pigs and generate a model of disease to test vaccines or treatment options. In this report, we challenged sows and five-month-old pigs with a swine isolate of SEZ from an outbreak of disease in the U.S. We also compared the swine isolate to a genetically similar guinea pig isolate and a genetically dissimilar horse isolate. Though all three isolates were from clinical cases of SEZ in animals, only the swine and guinea pig isolates caused disease in pigs. The swine isolate caused the most severe disease and resulted in 100% mortality in both age groups. The guinea pig isolate also caused severe disease, though most five-month-old pigs recovered. Pigs exposed to the horse isolate showed no signs of illness. The differences in disease between the closely related guinea pig and swine isolates may be due to adaption of the swine isolate to causing infection in pigs, which should be investigated in future studies. This study describes SEZ infection in pigs and develops an animal model for SEZ disease that can be used in future research to evaluate vaccine efficacy and treatment strategies to limit the impact of this severe disease in pigs.

Technical Abstract: Streptococcus equi subspecies zooepidemicus is a commensal bacterium of horses and causes infections in mammalian species, including humans. Historically, virulent strains of S. zooepidemicus caused high mortality in pigs in China and Indonesia, while disease in the U.S. was infrequent. More recently, high mortality events in sows were attributed to S. zooepidemicus in North America. The S. zooepidemicus isolates from these mortality events have high genetic similarity to an isolate from the outbreak in China. Taken together, this may indicate S. zooepidemicus is an emerging threat to swine health. To generate a disease model and evaluate the susceptibility of healthy, conventionally raised pigs to S. zooepidemicus, we challenged sows and five-month-old pigs with an isolate from a 2019 mortality event. Pigs were challenged with a genetically similar guinea pig isolate or a genetically distinct horse isolate to evaluate comparative virulence. The swine isolate caused severe systemic disease in challenged pigs with 100% mortality. Disease manifestation in sows was similar to field reports: lethargy/depression, fever, and reluctance to rise. The guinea pig isolate also caused severe systemic disease in pigs; however, most five-month-old pigs recovered. In contrast, the horse isolate did not cause disease in pigs and was readily cleared from the respiratory tract. In conclusion, we were able to replicate disease reported in the field. The results indicate differences in virulence between isolates, with the highest virulence associated with the swine isolate. Additionally, we generated a challenge model that can be used to evaluate virulence factors and disease prevention strategies.