Epidemiological Investigation and Pathogenicity of Streptococcus suis in Eastern China

Background: Streptococcus suis , a zoonotic Gram-positive bacterium, is the etiological factor for septicemia and pneumonia in humans and pigs, posing a global public health threat. Currently, limited studies have investigated S. suis infections in major pig farms. This study investigated the serotypes, virulence genes, and pathogenicity of the isolates in 89 pig farms across 12 regions from 2022 to 2024. Results: The overall infection and isolation rates were 59.59% and 16.1%, respectively. The infection rate was the highest in Guangdong (72.41%) and the lowest in Hubei (43.75%). Suckling piglets, nursery pigs, fattening pigs, and pregnant sows were susceptible to S. suis infection with infection rates being as high as 60%. The infection rates in spring, summer, autumn, and winter were 70.72%, 60.67%, 40.62%, and 68.97%, respectively. Serotype analysis of 137 isolates revealed increased serotype diversity in coastal provinces, especially in Guangdong, Jiangsu, and Shandong. Serotype 1 was detected in Liaoning. The most prevalent serotype was NT (21.01%), especially in Anhui, Guangxi, and Guangdong, followed by Serotype 2 (20.29%) and Serotype 7 (18.12%). Virulence gene analysis revealed that the occurrence of gdh , gapdh , and orf2 (>89%) was high, whereas that of 89k and epf was low (≤ 28.47%). Serotypes 1 and 7 frequently harbored mrp and gdh but often lacked 89k and epf . Serotypes 2 and NT harbored all tested genes with low 89k occurrence rates. The occurrence rates of sly and epf (≤43.75%) were low in serotype 9. Animal challenge experiments demonstrated that Serotype 2 induced acute death in Landrace pigs aged 42 days with a mortality rate of 100%. In contrast, Serotype 7 was associated with low mortality rates (28.57%) and induced mild pathological symptoms, including pneumonia and pericarditis, and yellow effusion in the thoracic cavity. Conclusion： This study provides useful insights for the prevention and control of S. suis infection in pig farms in China.