Study of Pathogenicity of Streptococcus suis Type 2 in SPF Pigs

Abstract: Streptococcus suis serotype 2 (S. suis 2) is an important zoonotic pathogen that can cause severe disease and even death in both humans and swine. However, currently less knowledge is available about them. Therefore, the aim of this study was to investigate pathogenicity, localiazation and dynamic distribution of S. suis 2 in tissues and organs of infected SPF pigs. Thirty 6wk old specific pathogen free (SPF) pigs were randomly divided into experimental groups (including 20 pigs) injected with either the Sichuan strain ZY05719 of S. suis 2(20 pigs) or aseptic medium (control, 10 pigs). The animals were sacrificed 12h, 1d, 2d, 3d, 4d, 5d, 6d, 7d, 8d, 10d after injection. Samples were collected from different tissues including tonsil, lymphoid node, liver, spleen, heart, lung, and brain. Samples were frozen and then sections were prepared for immunofluorescence and bacteria isolation studies. This study revealed that S. suis 2 are mainly located in tonsil crypts, where they could be readily detected soon after 12h of infection. The fluorescent cell-like bacterial particles could also be seen in the marginal zone and periarterial lymphoid sheath of spleen, the germinal center and cortex of lymph node, the hepatic sinusoid of liver, interstitium of lung; furthermore, bacteria were also present in the meninges of the brain where their level fluctuated over the post-injection period of observation. However, overall dynamics of bacteria did not substantially change in the various parenchymatous organs such as liver, kidney, lung and spleen, where the number of bacteria rose steadily for 3 days following infection and declined after the 6th day post-infection. And clear-cut associations were detected between pathogens and histological lesions o