Screening of strains for inactivated vaccine against Streptococcus suis serotype 4

Abstract:

[Background] The increasing isolation rate of Streptococcus suis serotype 4 (SS4) causes serious harm to animal farming and public health, and there has been no effective vaccine for SS4 so far. [Objective] The purpose of this research was to screen out SS4 vaccine strains with strong pathogenicity and desirable antigenicity and genetic stability. [Methods] Seven SS4 isolates (code A1-A7) were selected as the tested strains. They were determined for the half lethal dose (LD₅₀), the IgG titer in serum of immunized mice and the immune protection rate by Reed-Muench method, enzyme-linked immunosorbent assay (ELISA) and challenge assay, respectively. Meanwhile, the histopathological changes of the mice organs were observed, and after continuous subculture, the 10th, 20th, 30th generations of the tested strains were subjected to pathogenicity and antigenicity tests. [Results] The LD₅₀ of A1-A7 for the mice were 2.19×10⁸, 1.76×10⁸, 1.83×10⁸, 1.01×10⁸, 4.05×10⁸, 1.19×10⁸, and 9.03×10⁷CFU, respectively. After 7 days of secondary immunization, the IgG titers of A1, A2, A3, A4, A6, and A7 immune group were 1:1 600, 1:1 600, 1:3 200, 1:6 400, 1:3 200 and 1:6 400, respectively, and the immune protection rates were 30%, 30%, 50%, 70%, 60% and 80%, respectively, with the pathological changes of A4 and A7 immune groups being milder than the other immune groups. After in vitro passing to the 30th generation, the LD₅₀ of A4 increased to 3.81×10⁸CFU, and the IgG titer and the immune protection rate decreased to 1:1 600 and 40%. By comparison the LD₅₀ of A7 elevated to 2.49×10⁸ CFU, and the IgG titer and the immune protection rate remained stable at 1:6 400 and 80%. Additionally, the pathological changes of A7 were milder compared with those of A4. [Conclusion] A7 (original number HBgu18-4) had strong pathogenicity and good antigenicity, with uniform and stable genetic traits. It could be used as a candidate strain for SS4 vaccine.