Isolation And Identification Of SS7 And The Study On The Pathogenicity Of SS7 And High Pathogenic PRRSV Coinfection

Streptococcus suis strains were isolated from samples of Streptococcus suis epidemic investigation in the clinic, and the serotype and virulence factors of which were detected by PCR assay. Two strains isolated from healthy pigs, SJZB4_SS7 strain and ZDG9_SS2 strain, two strains isolated from ill pigs, WC0711_SS7 strain and JMS0710_SS2 strain, and two standard stains, 8074_SS7 strain and R735_SS2 strain, were selected to be detected the pathogenicity to zebra fish. The result showed that all the six strains of S.suis were low pathogenic, and the WC0711 strain of SS7 has the highest pathogenicity to zebra fish among these six strains.Glutamate dehydrogenase(gdh) gene is one of the most conservative genes of S.suis in all serotypes, and the GDH protein has several epitopes that can stimulate the body to produce specific antibodies. An indirect-ELISA assay was developed to detect antibody to Streptococcus suis using the recombinant protein derived from the epitopes region of GDH protein. The result showed that the assay was specific sensitive and repeatable. This indirect-ELISA could detect antibodies to all the serotypes of Streptococcus suis. This assay would provide a simple and rapid means for monitoring SS infection.PRRSV isolated HuN4 at passage generation 25, the titers of which was calculated as 2*104.97TCID50/ml, and SS7 isolate WC0711, which contained 1.1*109 colony-forming units(cfu)/ml were used as the challenge inoculum in the coinfection experiment. Forty-two 4-week-old crossbred piglets, seronegative for PRRSV and SS, were randomly assigned to four groups. Group 1(n=8) pigs served as uninoculated controls, group 2 (n=12) pigs were inoculated with HuN4 on day 0 of the study, group 3 (n=10) pigs were inoculated with WC0711 on day 7, and group 4 (n=12) pigs inoculated with HuN4 on day 0 and WC0711 on day 7 respectively. Clinical signs, bacteremia and viremia, serology, distribution of the two antigen, macroscopic and microscopic lesions were detected and evaluated respectively. We found that WC0711_SS7 can only cause temporary respiratory signs and mild lesions in lung, can not cause other signs such as arthrophlogosis and meningitis, and also it can not cause death. The mortality rate in group 2 was 16.7%, group 3 was 0% and group 4 is 80.0%. Pigs in group 2 mainly showed signs of respiratory system and CNS disease. Died pigs in group 4 showed typical symptoms such as high-fever, anorexia, red discoloration of the ears and body. It was very similar to the clinical signs in the field. The distribution of SS7 in different viscera of group 4 was higher than that of group 3, and the distribution of PRRSV in brain and stomach of group 4 is higher than that of group 2, while the distribution in other viscera are basically identical to that of group 2. Bacteremia had been detected in group 4 and sustained to the end of the study, while group 3 had no bacteremia. Viremia in group 2 and 4 had no significant deviations. The lesions of viscera of pigs in group 4 are significantly severe than group 2 and 3 macroscopically and pathologically. According to the results, we considered that the PRRSV infection has induced the increase in susceptibility to Streptococcus suis serotype 7 infection, and also the coinfection with SS7 increased the pathogenicity of PRRSV to piglets.