Immunization of pigs against Streptococcus suis serotype 2 infection using a live avirulent strain.

Streptococcus suis capsular type 2 is still an important cause of economic losses in the swine industry. At the present time, vaccination of pigs against this infection is generally carried out with autogenous bacterins and results are equivocal. In this study, the protective effect of a live avirulent S. suis type 2 strain (#1330) which had induced a good protection in mice, was evaluated in swine. The experiment was performed in triplicate using 4 week-old piglets. A total of 15 piglets were vaccinated 3 times, 15 others were vaccinated 2 times, and 15 piglets were injected 3 times with sterile Todd-Hewitt broth. Using an indirect ELISA, an increase in the IgG response to S. suis antigens was noted in 27 of the 30 vaccinated piglets. On day 21 post-vaccination, all animals were challenged intravenously with a virulent S. suis type 2 strain (#999). In the 2 vaccinated groups, 26 animals were fully protected. Only 1 out of the 15 piglets vaccinated 3 times developed mild clinical signs. In the group vaccinated twice, 3 piglets showed clinical signs and 1 of them died after the challenge. In the control group, 7 animals died out of the 11 with clinical signs of infection. In conclusion, a protective immunity was observed in swine when using strain 1330. However, more studies are needed to assess the use of a live S. suis strain in a vaccine for pigs.