Studies on the immunogenicity of Streptococcus equi vaccines in foals.

This research article investigates the effectiveness of different types of Streptococcus equi vaccines in producing protective antibody levels in Standardbred foals. The study found that vaccines made up of formalin-treated or heat-inactivated whole Streptococcus equi cells or partially purified M-protein resulted in higher protective antibody levels compare to other commercial preparations.

Experiment Methods

• The researchers used formalin-treated or heat-inactivated whole Streptococcus equi cell vaccines, partially purified M-protein of S. equi, a beta-propiolactone killed whole S. equi cell bacterin and a cell-free extract of S. equi cells to administer to Standardbred foals.
• Serological methods were used to measure the foals’ immune response to these vaccines, specifically looking at the production of passive hemagglutination antibodies.

Results and Analysis

• The team found that the mean passive hemagglutination antibody levels in foals given either four doses of formalin-treated whole cell vaccine or an initial dose of formalin-treated followed by three doses of heat-inactivated vaccine (with or without levamisole) were significantly higher two weeks after the final dose.
• These antibody levels were higher in foals given formalin-treated whole cell vaccine compared to those given commercial bacterin.
• However, the passive hemagglutination antibody levels in all the groups decreased after 12 to 16 weeks following the fourth dose of the vaccine.
• Foals given a commercial cell-free extract did not show a significant increase in antibody levels even up to four weeks following the third dose.
• A group of six pony foals that were immunized with partially-purified M protein showed mean passive hemagglutination antibody levels lower than those found in foals given whole cell vaccines.
• During a challenge experiment with S. equi, two of the six foals vaccinated with partially-purified M-protein and all three controls developed clinical disease. In this group, the passive hemagglutination antibody levels of vaccinated foals increased after challenge, and at 28 days after the challenge, the antibody levels of vaccinated and recovered controls were similar.

Conclusion

• The study concludes that certain formulations of Streptococcus equi vaccines, in particular those using formalin-treated or heat-inactivated whole cells, result in increased protective antibody levels in foals.
• However, these levels appear to decrease around 12 to 16 weeks after the final vaccine dose. The effectiveness of commercial cell-free extract and partially-purified M-protein vaccines appears to be lower.