Serum bactericidal responses to Streptococcus equi of horses following infection or vaccination.

The research focuses on the response of horses to Streptococcus equi, which is either through infection or vaccination, examined via a test based on horse blood. Surprisingly, it suggests the overrated importance of serum bactericidal antibodies in protecting horses against strangles.

Methodology

• The study adopted an indirect test which was blood-based, aiming at understanding the bactericidal responses in horses after they have been either infected or vaccinated with Streptococcus equi.
• Observations were made over a period of time to record the development and effects of bactericidal antibodies. It was found that these antibodies appeared between the second and fourth weeks, reaching their peak usually by the eighth week.

Key Observations

• The research noted that infection without clinical signs of abscessation also effectively induced strong bactericidal responses.
• Though horses had recovered from strangles or had been immunised with a commercial bacterin, the study revealed that their serum bactericidal activity had reduced eight months post-vaccination.
• A quick and significant increase in serum bactericidal activity was observed in horses that had developed strangles eight to 10 months after getting vaccinated.

Vaccination and Clinical Strangles

• Yearlings that were immunised with commercial S equi vaccines comprising either M protein or bacterin developed clinical strangles within half a year of vaccination, despite most of them having displayed strong serum bactericidal activity few weeks prior to the disease onset.
• In a similar fashion, seven ponies, which were yearlings, were hyperimmunised with an experimental vaccine rich in M protein. Even though they had exhibited strong bactericidal activity in their sera at the challenge time, they were found to be highly prone to an intranasal challenge of the S equi.

Conclusion

• The study concludes by suggesting that the function of the serum bactericidal antibody in protecting the horse from strangles might have been overvalued. This has been based on the observation that despite having strong bactericidal activity, horses were still susceptible to clinical strangles.