Investigations towards an efficacious and safe strangles vaccine: submucosal vaccination with a live attenuated Streptococcus equi.

This research paper explores the development of an effective and safe vaccine for Strangles, a contagious disease in horses. Several vaccine variants and administration methods were tested in foals. Although most vaccines were either ineffective or caused adverse reactions, a live attenuated mutant vaccine administered submucosally in the upper lip was found to be both safe and effective.

Investigative Procedures

• The researchers tested several distinct vaccines and various vaccination methods in foals.
• They then evaluated the protection level these vaccines provided by subjecting the foals to an intranasal challenge with invasively infectious Streptococcus equi bacteria.
• Protection levels were assessed via clinical, postmortem and bacteriological examinations, ensuring a comprehensive analysis of the test results.

Findings

• Inactivated vaccines containing native purified M-protein or entire S equi cells, each administered at least twice intramuscularly at four-week intervals, did not offer protection against the disease.
• Some live attenuated S equi mutants were deemed safe but ineffective when administered intranasally at least twice at four-week intervals, while others led to instances of strangles.
• A different live attenuated deletion mutant, when administered intramuscularly, resulted in complete protection but also caused local reactions at the site of vaccination, making it unacceptable.
• A notable exception was a specific live attenuated mutant vaccine, which when administered submucosally on the inside of the upper lip, showed promising results offering safety and efficacy in foals as young as four months of age.

Side Effects and Postmortem Findings

• These submucosal vaccinations caused small, temporary swellings that completely disappeared within two weeks.
• The post-mortem examinations found no vaccine remnants or abnormalities at the site of the vaccination, also highlighting the potential safety of this vaccination method.

Summary

• The research indicates crucial progress in the pursuit of a potent and safe strangles vaccine. Although most vaccines tested displayed shortcomings, one live attenuated mutant when administered submucosally was identified as a potential candidate, showing safety and effectiveness even in very young foals.