Evaluation of a commercially available modified-live Streptococcus equi subsp equi vaccine in ponies.

The study examines the effects of a commercially available vaccine against Streptococcus equi subsp equi in ponies, evaluating its safety, duration of immunity, and any potential adaptation of the vaccine strain. However, the vaccine was found to cause substantial illness and some fatalities in younger ponies, even though all vaccinated ponies developed antibodies against the bacterium.

Research Method

• The research involved two groups of ponies: group 1 comprised five ponies aged between 1.5 to 8 years, while group 2 had four ponies that were 6 months old.
• All ponies were given the vaccine, with a booster dose administered 2 to 3 weeks later.
• At the time of the booster vaccination, an equal amount of a tetracycline-resistant wild-type strain of S. equi was also administered to observe the interactions between the vaccine strain and the wild strain of bacteria.
• The ponies were monitored for 50 days, with the recovery of all bacterial strains tracked using bacteriologic culture and PCR assays.

Findings and Interpretation

• The older ponies in group 1 were found to have pre-existing antibody titers against S. equi, even though they hadn’t been knowingly exposed to the bacterium before.
• The younger ponies in group 2 were immunologically naïve, having no previous exposure or immunization against S. equi.
• Despite the different starting conditions, the vaccine led to detectable increases in anti-S. equi antibody titers in both groups of ponies, indicating an immune response.
• Ominously, while the ponies in group 1 did not display clinical symptoms linked to S. equi, all of the ponies in group 2 developed abscesses in their retropharyngeal lymph nodes. One pony even developed severe disease and had to be put down, indicating that the vaccine may have substantial negative side effects in naive ponies.
• The vaccine strain was recoverable from the tissue of ponies in group 2 for up to 24 days post vaccination, showing persistence in the body.
• Despite this, the researchers found no evidence that the vaccine strain had recombined with other bacteria or reverted to a virulent form.

Conclusion

• Given that the vaccine successfully elicited anti-S. equi antibodies in all the ponies, it demonstrated a level of effectiveness in inducing immunity.
• However, the illnesses and fatality observed in group 2 imply a significant risk associated with the vaccine in young, immunologically naive ponies.
• Consequently, while the vaccine strain did not display any unpredictable changes such as recombination or reversion to a more virulent form, the researchers hinted at potential limits to its widespread use, especially among younger equine populations.