Objective-To evaluate a commercially available modifed-live Streptococcus equi subsp equi vaccine for safety and persistence in vaccinated ponies and to detect recombination or reversion events in the vaccine strain. Animals-5 ponies that were 1.5 to 8 years old (group 1) and 4 ponies that were 6 months old (group 2). Procedures-Ponies were vaccinated, with a subsequent booster vaccination 2 to 3 weeks later, and monitored for 50 days. At booster vaccination, an equal amount of a tetracycline-resistant wild-type strain of S equi was administered. Recovery of all strains was performed by use of bacteriologic culture and PCR assays. Results-Ponies in group 1 had background antibody titers against S equi antigen before vaccination despite the lack of known exposure to S equi. Ponies in group 2 were immuno-logically naïve. Increases in anti-S equi antibody titers were detected in both groups. Ponies in group 1 did not have clinical signs of disease caused by S equi. In group 2, all ponies developed abscesses in retropharyngeal lymph nodes; 1 pony developed severe clinical disease and was euthanized. The vaccine strain was recovered from ponies in group 2 for up to 24 days after vaccination. Conclusions and Clinical Signifcance-Although the vaccine was successful in inducing IgG antibodies against S equi in all ponies, fndings suggested that the vaccine may have caused substantial morbidity and some deaths in the young ponies. In young ponies, the vaccine strain persisted in tissues for weeks; however, no evidence of recombination was detected.
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