Use of repetitive sequence-based polymerase chain reaction to characterize Streptococcus equi subspecies Equi isolates cultured from horses experiencing post-vaccinal reaction

Strangles caused by Streptococcus equi subspecies equi is a continuous challenge to horse owners and practitioners. This led to great effort in developing vaccines against this disease. The goal of this study was to characterize S. equi isolates obtained from horses suffering from post vaccinal reactions following the use of an intranasal vaccine Pinnacle IN™, Fort Dodge Animal Health. Fifteen S. equi isolates obtained from several distinct post vaccinal reaction episodes in Colorado, Florida, Kansas, Kentucky, Minnesota and Ohio, USA. Clinical signs in these post vaccinal reactions episodes included fever, localized neck abscesses, strangles, or purpura hemorrhagica. S. equi was cultured from neck abscesses and/or lymph nodes. Repetitive element based polymerase chain reaction (rep-PCR) was performed on the post vaccinal reaction isolates and the vaccine strain. DNA analysis showed that all of the post vaccinal reaction isolates except one obtained from various geographic locations were identical to the intranasal vaccine strain. One isolate was found to be genetically distinct from the intranasal vaccine strain using the rep-PCR. These findings clearly suggest that the horses exhibited complication as a result of vaccination except in one incidence. Whether these complications occurred from a return to virulence by the organism, immune reaction of the host or inappropriate administration of the vaccine is unclear.