Vaccine Strain <i>Streptococcus equi</i> subsp <i>equi</i> Causing a Focal Intramuscular Abscess in a Healthy Adult Horse

Prevention of strangles infection, caused by Streptococcus equi subsp equi , commonly involves the use of intranasal, live attenuated vaccines. However, the vaccine strain of S. equi subsp equi has also been linked to disease in recently vaccinated animals. In this case report, failure to follow recommendations for the administration of the S. equi subsp equi intranasal vaccine resulted in a transfer of vaccine strain S. equi subsp equi to the site of administration of an intramuscular vaccine with subsequent abscessation. While the most likely transmission route was due to an in‐contact horse that sternutated intranasal vaccine onto the intramuscular vaccination site of the affected horse, the vaccine administrator’s failure to wash hands after intranasal vaccine use per the manufacturer’s guidelines may also be possible. The strain isolated from the abscess was confirmed to be the vaccine strain using whole genome sequencing. This case highlights the importance of administering vaccines following manufacturer and best practice guidelines. It also provides support to previous evidence that the vaccine strain does not revert to wild type and is capable of causing clinical disease if handled improperly.