Injection site abscesses associated with commensal and environmental bacteria following intramuscular vaccination in horses

• Injection-site abscesses occurred in 5 of 70 horses after intramuscular vaccination without skin disinfection. • Commensal and environmental bacteria, not vaccine components, were implicated as the causative agents. • Disinfecting the injection site is recommended to minimise adverse events in equine field settings. Five previously healthy horses in Korea, including a 19-year-old mixed-breed gelding, a 16-year-old mixed-breed mare, a 13-year-old Thoroughbred gelding, a 12-year-old Belgian warmblood mare, and a 12-year-old Andalusian mare, developed subcutaneous abscesses in the left neck after receiving intramuscular vaccination without prior skin disinfection. The vaccination used was Equivac® 2 in1, containing Clostridium tetani toxoid and Streptococcus equi cell-free extract. Within one week, the horses developed localised swelling and/or abscessation at the injection site. One case involved a ruptured abscess, while the others presented with warm, mildly painful, and non-fluctuant swellings. No foreign bodies were palpable at the injection sites. Both aerobic and anaerobic bacterial cultures were performed using 5 % defibrinated sheep blood agar, and the isolates were identified as Bacillus cereus, Corynebacterium glutamicum, Aerococcus viridans, Acinetobacter pseudolwoffii, Arthrobacter koreensis, and Paenarthrobacter aurescens , all of which were consistent with commensal or environmental flora. PCR testing was negative for Streptococcus equi . In this cohort, post-injection abscesses occurred in 5 out of 70 horses (7.1 %) and were associated with commensal and environmental bacterial species, suggesting these cases likely resulted from procedural lapses rather than a direct vaccine-related reaction. Although the benefit of alcohol swabbing is debated in human medicine, this practice may offer important benefits under variable hygienic conditions in equine field settings. Accordingly, disinfecting the injection site with an alcohol- or disinfectant-soaked swab should be considered a routine precaution to minimise post-injection complications in horses.