Vaccination-associated lameness in warmblood horses after intramuscular injection

Background Intramuscular vaccination is a routine component of equine medicine, but local muscle soreness may transiently affect gait symmetry. Objective data on vaccination-associated gait changes in horses are lacking. Objectives To investigate whether intramuscular vaccination induces measurable gait asymmetries depending on injection site, to inform recommendations on vaccination site selection and short-term exercise management. Methods In this prospective, randomised, blinded, placebo-controlled study, eighteen clinically sound Warmblood horses were enrolled and received an intramuscular vaccination or a 2.0mL saline injection into either the musculus pectoralis descendens or musculus semitendinosus. Objective gait analysis using body-mounted inertial measurement units was performed at baseline and 8, 24, 48, 72 and 96 hours after injection. Vertical displacement asymmetries of the head, withers and pelvis were analysed using predefined clinical relevance thresholds. Results Fourteen horses were included in the final analysis (pectoralis: nexperimental=8/ncontrol=5; semitendinosus: nexperimental=6/ncontrol=3). Vaccination into the musculus semitendinosus resulted in a transient increase in hindlimb push-off asymmetry. Mean pelvic push-off asymmetry increased from 5.47 mm at baseline to 10.57 mm at 48 hours post-vaccination (P < 0.001) and returned to baseline by 96 hours. No clinically relevant changes in gait symmetry were detected following vaccination into the musculus pectoralis descendens or after saline injection at either site, despite an isolated statistically significant change in the semitendinosus control group at timepoint 96. Conclusion Vaccination into the musculus semitendinosus resulted in a transient increase in hindlimb push-off asymmetry after 48 hours. These findings support a short reduction in training for at least 72 hours following vaccination.