Vaccination-associated lameness in warmblood horses after intramuscular injection.

Overview

• This study investigated whether intramuscular vaccination in Warmblood horses causes measurable changes in gait symmetry depending on the injection site.
• The goal was to provide evidence-based recommendations on vaccination site selection and post-vaccination exercise management.

Introduction & Background

• Intramuscular injections for vaccination are common in equine medicine.
• Local muscle soreness following vaccination might temporarily affect how horses walk or trot, specifically causing asymmetry in gait.
• Before this study, there was little objective data quantifying these gait changes and how they relate to injection location.

Objectives

• To determine if intramuscular vaccination causes measurable gait asymmetries in Warmblood horses.
• To assess whether the injection site (pectoralis descendens or semitendinosus muscles) influences the degree or type of gait disturbance.
• To inform best practices for vaccination site choice and recommendations on short-term exercise following vaccination.

Methodology

• Study design: Prospective, randomized, blinded, placebo-controlled.
• Subjects: Eighteen clinically sound Warmblood horses.
• Interventions:
  • Vaccination or saline injection (placebo) was administered intramuscularly.
  • Injection sites were randomised to either the musculus pectoralis descendens (chest muscle) or musculus semitendinosus (hindlimb muscle).
• Data Collection:
  • Objective gait analysis was conducted using body-mounted inertial measurement units (IMUs).
  • Measurements were taken before injection (baseline) and at multiple time points afterward: 8, 24, 48, 72, and 96 hours.
  • Parameters analyzed included vertical displacement asymmetries of the head, withers, and pelvis, focusing on clinically relevant thresholds.
• Final analysis included 14 horses from original 18 due to exclusions or dropouts:
  • Pectoralis group: 8 vaccinated, 5 saline controls.
  • Semitendinosus group: 6 vaccinated, 3 saline controls.

Results

• Vaccination into the musculus semitendinosus (hindlimb muscle) caused a transient increase in hindlimb push-off asymmetry:
  • Mean pelvic push-off asymmetry rose from 5.47 mm at baseline to 10.57 mm at 48 hours post-vaccination (statistically significant with P < 0.001).
  • Asymmetry then resolved by 96 hours post-injection, returning to baseline values.
• No clinically relevant gait symmetry changes were observed in horses vaccinated in the musculus pectoralis descendens (chest muscle) or those receiving saline injections at either site.
• An isolated statistically significant change was noted in the saline semitendinosus control group at 96 hours, but this was not clinically meaningful.

Conclusions and Implications

• Intramuscular vaccination in the semitendinosus muscle can cause a temporary increase in hindlimb gait asymmetry, likely due to transient muscle soreness or inflammation.
• Pectoralis muscle vaccinations did not induce measurable gait disturbances.
• Based on these findings, it is advisable to reduce or modify training/exercise of Warmblood horses for at least 72 hours after vaccination, especially when administered in the semitendinosus muscle.
• This research supports an evidence-based approach to injection site selection and post-vaccination management to avoid unrecognized lameness or injury.