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Vaccination is an
essential part of preventative healthcare in horses and plays a major role in
protecting equine populations from infectious diseases such as influenza and
tetanus. Most vaccines are administered by intramuscular injection, meaning the
vaccine is delivered directly into a muscle. Although vaccination is considered
safe and routine, mild side effects can occasionally occur.
A recent study investigated whether intramuscular vaccination into either the semitendinosus muscle or the pectoralis descendens muscle could temporarily affect a horse’s movement and gait.
The semitendinosus is one of the large muscles located in the horse’s hindquarters. It forms part of the hamstring muscle group and contributes to propulsion during movement, particularly when the horse pushes off the ground during walking, trotting, or cantering. Because of its size and accessibility, this muscle is sometimes used as an injection site for vaccines or medications. The pectoralis descendens (also known as the cranial superficial pectoral muscle) forms part of the ‘thoracic sling’ between the front legs that supports the torso. It is a common site for intra-muscular Injections.
The study found that routine intramuscular vaccination into the semitendinosus resulted in a “transient, measurable push-off-type hindlimb asymmetry.” In simpler terms, the vaccinated horses showed a temporary unevenness in the way they used their hindlimbs during movement. This asymmetry was most noticeable 48 hours after vaccination but had resolved by 96 hours.
A “push-off-type” asymmetry refers to changes occurring during the propulsion phase of movement. During locomotion, the hindlimbs generate the force needed to push the horse forward. If one hindlimb produces less force because of soreness or stiffness, the horse may shift more weight onto the opposite limb. This can create a subtle unevenness in gait that may resemble mild lameness.
The asymmetry observed in the study was described as measurable, meaning it could be detected using objective gait analysis techniques. Modern gait analysis often uses motion sensors or inertial measurement units attached to different parts of the horse’s body. These devices can identify very small changes in movement that may not always be visible to the human eye. Such technology is increasingly used in equine sports medicine to improve the accuracy of lameness detection.
The research team did not detect any clinically relevant changes in gait symmetry following vaccination into the pectoralis descendens or after saline (control) injection at either site.
Importantly, the effect observed after vaccination in the semitendinosus muscle was temporary and short-lived. The horses returned to normal symmetry within four days of vaccination, suggesting that the effect was likely related to temporary muscle soreness or mild local inflammation at the injection site rather than any serious injury. Mild inflammation is a normal immune response following vaccination, as the body reacts to the vaccine and begins developing immunity.
This finding is particularly relevant for veterinarians, owners, trainers, and equine therapists because temporary gait asymmetry following vaccination could potentially be mistaken for orthopaedic lameness or poor performance. Horses competing or undergoing lameness investigations shortly after vaccination may therefore show altered movement patterns unrelated to musculoskeletal disease.
The study also highlights the importance of appropriate vaccination scheduling. Since measurable asymmetry occurred around 48 hours after injection, strenuous exercise or competition immediately after vaccination may not be ideal. Many veterinarians already recommend a short period of lighter work following vaccination to allow horses time to recover from any mild soreness or systemic effects.
Overall, the study suggests that intramuscular vaccination into the semitendinosus muscle may cause short-term hindlimb asymmetry, but the effect appears mild, temporary, and self-resolving in healthy horses.
For more details, see:
J. Lenarz, I.H. Smit, M. Rhodin, C. Lischer, M.C. Fugazzola,
Vaccination-associated lameness in warmblood horses after intramuscular injection,
Journal of Equine Veterinary Science (2026), Vol 159,105820.
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