Shivers in Mules: Trembling, Hindlimb Spasms, and Backing-Up Problems

Shivers is a chronic neuromuscular movement disorder described most often in horses, but the same movement pattern can be recognized in mules and other equids. The classic sign appears when the animal is asked to back up: one hind limb may suddenly lift too high, swing outward, tremble, and pause before being placed back down. Some animals also struggle when a hind foot is picked up for trimming or shoeing.

In many cases, the mule looks fairly normal walking forward, especially early on. That can make the problem easy to miss until backing, turning tightly, loading, or farrier work becomes difficult. The signs may affect one hind limb more than the other at first, but some cases become more obvious over time.

Shivers is considered a progressive condition in many equids, although the pace varies. Some mules may stay mildly affected for years, while others develop increasing trouble with hoof handling, backing, or athletic work. Because several painful or neurologic problems can look similar, your vet should evaluate any mule with trembling, spasms, or backing-up problems.

The exact cause of shivers is not fully understood. In horses, current evidence supports a neuromuscular disorder rather than a simple training issue or isolated hoof problem. Research and referral-center guidance suggest the abnormal movement comes from dysfunction in how the nervous system coordinates the hind limbs, especially during backing and limb lifting.

That said, not every mule that jerks a hind leg has shivers. Orthopedic pain can mimic it. Hock pain, stifle problems, back or pelvic pain, hoof pain, fibrotic myopathy, upward fixation of the patella, and stringhalt can all create abnormal hindlimb motion. Neurologic diseases such as EPM or generalized weakness disorders may also need to be ruled out.

Management factors can influence how obvious the signs look, even if they do not directly cause the disease. In affected equids, episodes may be more noticeable after stall rest, with stress or excitement, or during situations that require prolonged hindlimb flexion. Some vets also assess vitamin E and selenium status because deficiencies or imbalances can complicate neuromuscular health, though supplementation should only be done under veterinary guidance.

Diagnosis starts with a careful history and movement exam. Your vet will want to know when the problem happens, whether it is getting worse, and whether it shows up during backing, turning, trailer loading, or hoof handling. Watching the mule back several steps is especially important, because true shivers is defined by the characteristic hindlimb movement during backing.

Your vet will usually perform a physical exam, lameness exam, and neurologic exam. This helps separate shivers from look-alike conditions such as stringhalt, hock or stifle pain, back pain, equine protozoal myeloencephalitis, or generalized weakness. Depending on the findings, your vet may recommend bloodwork, vitamin E or selenium testing, and imaging such as radiographs or ultrasound of the hocks, stifles, pelvis, or back.

There is no single blood test that confirms shivers. Instead, diagnosis is often clinical and by exclusion. In straightforward mild cases, your vet may diagnose the condition from the classic backing pattern and a normal enough exam otherwise. In more complex or progressive cases, referral evaluation may be the safest way to build a practical care plan.

There is no proven way to fully prevent shivers, because the underlying cause is not completely understood. Still, good management may reduce how often signs show up and may help affected mules stay safer and more comfortable. In equids with shivers, regular turnout and consistent exercise are often recommended because signs can become more obvious after prolonged confinement.

A practical prevention-minded plan focuses on early recognition and trigger control. If your mule starts hesitating when backing, snatching a hind foot away, or trembling during trimming, have your vet evaluate the problem before it becomes a bigger safety issue. Prompt attention can also uncover painful conditions that mimic shivers and may be more treatable.

Routine hoof care, appropriate conditioning, and a balanced diet matter too. Work with your vet before adding vitamin E or selenium, since supplementation should match the mule’s actual needs and regional forage status. For mules already diagnosed with shivers, prevention is really about preventing setbacks: keep handling calm, avoid long periods of stall rest when possible, and build a hoof-care routine your mule can tolerate.