Polysaccharide Storage Myopathy in Donkeys: PSSM, Stiffness & Muscle Disease

Polysaccharide Storage Myopathy, often called PSSM, is a muscle disease in equids where muscle cells store glycogen abnormally. Glycogen is a normal energy source, but in PSSM it can build up in a way that interferes with how muscle fibers work during exercise. In horses, this can lead to muscle pain, stiffness, weakness, and episodes of tying up. Donkeys can show similar signs, although published donkey-specific data are limited.

In practical terms, a donkey with suspected PSSM may seem sore, unwilling to move forward, or stiff after work, transport, weather changes, or time off. Some animals have mild, recurring performance problems. Others have more dramatic episodes with sweating, trembling, hard painful muscles, or dark urine from muscle breakdown.

Your vet may talk about PSSM1 and PSSM2. In horses, PSSM1 is linked to a known GYS1 gene mutation, while PSSM2 is a broader label used when signs and muscle biopsy findings suggest abnormal muscle glycogen storage but the GYS1 mutation is not found. Because most research is in horses, diagnosis and management in donkeys often rely on equine evidence plus the donkey's individual history, body condition, and workload.

In equids, PSSM is tied to abnormal handling of glycogen inside muscle cells. In PSSM1, a mutation in the GYS1 gene causes muscle to keep making and storing glycogen too actively. In horses, only one copy of the mutation can be enough to cause disease. That means some affected animals inherit the tendency rather than developing it from management alone.

For PSSM2, the picture is less clear. In horses, this label is used when clinical signs and muscle biopsy findings fit a storage myopathy but the GYS1 mutation is absent. Some cases once called PSSM2 may overlap with other muscle disorders, so your vet may recommend a careful workup instead of assuming every stiff donkey has PSSM.

Diet and management can make signs worse even when they are not the root cause. High-starch or high-sugar feeds can increase insulin responses, and in horses with PSSM1 that can promote more abnormal glycogen storage. Irregular exercise, obesity, sudden workload changes, stress, and long periods of stall rest may also increase the chance of muscle soreness or tying up.

Because donkey-specific studies are sparse, your vet will usually interpret these risk factors through an equine lens while also accounting for donkey metabolism. Donkeys often maintain weight easily, so overfeeding concentrates can create extra problems beyond the muscle disease itself.

Diagnosis starts with a full history and exam. Your vet will want to know when the stiffness happens, what your donkey eats, how often they work, whether there were recent days off, and if there have been episodes of sweating, trembling, or dark urine. Because many hoof, joint, neurologic, and metabolic problems can also cause stiffness, ruling out look-alike conditions matters.

Bloodwork is often the first step. Muscle enzymes such as creatine kinase (CK) and aspartate aminotransferase (AST) can rise after muscle damage. In equine medicine, vets may also use a before-and-after exercise blood test, along with a CBC, chemistry panel, and sometimes vitamin E, selenium, or diet review, to sort out chronic tying-up cases.

If PSSM1 is suspected, your vet may discuss genetic testing for the GYS1 mutation. When the diagnosis is still uncertain, a muscle biopsy may help. In horses, biopsy can show abnormal glycogen staining patterns that support PSSM or point toward another myopathy. In donkeys, biopsy interpretation may still rely on horse-based criteria, so results need to be matched with the clinical picture.

Typical 2026 U.S. cost ranges are about $180-$350 for a farm call and exam, $120-$300 for baseline bloodwork, $80-$180 for follow-up muscle enzyme testing, $60-$140 for genetic testing when available, and roughly $600-$1,800 for referral-level workups or biopsy. Your vet can help you choose a stepwise plan that fits both the medical need and your budget.

Not every case can be prevented, especially if a donkey has an inherited tendency. Still, good management can reduce flare-ups and may lower the risk of severe episodes. The main goals are consistent daily movement, careful weight control, and a forage-based diet that avoids unnecessary starch and sugar.

Work with your vet to review hay, pasture access, treats, and any concentrates. Many donkeys do not need grain at all, and overfeeding energy-dense feeds can worsen both obesity and muscle problems. If your donkey needs extra calories, your vet may suggest safer ways to provide them rather than relying on high-starch feeds.

Avoid the stop-and-start pattern that often triggers muscle soreness in equids. Long periods of inactivity followed by sudden work can be a problem. Regular turnout, gradual conditioning, and steady routines are usually safer than weekend-only exercise. If your donkey has already had a tying-up episode, ask your vet for a written return-to-work plan.

If there is a known family history of PSSM1 in related equids, discuss breeding and testing questions with your vet. Prevention is not only about feed. It is also about matching workload, body condition, and monitoring to the individual donkey so small warning signs are caught early.