Shivers, Stringhalt, and Australian StringhaltBy Kentucky Equine Research Staff · November 2, 2002
Shivers, stringhalt, and Australian stringhalt are names for three similar types of abnormal hind limb movement in the horse. The signs, causes, progression, treatments, prognosis, and most commonly affected breeds are somewhat different for each condition.
In classic cases of shivers, the horse exhibits trembling of one or both hind legs, elevation and trembling of the tail, and sometimes quivering in the ears and eyelids.
Stumbling, lack of energy, reluctance to back, and stomping of the hind limbs may be early indications of a growing problem, but these signs are often overlooked or attributed to other causes. Shivers occurs most frequently in draft horses and warmbloods, although the condition has been seen in other breeds as well. There is considerable evidence of heritability. One researcher reports a higher incidence in stallions and geldings than in mares.
Signs are especially noticeable when the horse is moved after a period of rest, is backed, or is asked to turn sharply. When the hind leg is lifted it is often held in a flexed position for several seconds and then swung out to the side before the hoof returns to the ground. Stress or excitement may contribute to the frequency with which the problem appears, and the abnormal movement and shaking tend to become more exaggerated with time. The signs are seen when the horse is walking, although not every stride is affected. While mildly affected horses may be able to continue working, horses with more severe cases are unable to perform due to quivering, weakness, and muscle wasting. Horses with severe shivers present a trimming and shoeing problem because they are unable to elevate either hind leg normally or to hold a leg up for farrier care.
Efforts to find the cause of shivers concentrated initially on nerve disorders, but recent studies have tied many cases to equine polysaccharide storage myopathy (EPSM or PSSM). Horses with this condition can have abnormal glycogen accumulations in their muscles, and diagnosis depends on a muscle biopsy to identify this defect. When traditional high-starch (grain) diets are replaced with a highfat feeding regimen, up to 80% of these horses show a good response within a few months. The recommended diet provides about 20 to 25% of the caloric intake in the form of fat. Vegetable oil, powdered animal fat, rice bran products, and specially formulated low-starch/highfat feeds such as Re-Leve can be used. The fat source may be mixed with alfalfa pellets as a carrier or to increase palatability. Management may include supplementation with selenium and vitamin E; daily turnout and light exercise rather than stalling; and continued access to hay and/or grass.
Although there seems to be a correlation, researchers are not yet ready to say that shivers and EPSM are the same condition. Horses diagnosed with EPSM may show a variety of signs that may or may not include shivers. It is theorized that EPSM may be manifested in various ways by different horses, possibly because the same muscle groups are not affected as severely in some animals as in others. Horses with signs of shivers should be examined by a veterinarian who can recommend diet and exercise programs and rule out other neuromuscular conditions.
Like shivers, stringhalt refers to a defect in hind leg movement. The condition may affect horses of any breed and may be seen in one or both hind legs. A horse with stringhalt flexes the affected leg so violently that, in extreme cases, the pastern actually contacts the belly. The hoof is then slapped down, hitting the ground with greater than usual force. The abnormal motion can be seen when the horse is walking or trotting. Stringhalt results from a problem with the nerve signals that cause muscle cells to contract.
Instead of a smooth progression of motion, all the cells contract quickly, causing very rapid flexion. Stringhalt sometimes develops after injury or as a manifestation f equine protozoal myeloencephalitis (EPM) or equine motor neuron disease, but more often the exact cause of nerve deterioration is unknown. The condition may be mild, visible only s a slight "hitch" in the stride, but often progresses until the horse is severely handicapped. As with shivers, stringhalt can make farrier care difficult or impossible for affected horses.
Unlike shivers, stringhalt can often be corrected or improved surgically. The procedure involves cutting and removing a small section of the lateral digital extensor tendon that runs over the hock.
Because of the way the hind limb is structured, this seemingly drastic measure eliminates violent hock contractions while allowing the leg to move in a normal manner. Almost all horses move better after this surgery although the degree of improvement is quite varied, probably being somewhat influenced by the underlying cause of the defect.
The dramatic hind leg flexing is the same, but the cause of the nerve damage in Australian stringhalt is different from the classic condition. Horses grazing drought-stressed pastures in Australia and New Zealand may consume one of several plants including flatweed (cat's ear), fireweed, mustard weed (skeleton weed), and dandelion. Nerve damage and muscle wasting have been linked to ingestion of a mycotoxin in these weeds, which may constitute a large percentage of pasture herbage after grasses have wilted in dry months. Australian stringhalt may affect any breed, although draft horses and Thoroughbreds account for the largest number of cases.
In addition to nerves in the hind leg, long nerves elsewhere in the body, especially those controlling the larynx, are commonly affected by the toxins. Some degree of laryngeal paralysis is common in horses with Australian stringhalt, resulting in horses that "roar." Phenytoin, dilantin, baclofen, and vitamins B and E have been used to reduce signs of the condition, and results have varied. Horses removed from contaminated pastures usually recover, although progress may be slow, taking up to 18 months before normal motion is restored.