There are numerous causes of increased muscle tone in large animals including forms of myotonia, ear tick infestation, and shivers in horses, as well as spastic conditions in cattle.28-38
A condition that demonstrates enhanced muscle tone in cattle is spastic paresis,32,33 or Elso heel. Commonly, calves between 2 and 7 months of age are affected and have an extremely straight angle to the hock and stifle.
Signs reflect a decreased capacity or inability to flex the hock because of continuous tension on the gastrocnemius muscle when standing. Involvement may be unilateral or bilateral. The Holstein Friesian breed is most commonly affected, although other breeds have been found to suffer the disorder.32-35 Spastic paresis may have a distinct familial pattern, but environmental exposure to toxins in utero have also been implicated as a cause.32-34,36Clinical signs similar to spastic paresis are seen in horses with shivers, which is most commonly seen in draft horse breeds, warmbloods and warmblood crosses older than 1 year of age, and Thoroughbreds, although it may occur in light horse breeds.37 Horses are generally taller than 16.3 hands and more often male than female. Suggested causes include genetic, traumatic, infectious, and neurologic diseases, although its exact basis is unknown. The disease primarily affects the hindlimbs and is characterized by periodic, involuntary spasms of the muscles in the pelvic region, pelvic limbs, and tail, which are exacerbated by backing up or picking up the hindlimbs. The affected limb is elevated and then abducted. It may actually shake and shiver, and the tailhead usually elevates concurrently and trembles. In more severely affected animals, on backing up the hindlimb is suddenly raised, semiflexed, and abducted with the hoof held in the air for several seconds or minutes. The tail is elevated simultaneously and trembles. After a variable period of time, the spasms subside, the limb is extended, and the foot is brought slowly to the ground. Surface electromyography indicates that both flexors and extensors fire simultaneously during backing up in horses with shivers, resulting in strong, painful muscle contractions and poorly timed relaxation.38 There are no distinct myotonic discharges on electromyography with shivers, indicating it is not a true myotonic condition.