Page 1136 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1111

VetBooks.ir flexion of the hock and stifle of one or both affected, or a group of horses, (depending on how
In ‘Australian stringhalt’ there may be one horse
hindlimbs, usually at a walk. Adult horses of any
breed may be affected. Stringhalt is usually an
associated with plant toxicity have a sudden onset,
individual disorder with one limb affected. It can many animals are grazing the pasture). Those cases
occur as outbreaks, known as ‘Australian string- and the severity of signs increases in the first days
halt’, that are likely to be associated with ingestion to weeks. In contrast to the sporadic cases, these
of neurotoxic plants. horses usually recover without treatment, although
the recovery time may be prolonged. Clinical and
Aetiology/pathophysiology neurological examinations are usually unremarkable
The aetiology of sporadic stringhalt is unclear. It in both the sporadic and epidemic form; however,
has been suggested that damage to the reflex arc or it should be noted, especially in sporadic cases, that
its connections, perhaps from trauma, may lead to stringhalt may also have a central cause and ruling out
the abnormal and characteristic hock flexion that is other CNS conditions, such as EPM, is important.
seen. Thus, a sensory or motor neuropathy, spinal
cord disease or myopathy may be responsible. In Management
some cases, injury to the metatarsus and tarsus has In the epidemic form most horses recover in weeks
preceded the development of clinical signs by several to months. If a toxic aetiology is suspected, horses
months. Injury may interrupt control of the muscle should be removed from the pasture they were graz-
spindle trigger mechanism. The epidemic form ing at the time of onset of clinical signs. The admin-
known as ‘Australian stringhalt’ is thought to involve istration of phenytoin (15 mg/kg p/o q24 h; q12 h in
plant toxicity, and ingestion of Hypochoeris radicata is severe cases) for 2 weeks may result in a more rapid
implicated in most cases, particularly under certain resolution of clinical signs. Treatment may have to
environmental conditions such as drought and over- be extended if clinical signs become worse. In the
grazing of poor-quality pasture. Despite the name, sporadic form, exercise, intra-articular administra-
development of reversible stringhalt associated with tion of steroids in trauma cases and surgical ther-
ingestion of Hypochoeris radicata has been reported in apy involving tenotomy or tenectomy of the lateral
many parts of the world including France, Germany, digital extensor tendon have been attempted with
the UK and South Africa. variable success.

Clinical presentation Prognosis
The condition is recognised from the characteris- Full recovery is rare in cases of sporadic stringhalt.
tic gait, but it is important to view the horse during Immediate improvement may be noted in some
standing, backing up, walking away, while lifting a horses following surgical intervention, although
hindlimb and during trot to differentiate the disor- response is variable. The prognosis is good with the
der from ‘equine shivering’ (see below). One impor- epidemic form. Most horses recover fully, although
tant difference between the two disorders is that the weeks to months may be required.
abnormal limb behaviour persists in trot with string-
halt cases, but not shiverers. In addition, hyperflex- EQUINE SHIVERING (SHIVERS)
ion when walking forwards is usually intermittent in
shivers, but more consistently observed in stringhalt Definition/overview
cases. Hindlimb abduction is not usually apparent in Equine shivering is a neuromuscular disease charac-
stringhalt cases. terised by intermittent muscle fasciculations of the
The disorder varies in severity from a grade 1 hindlimbs, so they appear to be shivering. It is pro-
affected horse with only mild exaggeration of flex- voked by a particular set of movements such as walk-
ion during backing, turning or lifting the hindlimb, ing forwards from a standstill, or picking up a hind
which is inapparent during exercise, to a grade 5 in foot. Shivering is a condition of unknown aetiology
which the affected limb may strike the abdomen with a suspected genetic origin that has been long
during movement. recognised in draught breeds.