Equine headshaking is a behavioral
problem that causes considerable frustration for veterinarians, riders,
owners and horses themselves. It is considered to be one of the most poorly
understood conditions affecting riding horses today. In many cases, the
horse is unridable, the cause is unknown, the prognosis is poor and the
condition is incurable (Cook, 1992). Although this is true, continued research
has provided some explanations and insight into this disorder.
Definition of Headshaking in Horses
The term headshaking is defined
as an abnormal behavior occurring when a horse shakes its head in the absence
of obvious extraneous stimuli, and with such frequency and violence that
it becomes difficult or dangerous to ride or appears to be distressed (Mair
and Lane, 1993). Headshaking in horses is considered a natural behavioral
pattern and part of their natural defense system when it is used to combat
attacks by biting flies. This same behavior may also be seen in horses
restrained prior to exercise; this is considered a normal sign of impatience
and anticipation. In both these cases, the headshaking behavior is considered
a normal physiological response (Mair and Lane, 1993). Headshaking behavior
characterized by sudden, uncontrolled, violent and apparently involuntary
head throwing (in the absence of external stimuli) is considered abnormal.
It occurs most often in exercising horses, it begins as soon as they are
warmed up (approximately ten minutes of work) and gets progressively worse
the longer they are worked. These horses are referred to as headshakers.
Besides intermittent, sudden
and apparent involuntary head tossing (usually in a vertical, nodding motion),
there may be extension and forward kicking from the forelimbs, sneezing,
snorting and an attempt to rub their nose on their leg, the riders leg
or the ground. The horse and rider are often thrown off balance when these
bouts of head tossing occur.
Frequency and Seasonality of Headshaking
The precise percentage of the equine population afflicted with this condition is unknown; however, the disorder is observed world wide and is not considered to be uncommon. Abnormal headshaking behavior appears to be more prevalent in dressage-trained horses than in others; however, it is also seen in race horses, hunters, jumpers, eventers and long-distance horses. This condition appears to affect all breeds and both sexes. It has been suggested that the incidence may be higher in Thoroughbreds and other purebred horses (Cook, 1992).
This condition appears to
be seasonal in its occurrence. The most common onset (or recurrence) is
in spring (March/April in Canada) and early summer (May/June); signs of
the disease appear to regress spontaneously in autumn and remain absent
until the following spring (Madigan et al, 1995). In a lower percentage
of cases, the problem develops in autumn and regresses spontaneously in
winter. In even fewer cases, the problem may persist all year round. In
these cases, it is likely that this behavior began as a seasonal problem
before developing into a year-round problem (Mair and Lane, 1993).
Development and Cause of Headshaking
Many explanations have been offered in attempt to explain the cause of equine headshaking. Since this problem is for the most part unresponsive to therapy, many have regarded headshaking as a stereotypic behavior problem. It is likely that horses of certain temperaments are unable to cope with the stress and irritation of being ridden and outlet this by shaking their head. This explanation is likely correct for some cases, but it is likely that, in the majority of cases, the behavior is secondary to an underlying disease that is causing some sort of pain or irritation (Mair and Lane, 1990). Specific diseases affecting the respiratory tract, eyes, nervous system, upper alimentary tract and musculoskeletal system are all generally accepted as underlying diseases that can trigger headshaking (Mair and Lane, 1993). It has been suggested that allergic rhinitis may be the cause of headshaking in the majority of cases, but there is currently no laboratory data to confirm this hypothesis. Allergic rhinitis may, however, explain why this behavior is seasonal.
Bit and tack problems have been the cause of a small percentage of headshaking cases. Changing the bit and tack to improve the horses comfort and eliminate pinching has, on occasion, been sufficient to eliminate headshaking.
Headshaking may be caused
by exposure to bright light. Madigan et al. (1995) were able to reduce
the severity of or eliminate headshaking in four of seven headshakers by
reducing the amount of light these horses were exposed to. It is suggested
that in some horses sunlight stimulates the facial sensory branches of
the trigeminal nerve (the infraorbital branch of the nerve) and produces
a nasal sensation; a phenomenon similar to the photic sneeze that has been
described in man for many years (Madigan et al, 1995).
Prevention, Treatment and Control of Headshaking
With most cases of headshaking (approximately 90%), detailed clinical investigations fail to identify the cause of the behavior and, furthermore, in the few cases where a possible cause has been determined, treatment and correction of the suspected cause commonly fails to correct the behavior (Madigan et al, 1995).
Drug therapies used to treat suspected allergic rhinitis cases have been very unsuccessful. Cypohetadine (a histamine) has proved to be useful in treating some headshakers; however, its effectiveness has not yet been fully determined. Surgical treatment by infraorbital neurectomy (destruction of the infraorbital branch of the trigeminal nerve) eliminates sensation in the upper lip and cheek which can eliminate some cases of photo triggered headshaking. This procedure is, at best, 30-40% effective (Mair and Lane, 1993).
Without knowing the exact
cause(s) of this condition it is difficult to determine measures that can
be taken to prevent it from occurring. The disease, however, tends to be
more pronounced on warm, sunny days, and is frequently at its worst when
the horse is ridden near trees (Mair and Lane, 1993). Avoiding riding in
these conditions may be beneficial. Another method used to control headshaking
involves placing a nylon stocking muzzle over the upper and lower jaws
of the horse. This, along with acupuncture, has been successful in alleviating
a certain number of cases.
Curing a horse of headshaking
is rare. There are no miracle cures for this condition as previously stated.
In many cases, the horse is unridable, the cause is unknown, the prognosis
is poor, and the condition is incurable (Cook, 1992).
A definitive guide
Cook, W.R. (1992). Headshaking in Horses: An Afterward. Equine Forum. 1369.
Madigan, J.E., Kortz G., Murphy, C. and Rodger, L. (1995). Photic Headshaking in the Horse: 7 Cases. Equine Veterinary Journal. 27 (4): 306-311.
Mair, T.S., Howarth, S. and Lane, J.G. (1992). Evaluation of Some Prophylactic Therapies for the Idiopathic Headshaker Syndrome. Equine Therapy. 10-12.
Mair, T. and Lane, G. (1993).
Equine Practice 2. Headshaking in Horses. Bailliere Tindall. 109-119.
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