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.
 

Clinical Signs

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.
 

Prognosis

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).
 

Related links

Headshaking: A definitive guide
 

References

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.