What is Headshaking?
Headshaking is a repetitious behavior that appears unexpectedly in adult life between 4 and 17 years of age. The nature of the headshaking varies, but generally it appears to be an uncontrollable urge to shake or toss the head up and down, left to right, or in circles. The neck is included in the movement in many cases, and the horse is difficult to ride.
There is no age, sex, breed or functional predisposition to the headshaking syndrome in horses. Headshaking behavior to a greater or lesser extent is dependent on various triggering factors, so the behavior may not be apparent in a veterinarian's clinic because the appropriate trigger factors may not be present. The behavior might also be suppressed by the psychological factors attributable to the unfamiliar environment.
Some affected horses manifest headshaking at rest and others during exercise. The problem is exacerbated during exercise and thus renders some horses useless for riding. Interestingly, there is little mention of headshaking in the old veterinary literature, implying that the condition is more prevalent now. There really isn't any consistent pattern in how the problem begins, but in some cases the onset coincides with allergy season. For these horses, headshaking can become progressively worse, and extend into the "off" seasons. In others, headshaking arises spontaneously with no apparent change of season, ownership, barn, country, or discipline.
In horses with a seasonal affliction there may be symptoms suggestive of an allergy: nose rubbing, a watery ocular and nasal discharge, low head carriage, sneezing, snorting, head pressing, pressing the muzzle into the ground. This behavior is so dramatic you'll think your horse is possessed. Some horses get so irritated or frustrated, that they will actually endangering the rider.
In addition to the effects of seasonal change, headshaking has also been reported to worsen in bright sunlight, on warm and humid days or when the wind blows into the affected horse's face. Some affected horses will actively seek out the shade. In a few, the clinical signs appear to affect one side of the horse's head to a greater extent than the other.
The headshaker appears to be having a reaction to an unusually irritating stimulus in the nasal passages. In some instances, affected horses act as if an insect has flown up their nose, although there is none present. Horses affected with mild clinical signs of headshaking may simply exhibit mild, periodic head movements and low-grade twitching of muscles in the face, but he may be rideable. Moderately affected horses may be rideable with some difficulty under special circumstances (cooler weather, protected from bright sunlight, and on non-windy days). More severely affected horses are difficult to control, uncontrollable or unrideable. Very severely affected horses exhibit dangerous fitful head tossing and cannot be ridden; consideration of euthanasia in these cases is not uncommon.