PRESENTING SIGNS

Owners typically present horses with a headshaking problem in spring, summer, or fall with recurrent, intermittent, sudden, and apparently involuntary bouts of head tossing that may be so extreme as to throw the horse and rider off balance. For this reason, headshakers can be dangerous to ride, and owners commonly report an inability to train or compete effectively on them. With such horses, sneezing or snorting, nasal discharge, and attempts to rub the nose on the ground, a forelimb, or other nearby objects frequently accompany the headshaking. Other common behaviors include clamping the nostrils as if to close them, wriggling or flipping the top lip, and engaging in protective behaviors to avoid contact between human handlers and the head (Table 23-1). Horses in the stages of a headshaking attack may be best described as acting as though an insect has flown up their nostril. Video footage of the behavior should be considered an essential prerequisite to assessment and is relatively simple to obtain.

Table 23-1 Reported Causes of Headshaking

Readers are referred to Mills DS, Taylor KD, Cooper JJ: Weaving, headshaking, cribbing, and other stereotypies. In Proceedings. Annual Convention of the American Association of Equine Practitioners 51:221-230, 2005. Available on-line at http://www.ivis.org/proceedings/aaep/2005/mills/chapter.asp?LA=1.for a more extensive list of putative causes.

Although the condition can occur at rest, headshaking is more obvious during exercise in most horses, especially at the trot and after a brief warmup period. The prevalence of headshaking remains unknown, but it seems to be increasingly common and has been reported in Europe, North America, and Australasia. The problem may start at any age, but the median age of onset is approximately 5 years. An association is often made with ridden exercise or regular work, but this may be coincidental.

Headshaking has been reported in both sexes and various breeds, including warmbloods, ponies, and crossbreds. It has been suggested that Thoroughbreds may be at particular risk, despite the fact that the condition is uncommon in racehorses, possibly because of their age or the nature of the discipline. The high representation of Thoroughbreds described in reports may be a reflection of their high value. It has also been suggested that geldings are overrepresented, but castrated males are also more common in the general working population, so this, too, may be a misleading assumption. The problem has been reported in horses of various disciplines and levels of sporting achievement, although horses used for dressage may be at increased risk. This may be a response to the bit or of the tension and flexion of the neck in dressage work. Alternatively, dressage riders may be more sensitive to the horse’s head movements because mild cases of head nodding could impact scoring in this sport, which requires stillness in head and body carriage.

Approximately 60% of headshakers appear to be affected only during the spring and summer months; the remaining horses are typically affected year round, either intermittently or persistently. In the latter group of horses, the condition may initially have been seasonal and the severity of headshaking may continue to worsen during the warmer months. Given the seasonal pattern of onset, many associations have been made, by owners and in the literature, between the season and the condition with little solid supporting evidence. The condition’s seasonality has been explained by the effects of increased exercise, heat, bright sunlight, and changes in air quality (such as the presence of irritants and allergens). In the United States, none of these trigger factors has been reported to reliably elicit the problem except exercise and exposure to ultraviolet (UV) light. In the United Kingdom, even ultraviolet light does not appear to consistently induce headshaking in horses known to have the condition. Increased workload during the spring and summer seems an unlikely explanation for the seasonality of the problem, and it is more likely that headshaking is more common in those months because of an accumulation of risk factors rather than because of any single factor.

Diagnosis is often complicated by the intermittent nature of the problem, which can result in absence of the behavior at the time of examination, especially if the horse was moved to a different location. This underscores the value of obtaining video footage of the problem. In determining the potential causes and treatment of an affected horse, the veterinarian should not overlook the intermittent nature of the problem.

EXAMINATION AND DIAGNOSIS

The intermittent and apparently involuntary nature of headshaking, together with the presence of other behaviors (see Table 23-1), suggests that headshaking is frequently a response to nasofacial irritation or pain, but it is important to appreciate that there is no single cause of headshaking and owners must be prepared for a potentially lengthy diagnostic process that ultimately reveals little. Case reports in the veterinary literature suggest that a number of disease syndromes can be associated with headshaking.

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