Trends Revealed in Headshaking

A recent study completed at the University of California, Davis, has given researchers more clues in understanding and alleviating headshaking in horses. The survey study suggested that headshaking appears more often in geldings beginning at the mean age of nine, that the condition is often triggered or aggravated in many horses by the same stimuli, and that a number of affected horses show the same characteristics behaviorally. The study also found that the conventional treatment of cyproheptadine has been beneficial in more than two-thirds of treated horses in the study.

Additionally, the study showed that Thoroughbreds were three times more likely to be headshakers than non-Thoroughbreds. A majority (91%) of the horses developed clinical signs in the spring or early summer that ceased in the late summer or fall.

The study, which appeared in the Journal of the American Veterinary Association, involved a survey of owners with a total of 109 horses affected by the headshaking disorder. Owners were asked about what triggers an episode, history, clinical signs, duration, seasonality, and response to treatments.

John Madigan, DVM, MS, Dipl. ACVIM, of the UC-Davis School of Veterinary Medicine, was the principal investigator in the study. "This is a disease that people thought was rider-driven," he explained. "There were previous implications that certain types of work were driving the horse crazy (and causing this disorder)."

But the disorder is purely a physiological reaction. Scientists believe that these horses shake their heads due to an abnormal stimulation of some of the branches of the trigenimal nerve, which provides sensation to the face and muzzle. "A pain for no reason comes into the face, mediated via the trigeminal nerve. We think that it's all pharmacologic and that there's no damage to the nerve system, but some chemical imbalance," Madigan explained. "We don't know why the nerve is triggered to fire, and they flip their heads, snort, itch, and make quick evasive movements."

Madigan said 89% of the horses in the study flipped their heads on a vertical plane, acting as if an insect were flying up the nostrils. Light caused the reaction in more than half of the horses. In past years, Madigan and collaborators termed this phenomenon "photic headshaking," which is similar to the "photic sneeze" in some humans, where light stimulation of the eye causes abnormal stimulation of facial and nasal muscles, causing a sneeze. In horses, however, the light exposure precipitates a violent headshaking reaction.

The study showed that episodes are also triggered by exercise and a number of different stimuli. "They're not all light-stimulated horses," he said. "We had three horses that if you show them long-stemmed hay, they act like they're getting shocked, so they have another stimulator into their cranial nervous system."

Behaviorally, "over 90% of the owners described their horses as normally reliable horses. Many of these owners said their horses were so out-of-character, and had been basically perfect until the headshaking began," Madigan said. This is one of the reasons Madigan says the research team thinks there's a biochemical reaction, possibly involving the serotonin levels.

Treatments included everything from antihistamines and acupuncture to neurectomies (cutting of the infraorbital nerve to interrupt nerve transmission). However, the only notable positive results were seen in the use of the conventional drug cyproheptadine, and different gadgets like masks for photic headshakers that shield the eyes from bright sun.

"We hope this study guides people so they don't put their resources into those particular remedies (which haven't shown to work), but try something that would be more likely to benefit," he said. "We need some better therapies, and now that we have a baseline (for what works and doesn't), we have ongoing research looking at treatment regimens for the horses," he said.

Chief clinical signs in 109 horses:

  • Shaking or flipping of the head in a vertical plane: 97 (89%)
  • Acting as if an insect were flying up the nostril: 96 (88%)
  • Rubbing the muzzle on objects: 82 (75%)

Less prevalent signs:

  • Anxious expression while headshaking: 67 (61%)
  • Excessive snorting: 70 (64%)


  • Cyproheptadine: Of 61 horses treated, 43 had moderate to great improvement (70%)
  • Antihistamines: Of 16 horses treated, one (6%) responded
  • Antimicrobials: Of 11 horses treated, two (18%) responded
  • Corticosteroids: Of 20 horses treated, three (15%) responded
  • Non-steroidal anti-inflammatory drugs: Of six horses treated, none responded
  • Melatonin: Of seven horses treated, two (29%) responded
  • Chiropractic: Of 28 horses treated, one (4%) responded and four (14%) responded slightly
  • Acupuncture: Of 25 horses treated, four (16%) responded and six (24%) responded slightly
  • Cloth or constricting piece of material on the muzzle: Improved clinical signs in 15 of 45 horses (33%)
  • Blindfolds or masks: Used in 75 horses, improved clinical signs considerably in 39 horses (52%) and slightly in six(8%)
  • Fly-control measures: Yielded improvement in three of the 109 horses (2%)

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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