Dr. John Madigan of the University of California--Davis, has done a great deal of research on photic headshaking in horses and has reported on these animals in several articles. Madigan and his fellow researchers believe horses shake their heads in response to bright light due to an abnormal stimulation of some of the branches of the trigeminal nerve, which provides sensation to the face and muzzle. The light stimulation of the eye causes an abnormal stimulation of some of the nerves of the face and muzzle and results in actual tingling or even pain sensation in some horses, which causes them to shake and rub their heads. The reason these horses have facial pain resulting from direct sunlight is still unknown. One theory is that a previous facial injury has resulted in abnormal nerve transmission. For these photic headshaking horses, avoiding bright light, keeping them in dark stalls, even riding at night eliminates this behavior completely. Other horses are helped by using eye protection while in bright light. In some cases, headshaking can be treated successfully with a drug called Cyproheptadine, which is a histamine- and serotonin-blocking agent. At the 1997 convention of the American Association of Equine Practitioners, Madigan reported on a study that was conducted on a group of 25 headshakers. Cyproheptadine administration resulted in moderate to great improvement in 76% of the horses.
Headshaking is the act of a horse flipping its nose into the air, sometimes even shaking its head from side to side, writes Dr. Christina S. Cable in the August edition of The Horse. In its pathologic manifestation, this behavior is performed frequently, if not constantly. It also often involves rubbing the muzzle and sneezing. If you own or care for a headshaker and the behavior persists despite fly control and properly fitted tack, you should contact your veterinarian. A thorough physical exam should be performed and the horse ridden so your veterinarian can observe the headshaking behavior. Questions you should be prepared to answer include: When does the behavior occur most frequently--while the horse is inside or outside? Is the headshaking better or worse on an overcast day or indoors? When did the behavior begin? If the headshaking is chronic, did the behavior resolve over the wintertime? In other words, does this seem to be a seasonal problem? Based on the answers to these and other questions, your veterinarian will formulate a plan to determine the cause of the headshaking. Causes include ear mites, ocular problems, fungal infection of the guttural pouch, middle ear infections, nasal and/or dental problems, and a disorder called photic headshaking, which becomes worse in sunlight.