Q. What causes horses to shake their heads, and how can this issue be resolved?
A. This is actually a question that could have quite an expansive answer. Headshaking can take lots of forms:
The head toss seen with play behavior and inter-male aggression;
The occasional shake seen after rolling or with insect avoidance;
Rhythmic nodding; and
Excessive, troublesome headshaking during work.
We do see some horses that standing at rest in their stall perform a rhythmic, unchanging, vertical nodding. This seems to be a stereotypy much like other movement stereotypies in confined horses, including weaving.. This should be distinguished from more dramatic and sometimes debilitating headshaking that may include incessant vertical tossing of the head, frequent snorting, and urgent attempts to rub the nose.
Both of these types of horses should first have a thorough physical exam by a veterinarian. A certain number will have a diagnosable medical condition causing this behavior, including bone, dental, sinus, or eye problems. Some could have persistent irritation from parasites or vegetation such as foxtails. Tack should be investigated, but unfortunately that is not so commonly the cause of the severe head shaker.
A large number of these severe head shakers fall into another category, essentially diagnosed by exclusion of all the other potential causes. This is seen often but not exclusively when the horse is at work. A subset of these horses seem to show signs only in daylight or brightly lit areas and are called photic head shakers. Another subset is linked to any number of trigger factors seemingly from the environment, and some are distinctly seasonal. Much research has been done linking these cases to problems with trigeminal nerve function. The trigeminal nerve, in part, conducts sensory information from the face, mouth, nose, and eye. A dysfunction in the nerve causes the horse in essence to be hypersensitive or perhaps even experience pain in response to stimuli from the environment that would otherwise be ignored by unaffected horses.
Treatments vary from physical protection from triggers, such as stockings over the nose or fly masks to cover the eyes, to a variety of medications. Veterinarians have even tried surgical procedures on the trigeminal nerve. All of these treatments have widely varying success rates along with adverse effects, not to mention many are not suitable for show horses.
It might be possible that headshaking can be emancipated from its initiating cause and become a stereotypy. However, with these severe head shakers it is much more likely that an underlying cause is still present even if attempts at diagnosis have been unrewarding.
As I've mentioned before, keeping a physical record of your horse's behavior, along with detailed notes on environmental factors that might be at play could help you recognize triggers (e.g., wind, sunlight, pollen, etc.) and assist with the diagnosis and treatment.
About the Author
Prior to attending veterinary school, Dr. Nancy Diehl completed a master’s degree in animal science while studying stallion sexual behavior. Later, she completed a residency in large animal internal medicine at the University of Pennsylvania’s New Bolton Center and worked in equine practices in Missouri and Pennsylvania. Diehl also spent six years on faculty at Penn State, where she taught equine science and behavior courses and advised graduate students completing equine behavior research. Additionally, Diehl has co-authored scientific papers on stallion behavior, early intensive handling of foals, and feral horse contraception. Currently she is a practicing veterinarian in central Pennsylvania.