Head shaking, that annoying condition that causes your horse to wiggle his head back and forth or rub his nose on the ground obsessively can be caused by several ailments.
Before you contact your veterinarian, make notes on the details of your horse's symptoms. Did the shaking start recently? It could be related to a seasonal spring allergy. A second, less understood cause has to do with the increased sunlight during spring, summer or fall. Called "Photic" syndrome, the increased sunlight stimulates a nerve in the horse's face that can cause a horse to exhibit allergy symptoms, only the shaking is caused by the sun.
Another possible issue is behavioral. Stereotypies-or repeated obsessive behavior such as cribbing and headshaking-are a response to stress.
A third possibility is an ill-fitting bit or pain somewhere during the riding process. If the headshaking only happens when the horse is under saddle, this may be a clue that it's related to tack or riding.
Try following these tips to root out the cause-and hopefully the cure-for headshaking:
1) Ride your horse in a different bit
2) Check the fit of your tack
3) Have a veterinarian do a comprehensive dental exam on your horse
4) Check your fly and gnat population and take steps to control it.
5) Ride your horse in a different saddle or bareback
6) Stable your horse with a compatible companion
7) Ask a friend to ride your horse-note if the headshaking continues. If it stops, it may be your riding.
If none of these efforts changes your horse's behavior, contact your veterinarian and ask for help.