Originally considered a bad habit, it is now thought that some horses are allergic to certain pollens, which causes them to headshake. However horses also normally react to flies, and headshaking may be a fly defence overreaction even if flies are not present.
The use of nose nets is thought to improve the problem. Nose nets are not allowed in all competitions. However British Dressage has approved of the use of some muzzle nets.
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