A pain in the head

IT is that time of year again when horses who are seasonally affected by trigeminal-mediated headshaking (TMHS) often show the most severe signs. But what exactly is this condition? What causes it, and what can we do about it?

The trigeminal nerve supplies sensation to the horse’s face and allows the horse to feel touch, itch, temperature and pain. For reasons we don’t yet understand with headshaking cases, a branch of the nerve supplying the nose and muzzle becomes hypersensitive and the horse experiences feelings that range from light tingling to electric shock-like pain, often with no obvious stimulus.

This causes the characteristic and now well-recognised signs of nose rubbing due to nasal irritation and sharp, mostly vertical “tics” and larger flicks of the head due to pain. The latter are often described as though an insect has