Is My Horse Difficult or In pain? Can Facial Expressions be Used to Determine the Presence of Pain?

Left eye has intense stare (PS 2) muscle tension above eye (PS 1) Left ear rotated outwards (PS 1) mouth wide open, exposing teeth tongue (PS 3) left nostril flared with angular sides, wrinkle below the nostril (PS 1) Lower muzzle tense (PS 1)
Veterinary News

There is undisputed evidence to show that owners, riders and trainers have a poor ability to recognise signs of pain manifest as lameness when horses are ridden. Approximately 47% of sports horses in regular work and assumed to be sound have pain-related gait abnormalities. In a dressage horse the early stages of lameness may only be manifest under certain conditions and may be misinterpreted as conflict behaviour, difficulties in learning a movement or ‘one-sidedness’. However, tension, evasions, unevenness in the contact, resistances, spookiness, difficulties with specific movements such as flying changes could all potentially be a manifestation of pain.