Tears running down the side of your horse's face is a common sign noted by many horse owners. There are multiple causes for excessive tearing. Some are important and require a veterinary exam while other causes may be temporary and can clear up without treatment. Remember, however, that the horse's eye is extremely sensitive and anytime a horse's eye is tearing, I believe you should have a vet examine him. If you take the wait and see approach and it is a serious problem, an eye infection can cause blindness in just a few hours so waiting is not a good idea. Excessive tearing can be due to over production of tears or to a lack of drainage. Lack of drainage is due to blockage of the nasolacrimal duct that leads from the eye to the nose. The duct may be infected, swollen, and blocked by mucous, or if the horse is young it may not even be formed correctly. Your veterinarian can flush the duct to see if it is open and functioning correctly. If it is and excessive tearing is still occurring, then too many tears are being produced, which is usually due to some irritation around or in the eye. Usually the inflammation causes pain and any time a horse is squinting, the eye needs to be examined.
Pain causing excessive tearing can either be due to inflammation on the outside, such as an infection or tumor in the membranes around the eye, damage to the outer surface of the eye itself, or pain can be due to inflammation inside the eye. Scratches on the cornea can cause pain and excessive tearing; diseases such as uveitis and glaucoma affect the inside, and both cause excessive tearing. Although allergies can cause too much tearing, your vet should examine all eyes that are tearing excessively.