A horse's tear ducts connect from both the upper and lower eyelid to a gland located in the corner of the lower eyelid. Long, narrow tubing runs from that gland--called a lacrimal gland--down the inside of the horse's nose and empties tears into the nasal cavity, writes Dr. Duane Fitzgerald of Michigan's Thornwood Equine Veterinary Care. Known as nasolacrimal ducts by veterinarians, these tear ducts can become occluded (blocked) and cause symptoms that need veterinary treatment to be relieved.
Veterinarians call the abnormal overflow of tears down the horse's face "epiphora." Typically caused by a blockage at either end of the tear duct, the tears can be either clear or mucopurulent (yellowish) depending on whether a secondary bacterial infection is involved. Blockage often results from external factors such as dirt or dust that become trapped in the medial canthus (corner) of the horse's eye, but traumatic injuries and nasal lesions including polyps and squamous cell carcinoma have been shown to obstruct the duct, report veterinarians Derek Knottenbelt and Donald Pascoe in "Diseases and Disorders of the Horse."
Bacteria feed on the excessive tearing that is symptomatic of an obstructed nasolacrimal duct. This secondary infection causes irritation and inflammation of the horse's eyelids and a yellowish discharge, and your horse can damage the eye by rubbing it to relieve itching, warn veterinarians Audrey Pavia and Kate Gentry-Running in "Horse Health and Nutrition for Dummies."
Diagnosis and Veterinary Treatment
Your veterinarian diagnoses blocked tear ducts by introducing a special dye into both of your horse's eyes and observing the small duct opening located on the floor of the nasal cavity. If there is no evidence of the dye within three to five minutes, the vet concludes that there is an obstruction and can proceed to treatment, states Dr. Fitzgerald
Common veterinary treatment involves the vet inserting a catheter or small, curved syringe into the nasal duct opening and flushing a sterile solution up the duct to the corner of the eye. This procedure pushes the obstruction out through the opening in the eyelid and relief can be seen within 24 hours. Occasionally, clearing the obstruction requires sedating the horse and surgically inserting a tube to create a new pathway that drains the tears into the horse's mouth, nasal cavity or sinus, advises the Merck Veterinary Manual.