There didn’t seem to be any associated pain or infection but it was a bit unsightly and the owner asked if the eyes could be checked out.

This grey and somewhat gloopy discharge from the eyes is a relatively common finding in horses, especially in middle-aged or elderly animals. It is actually composed of tears. The pink tissue (conjunctiva) that lines the eyelids and socket contain lacrimal glands that produce tears constantly. These tears are spread across the surface of the eye (the cornea) every time the horse blinks.

The tears form a triple layer across the cornea: an aqueous (“watery”) layer, a layer of mucus and an “oily” layer of lipids. This tear film is essential for eye health. It allows oxygen and nutrients to diffuse into the cornea and removes waste products. It traps any debris that comes in contact with the surface of the eye. It also lubricates the eye and aids vision by directing light falling on the cornea into the eye without distortion.

A system of small tubes called the nasolacrimal ducts drain the tear film from the corneal surface. Tiny openings are located on the inner surfaces of the upper and lower eyelids, close to the corner of eye. These openings drain the tears into a pouch called the lacrimal sac. The nasolacrimal ducts run from these sacs down the horse’s face under the skin and opens into each nostril via openings called the nasal punctae. These openings can be seen as a pink spot in the skin the floor of the nostril, close to the pink lining of the nasal passages (Figure 1).

In the normal animal, the excess tears drain away down these ducts into the nostrils and the skin below the eyes remains dry. The rates of tear production and drainage are balanced so the entire film is replaced on average every seven minutes. Total tear production in a normal adult horse is in the region of 50ml per day. However this tear drainage system can sometimes become blocked or narrowed, resulting in the overflow of tears out of the eye and down the face. In most instances this discharge is grey and sticky but if infection is also present, it may be yellow and cloudy.

The presence of discharge on the face can attract flies in the summer months, leading to further irritation. One or both eyes may be affected. Some horses show no apparent signs of discomfort whilst others may develop some hair loss and skin irritation on the face, leading to head tossing or rubbing of the eyes on the forelimbs.

Occasionally a foal may be born where the nasolacrimal duct has a blind ending in the nostril. Excessive tear staining on the face will be apparent in these animals in the first few weeks and months of life. In most cases this condition can be corrected by a minor surgical procedure under local anaesthesia and sedation to open the duct and place a catheter in it. This catheter is left in place for a few weeks while the new opening heals around it.

EYE CHECK

Tear staining can also be caused by eye injury or disease so it is important that the eyes are first checked to ensure they are healthy. Inspection of this mare’s eyes revealed no signs of eye pain or inflammation. Her vision was normal and the internal structure of the eyes was checked with an opthalmoscope to rule out any conditions affecting them.

These normal findings pushed blockage of the nasolacrimal ducts to the top of the list of suspected causes. The diagnosis was confirmed by performing a Jones test. This involves the application of a few drops of orange fluorescin stain to the surface of the eyes. This stain turns bright green once it comes in contact with tears. In a normal horse, luminous green discharge will therefore be seen to run from the nostrils within a few minutes of drop application (Figure 2). Even with some sedation on board to lower her head, this mare’s nostrils stayed resolutely green-free and instead luminous tears started to roll down her face!

A small and pliable sterile catheter was gently threaded into the opening of the blocked duct and attached to a syringe of sterile saline. Some firm pressure on the syringe was required to flush the ducts clear. Success was confirmed when the saline was seen to squirt from the tiny openings on the eyelids. The procedure was then repeated on the opposite nostril. A chat with the client a few weeks later confirmed that the grey discharge had disappeared.

Karen Dunne MVB MA Cert EM (Stud Med) is a veterinary surgeon and veterinary nursing programme director at Dundalk Institute of Technology. She is a member of the Equine Group of Veterinary Ireland.

Email: hq@vetireland.ie

Telephone: 01-4577976