Corneal ulcers in horses

Corneal ulcers account for the majority of ophthalmological consultations in horses. Usually caused by trauma, they generally have a good prognosis but require appropriate treatment and close monitoring.

1. What is an ulcer and how is it diagnosed?

The eyeball consists of two parts, the sclera (white of the eye) and the visual part. The latter is protected by a membrane called the cornea.

In horses, the sclera is barely visible, and the cornea covers almost the entire accessible part of the eye, even when the eyelids are wide open.

An ulcer is an erosion of the cornea, similar to a wound on the skin. The analogy with a wound helps to understand that an ulcer, like any injury:

- is mostly traumatic, although some viral keratitis can cause ulcers.

- may be more or less deep

The symptoms are fairly non-specific; the eye is usually closed, painful, and has a discharge that varies in thickness.

However, diagnosis is not as straightforward as it is for a skin wound. As the cornea is transparent, fairly superficial lesions are not always easy to identify. The veterinarian therefore uses a dye, fluorescein, which attaches itself only to the superficial damaged areas of the cornea. The ulcer then appears as a fluorescent green spot on the eye.

2. What are the possible treatments?

As we have seen, an ulcer is equivalent to a wound, so the veterinarian will seek to improve and accelerate healing.

The main problems encountered will be very frequent corneal infection, associated with intense pain causing itching, which can aggravate the ulcer.

The most common treatment involves administering antibiotic and healing eye drops to the horse. Atropine is also prescribed to dilate the pupil, limit pain, and reduce after-effects. The frequency of administration is generally the key to success, but this can be difficult depending on the horse's cooperation and the owners' availability.

For horses that are particularly resistant to local treatment, the veterinarian may need to insert a sub-palpebral catheter. This small flexible tube attached to the mane ends under the horse's eyelid and allows eye drops to be administered frequently without approaching or opening the eye.

Local treatments are usually combined with systemic anti-inflammatory drugs to manage pain.

Medical treatment may fail if the bacteria resist antibiotics or if the cornea is sluggish, meaning that it does not heal.

In this case, the first step may be to perform surgery under sedation and local anesthesia to debride the ulcer. The veterinarian will remove the cornea at the edges of the ulcer if it is detached, which limits the spread of eye drops. They may also scarify the cornea to stimulate healing.

When the ulcer is too deep or does not respond to previous treatments, surgery is the best option.

The cornea does not heal as well as skin because it is fragile and tears more easily. The surgery therefore involves performing what is known as a conjunctival graft. The surgeon uses a small flap of conjunctival tissue (from the inside of the eyelid) which is attached to the cornea to completely cover the ulcer. This protects the cornea and improves its blood supply for proper healing. Once the eye has healed, the part that attaches the flap to the eyelid can be cut away, and the connective tissue will then regress and disappear.

In the case of a perforating ulcer, i.e., if the entire thickness of the cornea is affected, known as a "perforated eye," only very early surgery can save the eye, provided that the lesion is small.

3. What next? Aftermath and consequences

Most ulcers have a good prognosis and leave no lasting effects.

However, especially in the case of deep lesions, the horse may be left with a scar. This appears as a white mark on the cornea. It does not bother the horse unless it is very large. The discomfort caused can be compared to that of dirt on a pair of glasses.

A relatively common complication is uveitis secondary to the ulcer. Even if the crisis is managed during ulcer healing, the eye may subsequently become sensitive and prone to recurrent uveitis.

To prevent ulcers, a semi-rigid protective mask is strongly recommended. This prevents trauma caused by a branch or awn under the eyelid, for example.

Pauline Cantet, equine veterinarian.