The horse’s eye is large and lies in a prominent and somewhat unprotected position in the skull. Given the horse’s propensity of flight under circumstances of fright, trauma to the eye is not uncommon. Also, given the many varied occupations of horses in some rather harsh environments, it is not unexpected that the horse incurs a moderate amount of eye disease. The eye is an extremely delicate and specialized structure, and any eye problem should be dealt with immediately to prevent any permanent damage that may impair vision and affect the horses safety to be ridden.
In most occasions this would be evident, as your horse will show clear signs of ocular discomfort like closure of the eyelids or excess discharge. However, sometimes these signs can be more subtle and difficult to notice. It is important that you stand in front of your horse and look at the angle of the eyelashes; this should be symmetrical in both eyes and almost parallel to the ground. When the eye is painful, the first thing that will happen is that the eye will sink into the socket and the angle of the eyelashes will drop in the affected eye.
As previously explained, an ocular problem is always an emergency and your vet should be contacted immediately. Minor ocular injuries today can turn into major disasters tomorrow, and your horses vision can be affected forever. The same principle applies for eyelid wounds and lacerations, as the eyelids have a vital function in spreading the tear uniformly over the ocular surface to keep it moist and protected.
Your vet will ideally require a dark stable to perform the examination. For assessment of ulcers on the surface of the eye your vet may use special dyes that make the injury evident; these dyes are fluorescein (which makes the ulcer bright green) or rose Bengal (which makes the areas of the surface of the eye not covered by tears turn pink). Sometimes it may be necessary to dilate and open the pupil with some drops to examine the back of the eye completely.
With some emergencies the surface of the eye goes completely opaque and it is impossible to examine the internal ocular structures. In this instance, your vet may elect to perform an ultrasound examination of the eye.
Eyelid lacerations are a relatively common injury, possibly due to the horses rapid head movement response when threatened. Lacerations of the upper eyelid are more common and more significant because the upper eyelid is primarily responsible for tear film distribution. Eyelid lacerations should be promptly and surgically repaired.
Ulcers of the cornea (surface of the eye) are also a common reason for veterinary attention. Again, the positioning of the equine eye, in combination with a horses disposition, predisposes the horse to corneal injury. Treatment of corneal ulceration is directed at controlling the infection and promoting corneal healing. Most non complicated corneal ulcers should heal in a few days with just medical therapy, i.e. antibiotic drops. Those which fail to heal in a relatively short time require more aggressive treatment.
Uveitis or inflammation of the uvea, which is the middle layer of the eye, is relatively common and can eventually lead to blindness so rapid treatment with topical and systemic anti-inflammatories is essential. Once a horse has had an episode of uveitis, it is likely that he/she will suffer further episodes in the future.
Horses can also develop glaucoma, where the pressure of the fluid inside the eye rises to abnormal levels, although it is uncommon and most of the times happens as a consequence to uveitis.
Cataracts or opacities of the lens are also relatively common. These can range from small scratches on the lens to complete opacities that can interfere with your horses ability to see with clarity.
Abnormalities of the fundus (back of the eye) and the retina can also be frequently encountered. Although most of the times these are just incidental findings detected during an ocular examination for a different reason (for example during a pre-purchase examination), sometimes they can affect the vision in variable degrees or even cause total blindness. You should ask your vet to examine the back of the eye if you suspect that your horses vision is compromised or if he spooks easily.