Horses cope surprisingly well with limited vision. They are not required to read or write, after all — their eyes are essentially designed to spot food, water and predators. Yet we’ve redesigned their roles, so they need to watch out for cross county fences, showjumps, arena edges and other obstacles.
Older horses often have a standard routine, rather like many older people, involving familiar surroundings, the same companions and few changes in their environment. This means that they tend to cope with deterioration to their vision without anyone noticing. Owners typically underestimate the severity of any visual defects and none of us really know what a horse can or cannot see.
Horses do suffer from age-associated vision changes, but rarely to the point of blindness. Even so, it is important to be aware of the potential problems and the associated human safety and horse welfare implications.
- The cornea — the clear “window” at the front of the eye — is vulnerable to injury at all ages, largely due to the prominent position of the eyes on the side of the head.Corneal disease is common and has the potential to be more serious in older horses. With time, changes can affect the ocular (eye) surface and cause deterioration in the tear film — the natural lubricant covering the eye. Tear deficiency and dry eye syndromes are rare in horses but are more likely with increasing age.
Older animals tend to develop corneal ulcers that are slow to heal, often near the lower eyelid. They may also be more prone to ulcers in the first place. There are suggestions of a reduction in corneal sensitivity, particularly with pituitary pars intermedia dysfunction (PPID, otherwise known as Cushing’s disease) sufferers, which can leave the eye more vulnerable to injury.
● Uveitis — inflammation within the eye — is reported to some degree in many older horses’ eyes upon ophthalmic examination. Before the cause was known, the intermittent nature of the disease was thought to be associated with the phases of the moon, hence its historical name “moon blindness”.
We now know that uveitis is linked to the immune system. After a lifetime of inflammatory episodes, the condition can be more problematic in older horses. Recent treatment innovations, such as local implants with the immunosuppressive medication cyclosporine, may help cases that cannot be managed with topical ointments, drops or other medication.
When an affected eye remains painful and unresponsive to treatment, removal remains the best option — even in an older horse. This can be done in the standing, sedated horse using local nerve blocks and is relatively straightforward.
● Glaucoma causes increased accumulation of fluid, raised pressures and swelling within the eye, usually occurring as a complication of equine recurrent uveitis. Recent studies reported that 65% of equine glaucoma cases presented in horses older than 15 years, so seek veterinary advice if your veteran develops a bulging eye.
● Cataracts are defined as an opacity in the lens within the eye. Senile cataracts can appear as the lens loses transparency with age, although horses often cope remarkably well, provided there is not a complete cataract that covers the lens. The lens also become less flexible with age; while spectacles will help humans, this is not an option for our equine patients.
Cataract surgery can correct significant visual impairment but should only be considered if there are no other concurrent eye conditions. Many cataracts are a complication of underlying inflammatory eye disease, so simply removing the cataract is unlikely to help if the eye is already damaged. It is also questionable whether it is the right thing to do in an older horse.
● The retina, at the rear of the eye, registers the image that the horse sees. Unfortunately, the retina deteriorates with age and treatment options are limited.
Studies on horses aged over 30 years revealed senile retinopathy (age-related damage) in nearly three quarters of them. It is unclear, however, how much the condition affects vision.
On the blink
Accurate visual assessment of any horse is surprisingly difficult. We still do not know exactly how much a healthy young horse can see, let alone an older animal with detectable visual problems.
A vet may use an obstacle test, blindfolding one of the horse’s eyes and then the other, as well as a full clinical examination. Physical examination may show cloudiness, discolouration or distortion in the shape of the eye.
The so-called menace response, which tests whether a horse blinks at an approaching hand, can be hard to interpret accurately as many horses blink in response to the resulting air movement. It is sometimes best to stand back and observe the horse’s behaviour pattern over time.
The relatively delicate skin around the eye should also be monitored, along with the pink conjunctival membranes in the eye socket that are especially sensitive in appaloosas and cream or coloured horses as they age. These are prone to irritation from dirt, dust and sunlight, among other things, and may benefit from protection offered by a well-fitting eye or fly mask.
Tumours such as squamous cell carcinomas, melanomas or sarcoids can appear around the eye. The key message with any lump or bump is to catch it early and seek veterinary advice — removal is simpler when the mass is small, especially if the eyelid is involved.
Many ophthalmic conditions are progressive and, given time, will begin to surface as your horse becomes a veteran. If you don’t look, you won’t find them, so it’s wise to arrange regular eye checks. Not all will cause problems, but it is always best to know so that the condition can be monitored and, if possible, treated.
Vision loss is often gradual and may go undetected. Ask yourself these questions and call your vet if you sense that something is amiss.
Is your horse losing confidence? Look for subtle changes — he may become uncertain in his gait, especially in dim light, or reluctant to walk over unfamiliar ground. He might bump into walls or fences, perhaps misjudging the width of a stable door and knocking himself.
Has he become bolshy or unpredictable? Walking over or barging into a handler could indicate altered vision. A horse will be more prone to accidents and injury if he is unable to assess hazards.
Is he less certain in the field? Be aware of behavioural or pecking order changes, especially among established herds. A horse with poor vision may follow others closely, tilting his head and using his ears as he relies on his hearing.
When ridden, is he unusually awkward? If his vision is changing, he may shy frequently and become less responsive to the aids or more reluctant to move forwards.
Is he ‘nosier’? He may start using touch, running his nose over novel objects — a good reason not to trim whiskers and eye feelers, as they provide vital sensory feedback.
Any noticeable signs? An eye that appears painful and is partially closed, or has obvious discharge or discolouration, needs veterinary attention.
Ref Horse & Hound; 11 April 2019