Insights into equine vision
The horse’s vision is an important topic in relation to the welfare of the horse and of course, the safety of the rider and handler. Unfortunately, there are many aspects of a horse’s vision that are not well understood. Dr Andrew Turner, specialist in veterinary ophthalmology educates us about some of the eye conditions that may befall horses, and how they are treated.
Consideration of the anatomy and physiology of the eye is important in the understanding of a horse’s vision. Through evolution, the horse eye has developed as a large globe with a horizontally oriented pupil.
The eyes are situated in a lateral position on the head. This provides the horse with superior peripheral vision, enabling it to see predators more easily. The pupil also allows a broad perception of light and movement, again necessary for a horse to avoid predators.
It is important to understand how the horse’s eyesight adapts to changes in light. The photoreceptor cells of the retina are called the rods and the cones. The rods are most active in dim light and can take 20 to 30 minutes to adapt when a horse moves from bright light to dim light. In practical terms, this is most important when horses move from bright light to much dimmer light.
For example, in preparation for an indoor showjumping event where the horse must move from a brightly lit outdoor practice area into a dim indoor showjumping arena. Poor performance in the arena may reflect the lack of time the horse’s eye has had to adjust to the dimmer light and it is therefore essential that indoor arenas are well lit. Despite this, a horse’s dim light vision is very good once adapted.
People often ask how well a horse can see in the dark. At night time we have all heard horses thundering around a paddock in the pitch dark, mostly avoiding objects in the paddock, and this is a clear indication of their ability to see in the dark!
Conversely, in bright light, a horse’s eyes adapt to reduce the effects of glare. The pupil narrows to protect the light-sensitive retina from very bright light. The small brown lumps on the pupil margin (granulae iridica, corpora nigra) are thought to reduce the effects of glare (dazzle).
The retina of the horse is also particularly well adapted to detecting moving objects rather than stationary ones. This enables the horse to see moving objects far away and accounts for the way a horse will stand motionless and gaze into the distance at a moving object humans are often unable to see.
Another common question is, what colours does a horse see? Horses are not colourblind. Broadly speaking, horses see two colours (dichromatic vision). The colours best seen by horses are blue and a colour between green and red. Red is seen as a dull grey colour, not as red. This is because it is the horse’s brain that perceives the colour, not the eye.
Humans see four basic hues: blue, green, yellow and red and a range of intermediate colours such as yellow green, reddish yellow, reddish blue and other combinations.
How does a horse’s visual acuity compare to humans? In human terms, the best vision is 20/20 and horses are thought to have the equivalent of 20/30 to 20/60 vision. But this difference is not particularly significant.
Horses have a good field of vision, with two well-known blind spots. There is a small blind spot at the back of the horse, so don’t ever quietly walk up behind a horse and touch the butt of the tail! And there is also a blind spot directly beneath the head and chin area to be aware of when handling and riding horses. It is important to understand this particularly in relation to jumping a horse. Basically, the horizontal visual field of a horse is about 350 degrees.
Horses can appreciate depth even with only one eye using specific visual clues. These include relative brightness, size, contour and areas of light and shadow. I clearly remember a horse that was totally blind in one eye completing the Adelaide Three Day Event with a clear round – and it was competing in the 4 Star Division – Olympic and World Championship level!
There are numerous eye injuries and diseases that potentially threaten a horse’s vision but with appropriate treatment, the horse retains useful vision. The most common issue is corneal disease including corneal ulcers and abscesses and corneal cloudiness associated with glaucoma or high pressure in the eye. The cloudiness that results may affect the horse’s vision, but it is difficult to determine the degree of vision deficit that results from the opacity.
Behavioural tests can be performed to evaluate the degree of vision impairment. This involves setting up an obstacle course and covering one of the horse’s eyes while observing the horse’s ability to negotiate the objects in the course. Then covering the other eye separately and observing any changes in the horse’s ability to negotiate the course with only that eye covered. Although not as revealing as a human reading a Schnellin eye chart, it is still, nevertheless, a useful method to evaluate vision impairment in the horse.
Diseases of the iris can also severely impact vision and these include post-inflammatory lesions following severe uveitis and also benign iris cysts. Many years ago I was asked to examine a horse that was shying as the rider transitioned from a trot to a canter. On examination, there was a large free-floating cyst in the anterior chamber of the eye (the space between the cornea and the iris). The cyst moved with the upward transition of the horse’s gait causing the horse visual discomfort. On removal of the cyst, the horse’s shying issue ceased.
Another vision-threatening eye problem involves cataract or opacity of the lens. Small opacities are often noted in the lens of the horse but most of these have no noticeable effect on the horse’s vision, whereas these small lens opacities can have a big effect on a person especially when driving a car at night. Complete lens opacity in a horse will have a serious effect on vision and in these cases, cataract removal may be advantageous.
Other vision-threatening eye diseases include the effect of high intraocular pressure or glaucoma on the optic nerve which transmits messages from the retina to the brain. On occasions we see severe retinal disease causing vision problems. Some years ago, I was asked to look at a horse that had apparently gone suddenly blind. When I looked at the horse, a severely diseased retina was observed in both eyes, but it was obviously a long-standing disease.
On further questioning, the owner revealed that she had recently taken the horse’s paddock-mate out of the paddock, leaving the horse on its own and without a guide. Finally, it has been observed that some spooky, clumsy, unpredictable horses that are poor jumpers have been found to have refractive errors, including short and long-sighted issues.
In summary, there are many conditions that can have a serious effect on a horse’s vision and most importantly we should be mindful of the safety of the horse, its handlers and the rider. If in doubt, seek a professional opinion and, if necessary, a referral to a veterinary ophthalmologist.