This article is provided by Appaloosa Project research team member Dr. Lynne Sandmeyer for the education of website visitors. It is protected by international copyright law, and no part can be reproduced without the express written consent of the author.
The Basics of Vision
- Light enters and travels through the clear cornea, anterior chamber, lens, and vitreous
- The cornea and lens function to bend (refract) the light so it is focused on the retina
- Light strikes the retina where it is converted into an electrical signal by photoreceptors (rods and cones) – this is called transduction
- Rods – specialize in night vision
- Cones – specialize in day and color vision
- The signal passes through other cells in the retina which process and modify it
- The signal is then transmitted to the brain via the optic nerve
- The brain processes the signal into a visual image
What Does the Horse See?
- Horses have large eyes and a wide horizontal pupil which allows large amounts of light to enter
- Horizontal pupil allows wider view of the horizon
- Visual perspective varies with head position and breed (eg. Miniature horse versus Clydesdale)
- Lateral position of the eyes gives a panoramic view
- Each eye can see about 180 degrees horizontally by itself
- Binocular overlap of 65 degrees (directed down the nose and toward the ground, ie. where it would feed)
- Total visual field of about 325 degrees when head is pointed forward
- Small blind spot directly behind the head and in forehead region
- Peripheral vision is adapted to detect motion
- The nose prevents the horse from seeing directly under the body and forward for about 4 feet in front of the legs
- Visual acuity: 20/30 vision (Snellen Chart): horse has 0.6X the acuity of humans, 1.5X acuity of dogs, 3X acuity of cats
- Better able to discriminate between objects on the ground versus 70 cm above ground
- Accommodation (ability to focus on near objects by changing the shape of lens) is limited – horse changes head position to bring near objects into focus
- Have stereopsis or depth perception
- Good vision in dark - 9:1 ratio of rods (photoreceptors specializing in night vision) to cones (photoreceptors specializing in day and color vision)
- Reflective tapetum increases light capture ability in the dark
- Color vision is poorly developed – can discriminate red and blue from gray, ability to discriminate green and yellow is variable
Types of Visual Disturbances
Inability of light to enter the eye and strike the retina
- Opacification of clear structures which may be caused by
a) Corneal infection, edema, scarring
b) Cloudiness of the aqueous humor
c) Lens opacities (cataracts)
d) Cloudiness in the vitreous
Inability to convert light into an electrical signal
- Disease of the retina (eg. degeneration or detachment)
Inability to transmit signal to the brain
- Disease of optic nerve, chiasm, or radiations eg. inflammation degeneration, tumors
Inability of brain to process the signal into a visual image
- Cerebral cortex inflammation, degeneration, tumors
Common Causes of Visual Disturbance in the Horse
Congenital Disease
- Congenital cataracts
- Congenital stationary night blindness
Acquired Disease
- Equine recurrent uveitis
- Corneal disease
- Glaucoma
- Head trauma and optic nerve degeneration
Management of Blind Horses
General Notes
- Horses have natural history as a prey species, wary temperament, when cornered may kick, strike or run
- Herd animals, visual cues important in establishing dominance order in a group. Individuals that ignore the visual cues of herd mates are often bitten, shoved, or kicked by dominant individuals
- Horses that become trapped in fences or other hazards tend to panic causing themselves injury
Signs Suggestive of Blindness
- Gradual onset in many cases – progressive uncertainty, especially in low-light
- May bump into walls or fences, show reluctance to walk over unfamiliar terrain
- Herd behavior may change
- May shy frequently when ridden
- Fear or anxious behavior (eg. circling, calling to herd mates, crashing into walls, ignoring restraints)
- Traumatize themselves by running into unfamiliar obstacles
- Balance and posture may be altered initially
- Behavior becomes more settled as they adapt over weeks to months
Adaptation to Blindness
- Temperament will affect how horses adapt to blindness
- Horses showing excess fear are dangerous – can injure themselves or handlers
- Animals with calmer temperaments may adapt well
- Blind horses can develop a “mental map” of their environment (know perimeter of different paddocks or pastures, run, play within the boundaries)
- Increased use of other senses: hearing, smell, touch (horses nose and lip has similar concentration of sensory nerves as the human hand)
Social Interactions
- Dominant sighted horses may “bully” a blind horse
- Blind horse may keep to itself and hang back from the herd and may lose weight because of poor access to food
- Occasionally one or more protective horses will act as a companion to the blind individual and guide the blind horse around obstacles
- If guide horse wears a bell on the halter the sound clues can provide guidance
- Blind mares with foals need to know the foal is nearby
- Foal wearing a bell on the halter may make mare more relaxed
- When restraining, hold the foal near the mare’s front end so she can touch, smell and hear the foal
How to Assist a Horse in Adaptation
1. Provide a Safe Environment
Remove hazards:
o Tape up bucket handle hooks
o Cover sharp nails
o Take down pieces of wire in stalls and paddocks
o Cut down low hanging tree branches
o Consider board fencing for paddocks
Initially: Use a treeless paddock with a board fence and a stall or run-in shed with smooth, solid walls
Later: remove hazards from pastures, use board or smooth fencing, maintain a safe stall or shed
- Demonstrate the boundaries of any new enclosure
- Encourage the horse to touch fences, gates and water sources with muzzle
- Post signs on stalls or paddocks to alert visitors that the horse is blind
2. Behavioural Considerations
- Identify stressors causing anxiety (primarily confinement and separation from other horses)
- Be aware: regardless of temperament, any horse that has lost vision can change its behavior quickly and spook if outside stimulus suddenly scares it
3. Companions
- Initially house blind horses with or without presence of a calm, sighted companion in a paddock or barn
- Later choose turnout companions that are non-threatening
- If a guide horse emerges, consider putting a bell on its halter
4. Horsemanship
- Steady handling, regular grooming
- Predictable schedules for meals and turnout
- Respond to voices, smell and touch of people they knew and trusted when visual so speak to horse when approaching it and working with it
- Use consistent phrases and inflections when teaching voice commands: “whoa”, “step up”, “step down”, “stand”
- Approach the shoulder area first and stay near it when working around the horse as this is the safest position
- Do not clip whiskers on muzzle
- Keep feed and water in same location
- Handle bilaterally blind horses from both sides of the body
- Avoid initiating alarming sounds or sensations on the blind side of unilaterally blind horses
- Practice loading on and off trailer
- Be patient
Use of Blind Horses
Riding? – Cannot be recommended, however, each horse and its circumstances are unique (safety is a top priority)
Pasture pets – Treasured family members
Trail horses – Strong bond with the rider
Broodmares - see above for information on management
Athletes – eg. Dressage: requires precise communication between horse and rider
Unilaterally blind – use as a sport horse with caution (have half the visual field and little to no depth perception)
Sources
Miller PE, Murphy CJ. Equine Vision: Normal and Abnormal. In: Equine Ophthalmology (ed. Gilger BC), Elsevier Saunders, St. Louis, 2005:371-408
Timney B, Macuda T. Vision and hearing in horses. J Am Vet Med Assoc 2001;218:1567-1574
Brooks DE. Equine Ophthalmology. In: Veterinary Ophthalmology 3rd edition. (ed. Gelatt KN), Lippincott/Williams & Wilkins, New York, 1999:1053-1116
Dwyer A. Practical Management of Blind Horses. In: Equine Ophthalmology (ed. Gilger BC), Elsevier Saunders, St. Louis, 2005:449-456