Horses are known to have one of the largest eyes of all mammals, with ostriches being the only other land mammal to have larger eyes. The horse’s eye (being 8 times larger than a human eye), gives them an advantage of detecting subtle movements as well as providing 350-degree vision, ideal for both day and night activities and to protect them from predators, as horses are essentially a prey animal. Their large eyes are situated on the side of their head to provide this superior vision; however, this does make the eye more vulnerable to trauma. Due to this vulnerability, horses are at risk of several eye conditions that can lead to severe issues. This makes the equine eye challenging but an interesting part of veterinary medicine.
Understanding the Functions of the Equine Eye
Horses are known to use two forms of vision: monocular and binocular. The left eye and the right eye work independently of each other and each produce a different image due to monocular vision. This allows the horse to see both sides of their body. Binocular vision allows the horse to see directly ahead of them using both eyes together, to create a single more detailed image. As a result, horses have amazing vision, but they can also develop a lot of eye conditions as well.
Common Eye Conditions in Horses
A common eye condition in horses is known as conjunctivitis. Conjunctivitis is the inflammation of the conjunctiva, the mucous membrane that covers the front of the eye and lines the upper and lower eyelids. Symptoms such as eye discharge, swelling, rubbing or redness of the eye, may be seen. This inflammation may be caused by primary causes such as disease, allergies, parasites, infection, and foreign bodies. Conjunctivitis may also be seen as a result of a secondary cause. According to Stephanie Collins (1), secondary causes are ones which affect another part of the eye but then go on to affect the conjunctiva. Conjunctivitis in this case, is not a causative diagnosis, but rather a clinical sign. If symptoms of conjunctivitis are seen, a veterinarian can perform a thorough eye examination to address the clinical symptoms with the use of appropriate products to relieve the pain and discomfort in the first instance and the use of eye lubricants will be of great value here too. The veterinarian will then address any possible underlying secondary causes such as Corneal Ulcers, Uveitis, and Glaucoma.
Corneal Ulcers in Horses
The cornea is a thin transparent membrane offering protection by covering the front of the eye. It has a vital role of allowing light to reach the retina, located in the back of the eye, and as such, the cornea plays a critical role for improved vision, as well as for protection. Abrasions or injury to the cornea are very common due to the position and size of the horse’s eye, and these can lead to a sore or ulcer known as a corneal ulcer. Without correct treatment, a horse’s vison can be affected by the ulcer as well as causing immense discomfort and pain. Signs of corneal ulcers can include conjunctivitis as well as cloudy discoloration of the cornea, excessive tearing, and swelling.
Treatment of corneal ulcers will depend upon the severity of the ulcer and a test will be undertaken to ascertain whether an ulcer is present. A veterinarian will place a fluorescein dye directly into the eye when an ulcer is suspected. The fluorescein dye will adhere to damaged areas of the cornea and will fluoresce green. By using an ophthalmoscope, the veterinarian can then determine whether the ulcer is superficial or deep. According to Dr. Kirk Gelatt (2), a superficial ulcer can be controlled with topical antibiotics. A similar treatment will be used for deep ulcers as well, but many require surgical grafts to strengthen and maintain the integrity of the cornea.
Uveitis in Horses
The uvea is a layer of the eye that contains three structures, the iris, (the colored part of the eye that controls and adjusts pupil size), ciliary, (the part of the eye that makes the aqueous fluid and changes the shape of the lens), and the choroid, (the middle part of the eye that supplies most of the blood and nutrients). Uveitis is the inflammation of the uvea and is another common underlying condition that can contribute to conjunctivitis.
Like corneal ulcers, uveitis can be extremely painful for horses. It can become chronic and begin to recur at regular intervals. This is known as Equine Recurrent Uveitis (ERU) also known as moon blindness. ERU is an auto-immune disease where the body’s immune system attacks its own eye tissues, and it is one of the leading causes of blindness in horses, especially if left untreated.
According to Dr. Sarah Wahlert (5) from the University of Minnesota, in the United States alone 10 to 25 percent of horses suffer from ERU and 25 percent of horses diagnosed are appaloosas. Appaloosas, paints, drafts, and warmbloods are all at a higher risk of developing ERU.
Common signs of uveitis are squinting, excessive tearing, redness, clouding of the cornea, and contraction of the pupil. Uveitis can affect your horse’s performance as well as their quality of life. If any signs are noticed, it is extremely important to have your horse examined by a veterinarian promptly.
Glaucoma
Glaucoma is another serious eye problem that can occur in horses. Glaucoma is where the optic nerve within the eye (that connects to the brain) becomes damaged, causing increased pressure within the eye and as a result, losses the ability to send images to the brain causing blindness. Glaucoma can be considered primary, secondary, or congenital. Primary glaucoma is usually considered genetic but can be difficult to prove in some cases. Congenital glaucoma is seen at birth due to developmental abnormalities. It rarely responds to treatment, but it is uncommon. Secondary glaucoma is caused by trauma to the eye and results in increased pressure in the eye due to poor drainage issues. Equine Recurrent Uveitis (ERU) is the most common reason for a horse developing secondary glaucoma.
Early signs of glaucoma can go unnoticed by owners. Mild edema, redness of the eyes, or pain are all signs that a horse is in the initial stages of developing glaucoma. If signs are noticed, it is important that a veterinarian gives a thorough eye exam of your horse. If it is left untreated it can be extremely painful and can result in the loss of sight or even the eye itself.
Summary
Horses’ eyes are large, but delicate structures and as such are prone to trauma. It is important to be observant of the normal appearance of their eyes in order to be aware of any changes that may occur. Several eye conditions can result in similar symptoms, so it is essential you have your veterinarian do a full work up each time any abnormal changes are noted. It is especially important that all eye conditions are treated as soon as possible as delay may lead to irreversible damage as well as causing undue pain and discomfort to the horse. In all situations, an ocular lubricant can help to provide relief from discomfort and irritation and is often provided as an adjunct to other forms of prescribed treatments.
References
https://www.horsevet.co.nz/news/article/62/equine-conjunctivitis/
https://brandonequine.com/wp-content/uploads/2020/05/conjunctivitis-and-your-horse.pdf
https://www.liverpool.ac.uk/equine/common-conditions/uveitis/
https://extension.umn.edu/hors...
Article written by Lauren Coldiron, Equine & Large Animal Inside Sales Representative at Millpledge Veterinary. A California native, she found her love for horses at a young age and started her riding journey. Her love for horses moved her to Texas to work for an accomplished equine veterinarian where she found her passion for equine rehabilitation. She started her own business as a certified equine sports massage therapist and has a strong dedication to helping horses feel their best. At Millpledge, this dedication shines through as she assists veterinarians in selecting the most suitable products, contributing to the health and well-being of horses.
