An all too common call that Equine Medical Services gets is a horse with a swollen, painful and runny eye. There are several things that can cause this condition in your horse and the following are some of the most common causes. First, a brief review of the anatomy of the eye, then a look at (pun intended) a couple of the most common problems seen with the horse’s eye.
The anatomy of the horse and eye placement on the side of the head gives the horse the ability to see almost entirely around their body. There are however, two blind spots; one directly behind their head backwards to the tail and beyond and one in the front of the head between the eyes. But this placement puts the eyes in a position to become injured.
The horse’s eye is very like our own with a couple of exceptions. The horses’ pupil is shaped more like a long narrow horizontal oval and they have a nictitating membrane or a “third eye lid”. The pupil shape gives the horse a better horizontal view of the environment and the third eyelid helps to protect the cornea, the most outer portion of the eye itself. The cornea is transparent so that the light from the environment can pass through to the back of the eye to the retina. The cornea is very thin, about 0.5mm, but made up of 5 different layers. The epithelial layer is the most outer layer and one of importance for this article. The cornea is also the most sensitive part of the body. The density of sensory pain fibers in the cornea are 300-600 times greater than the skin and 20-40 times greater than the dental pulp. Behind the cornea is the anterior chamber, then the lens is next. The iris surrounds the lens on all sides and moves across the lens to allow the right amount of light into the back portion of the eye, through the posterior chamber, to be focused on the retina. The action of the iris as it increases or decreases in size is what we see as the size of the pupil dilating or contracting. The lens also moves slightly to focus the light and the image being transmitted onto the retina. As the light encounters the retina, the retina transforms the image light energy into chemical energy then into electrical energy to be transmitted down the optic nerve to the brain to be distinguished as images. Around the eye is the conjunctiva, the pink portion of tissue around the eye that is the attachment from the globe to the eyelids. Then there are the eyelids and the sclera or the white portion of the eye. Under the sclera is the layer called the uvea. This layer contains the blood vessels and usually is not seen.
Unfortunately, a lot can go wrong with the eye and that can become a serious situation that ultimately can cost the sight or the loss of the eye. One of the most common problems encountered on a day to day basis is corneal scratches and ulcers. Scratched corneas can lead to corneal ulcers and by the time these are noticed, most scratched corneas have developed into ulcerations. Ulcerations are very painful, and the horse will avoid opening its eye and will have a lot of excess tearing. The one test we can do to see if the cornea is injured is a fluorescein stain exam. This stain does not react with a normal cornea. But a cornea that has the first layer injured, the stain will give that portion of the cornea a fluorescing green glow. Depending on the degree of severity of the ulceration, there is typically some sort of infection to treat along with a degree of inflammation. The treatment is usually a topical antibiotic ophthalmic medication for the infection, a topical atropine ophthalmic medication, and systemic non-steroidal anti-inflammatory drugs (Banamine or Phenylbutazone “Bute”) to deal with the inflammatory portion. More severe case may need more aggressive treatment like surgical removal of abscesses that occur and/or removal dead tissue, and/or possibly placing a tissue graft to help the cornea to heal. The more complicated cases may need more frequent drug delivery due to the washing away of any topical drugs because of the washing effect of the tears.
Another common problem observed with the eye is conjunctivitis and uveitis. Both are inflammatory conditions of the conjunctiva and the uvea. Both conjunctivas and uveitis can be seen with corneal ulcers, but they can also be in other cases. Conjunctivitis is found when there is some inflammatory response to infections caused by flies, irritation from the wind blowing dust into the eyes, and other foreign bodies that find their way past the eyelids.
Uveitis can occur if the entire eye is inflamed. One type of uveitis is a condition called equine recurrent uveitis (ERU) or “moon blindness”. The signs of this condition are pain, squinting of the eye, increased tear production, and in severe cases cloudiness of the eye. There is not one cause of this condition, but rather many proposed causes to include bacterial or viral infections with a delayed autoimmune response by the horse. Some cases can be severe enough that the horse goes blind in the affected eye. The Appaloosa breed, to include Ponies of America, does have a higher risk of developing ERU due to genetic predilection. Treatment for both conjunctivas and uveitis is anti-inflammatory drugs, both non-steroidal and steroids, topically and systemically, and topical antibiotics if deemed necessary.
One other common problem with eyes is blunt trauma to the eye area. Lacerations to the eyelids or tissue surrounding the eyes is a result of blunt trauma. Depending on the severity of the blunt force, mild to moderate swelling of the area around the eye and conjunctivitis are typically seen. However, with more severe trauma, detached retinas, interocular hemorrhage, and even rupture of the globe can occur. Treatment depends on the severity of the trauma, the condition of the eye and if any laceration repair if necessary.
The horse’s eyes are a unique sensory organ and can be very vulnerable to injury. If you suspect your horse has any sort of eye problem, time is of the essence to protect the health of the eye and in a worst-case scenario prevent the loss of sight or even the loss of the eye itself.