Equine recurrent uveitis (ERU) is a devastating disease in horses that can be difficult to diagnose and even harder to treat. This chronic inflammatory disease of the eye is typically characterized by recurrent episodes of uveitis (inflammation of the inside of the eye) separated by periods of inactivity.
While ERU is the most common cause of blindness in horses and can affect any breed, it’s most commonly seen in Appaloosas and Dutch Warmbloods and draft horses.
ERU is also referred to as “moon blindness” because it was once thought that the inflammatory episodes coincided with the phases of the moon. The disease should not be confused with congenital stationary night blindness, another disease of the eye that also commonly affects Appaloosa horses.
Veterinarians typically divide equine recurrent uveitis into three different forms:
• In the classic form, a horse will show episodes of inflammation separated by periods of inactivity. As the horse ages, the inflammation occurs more frequently and is harder to control.
• In the insidious form, the horse’s eye is always experiencing low-grade inflammation that can be hard to identify. Most commonly seen in Appaloosa horses, this form also tends to worsen over time and doesn’t respond well to therapy.
• In the posterior form, the inflammation occurs further back in the horse’s eye. It’s most commonly diagnosed in Dutch Warmblood horses.
In all cases, the disease will affect both eyes over time. Each bout of inflammation causes further damage to the eye that will eventually result in blindness.
What Causes ERU?
The exact cause of ERU hasn’t been determined, but we do know that it is an immune-mediated disease. The horse’s own inflammatory mediators are targeting proteins in its eye. The horse’s autoimmune response may be “turned on” by previous infections with the bacterium Leptospira. In research models, scientists have successfully used Leptospira to induce ERU in some horses.
Because some breeds are predisposed to developing ERU, researchers suspect there is also a genetic component to the disease. There is no single gene responsible for ERU, but scientists have identified genetic markers that are associated with an increased risk of developing the disease.
Researchers at the Western College of Veterinary Medicine (WCVM) are trying to better define the disease in Appaloosa horses by surveying the local horse population in Saskatchewan, Manitoba and Alberta. Genetic information from these horses may provide insight into why ERU is so prevalent in this particular breed.
What are Clinical Signs of ERU?
ERU can occur at any life stage, but the clinical signs most often appear when the horse is middle-aged and continue to progress throughout its life. Appaloosa horses can be diagnosed with ERU as young as five or six years old and can go blind as early as 12 years of age.
Classic signs of ERU are signs of ocular pain: your horse may squint, have excessive tears and/or become head shy. The horse’s eye may appear to be sunken in and red, and it may be warm to the touch. If you look closely, the pupil is often small and the eye may be cloudy.
Symptoms of ERU can be difficult for the untrained eye to notice, especially with the insidious form of the disease. Results of a recent survey showed that many Appaloosas had severe damage to their eyes because of inflammation, but few owners knew their horses were affected due to the subtle nature of this form of the disease. Having your veterinarian screen your horse for ERU, especially if it is a susceptible breed, may help you detect the disease in its early stages. It’s important to talk to your veterinarian if you observe the slightest abnormalities in your horse’s eye. Early treatment will alleviate discomfort for your horse and may delay the onset of blindness. Signs can be subtle, so it’s better to err on the side of caution.
How is ERU Diagnosed?
The key part of ERU is that it’s a recurring disease: more than one episode of uveitis must be documented for your veterinarian to suspect ERU. A single episode of uveitis can be caused by many other things such as trauma or infection.
After a complete physical examination of your horse, your veterinarian will perform an ophthalmic exam to get a good look at your horse’s eye. The exam is non-invasive and can be performed safely and quickly. The ophthalmic exam may include the following steps:
- Sedation to make the exam easier and safer for your veterinarian and your horse.
- A nerve block of the eyelid to prevent the horse from blinking or squeezing its eyelid shut.
- Pupil dilation (with the use of atropine eye drops), making it easier to visualize structures within the eye.
- Assessing the volume of tears produced by your horse.
- Measuring the pressure within your horse’s eye (ocular pressure tends to decrease with uveitis).
- Using a fluorescein stain to detect ulcers in the horse’s eye (ocular ulcers may affect the way your horse is treated).
The veterinarian may also perform general blood work and laboratory tests to detect infections caused by Leptospira or other bacteria (which can cause a single episode of uveitis). These tests are important to rule out other potential causes of uveitis in your horse. Your veterinarian may enlist the aid of a board-certified veterinary ophthalmologist to confirm the diagnosis of ERU and to help develop a treatment plan that’s most suitable for your horse. If you own an Appaloosa horse, it may be wise to have your veterinarian examine your horse’s eyes at its regular check-ups.
How can ERU be Treated?
Once your horse is diagnosed with ERU, your veterinarian will initiate treatment to control the inflammation in the eye. This can be time consuming, particularly in the beginning when your practitioner is trying to get the inflammation under control. What’s important to know is that once your horse is diagnosed with ERU, it will need to be monitored and treated for the rest of its life.
Anti-inflammatory drugs are initially used to control inflammation in the eye. Topical products applied directly to the eye are most common, but it may be necessary to add systemic medications such as flunixin meglumine (Banamine®), phenylbutazone or Aspirin®. Systemic anti-inflammatory medications are particularly recommended for the posterior form of the disease as topical medications are unable to penetrate to the back portion of the eye.
Anti-inflammatory drugs can predispose a horse to the development of gastric ulcers, especially if they are stressed. Gastrointestinal protectants such as omeprazole (Gastrogard®) may be used to reduce the likelihood of side effects.
Eye drops to dilate the pupil may also be given to your horse as a short-term measure to reduce the pain from ocular muscle spasm and to prevent further damage to the eye — including adhesions of the iris to the lens. Veterinarians typically use atropine to dilate the horse’s pupil. While this drug reduces movement in the horse’s gut and can cause colic, the risk is minimal because of the low dosage used in these cases.
Your veterinarian will need to frequently re-check your horse’s eye and monitor the regression of uveitis. Once the eye inflammation is under control, the anti-inflammatory drugs are slowly tapered off. This is a critical period to monitor your horse for any signs of recurrence.
Horses diagnosed with the classic form of ERU can go into a quiescent (inactive) phase in which they don’t show any signs of the disease. After initial treatment, you will need to continuously monitor your horse throughout its life for “flare ups” – clinical signs of ERU that need to be re-treated. Even with good management, the eye inflammation will increase in severity and frequency as the horse ages.
Horses with this form of the disease may also be candidates for treatment with slow-release cyclosporine implants. These implants can reduce the severity and frequency of flare ups, but can only be placed surgically when the eye is in a quiescent phase.
It can be very difficult to manage the insidious form of ERU in horses. These animals often require treatment with anti-inflammatory drugs for the rest of their lives due to the chronic, low-grade inflammation that they experience in one or both eyes.
What Happens if my Horse Becomes Blind?
All horses diagnosed with ERU will eventually become blind from the consequences of inflammation. These can include cataract formation (an opacity in the lens of the eye), adhesions of the iris to the lens (the iris closes the pupil which lets light into the eye) as well as retinal detachments and degeneration – resulting in an inability of the eye to send images to the brain.
If your horse’s eye is painful and non-functional, it should be removed. An experienced veterinarian can perform an enucleation – removing a horse’s eye from the socket and sewing the eyelids closed.
Evisceration, a more cosmetic procedure, can be performed by a veterinary ophthalmologist. The specialist removes the inner contents of the eye and replaces them with a silicone ball. While the eye won’t look normal, it can still move and blink. For show horses and other equine cases where esthetics are important, there is also the option of creating a more realistic-looking prosthetic eye to replace the original organ. At this point, it’s important to consider the welfare of your horse and your ability to manage a blind horse since both eyes will eventually become affected by the disease.
How do you Manage ERU?
While ERU can’t be prevented, management practices that reduce irritation to the eye and stress for your horse can help to reduce the frequency and severity of inflammation in the eye:
- Manage your horse’s environment: Control insects and rodents. Reduce dust, especially in bedding and feed. Decrease your horse’s exposure to sun and flies by using a fly mask and providing shade. Eliminate sharp objects in your horse’s environment such as low tree branches and hooks in stalls.
- Manage your horse’s health: Provide your horse with regular hoof and dental care and establish an appropriate deworming and vaccination schedule. Maintain a proper diet and avoid feeding your horse from hay nets in order to reduce exposure to dust and potential trauma to the eye. Reduce your horse’s training schedule.
If you have a horse diagnosed with ERU, it’s also important to be vigilant about monitoring for clinical signs such as squinting, excessive tearing or redness of the eye. Call your veterinarian if you suspect a recurrence or flare up of the eye condition.