Uveitis is an ocular (eye) disease frequently encountered in animals. Typically, this inflammation occurs due to leaky blood vessels in the iris (the colored portion of the eye). Whether dog, cat, or horse, a diagnosis of uveitis raises concern for underlying systemic disease, and comprehensive diagnostic evaluation is recommended. In dogs, tick-borne diseases, fungal conditions, and cancer are the most common systemic problems that can lead to uveitis. For cats, the most common diseases include feline infectious peritonitis, feline immunodeficiency virus, feline leukemia virus, tick-borne diseases, and cancer. Uveitis can be caused by a one-time event such as trauma to the eye. In these instances, the condition might never happen again and might not create future problems. However, in the recurrent form of the condition, the disease can lead to permanent damage and eventually blindness. This blog will focus on the symptoms, causes, treatment, and prognosis for horses with uveitis.
What is uveitis?
Uveitis is inflammation of the eye’s uveal tract, a layer of tissue that lies between the eye’s outer layer (including the cornea) and its inner layer (the retina) and includes the iris, the ciliary body, and the choroid. This tissue is delicate, and when it’s inflamed, the effects can be painful. The iris sphincter muscle contracts and causes the pupil to close, resulting in a constricted pupil. The ciliary body muscles spasm, creating a great deal of pain. The blood capillaries will become leaky, and that releases proteins and cells, which results in the signs and symptoms of the disease.
What causes uveitis?
Uveitis in horses can occur because of blunt or penetrating eye trauma with or without corneal ulcers. These “single episode” cases are considered easier to treat and the prognosis, if caught in its early stages, is good. Repeated episodes of uveitis are more serious and difficult to treat. The exact cause of the disease is not exactly known, but bacterial (leptospirosis), viral, and parasitic infections have been associated with uveitis and their involvement of the initial inflammatory event is suspected.[1] Recurrent uveitis is a complex autoimmune disease that is influenced by genetic and environmental factors. The disease tends to increase in severity with repeated episodes. Progression of the disease can result in permanent damage to the structures of the eye.
What is Equine Recurrent Uveitis?
Equine recurrent uveitis (ERU), also known as moon blindness, is the most common cause of blindness in horses worldwide. It affects 2-25% of horses globally, with 56% of affected horses eventually becoming blind. ERU has been around for a long time. The term “moon blindness” originated in the 1600s when farmers thought the inflammatory episodes were linked to the phases of the moon. This is not the case; however, the disease can wax and wane. ERU can be difficult to manage and currently, there is no cure. ERU is defined as repeated episodes of inflammation of the uveal tract of the eye (the middle layer), involving one or both eyes. Another type of recurrent uveitis, called “insidious uveitis” is characterized by a consistent low-grade inflammation that causes cumulative damage to the eye. Cumulative damage caused by ERU can lead to cataracts, glaucoma, and eventually blindness. Although not all horses that experience a single episode of uveitis will develop ERU, they are at risk for the disease.[2] The disease is more often seen in Appaloosas, who are 8 times more likely to develop ERU than other breeds. The “LP genetic test” can be used to evaluate ERU risk in Appaloosas.
The symptoms of uveitis include:
- squinting/tearing
- pain, sometimes severe
- light sensitivity (constricted pupils),
- swollen or red eye
- cloudy appearance or bluish haze over the cornea (i.e. yellowing of a normally blue iris, or darkening of a brown iris.)
- the white of the eye might appear bloodshot or you might see pus or yellow deposits under the cornea.
- clumsy behavior/self-trauma
- resistance moving from one place to another
Graphic courtesy of www.horsejournals.com
What is the treatment for uveitis?
Treatment for uveitis begins with a comprehensive series of diagnostic tests to determine the root of the problem. Diagnosis of a vision problem begins with a complete eye examination. In some cases, referral to a veterinary eye specialist (ophthalmologist) is necessary. Every patient suspected of uveitis should have full bloodwork (CBC and chemistry), urinalysis, and testing that checks for the presence of tick-borne diseases as well as leptospirosis. Additional diagnostics such as chest x-rays, abdominal ultrasound or other disease testing may be recommended.
The treatment goals for single-episode and recurrent uveitis are to reduce inflammation, control pain, and restore balance to the immune system. It is important that episodes of inflammation be detected early so appropriate treatment can be applied and the consequences of inflammation may be reduced. Treatment will take several weeks to months to complete. Conventional treatment options generally include topical and/or systemic non-steroidal anti-inflammatories (like Banamine or Equioxx), corticosteroids, and medications to dilate the pupil (to reduce pain). If the horse is found positive for leptospirosis, a two to four-week course of antibiotics will be prescribed.
Dispensing medication to a horse can be tricky, especially if he/she is experiencing pain. A device called a “sub palpebral lavage catheter” uses a port that is surgically inserted into the horse’s eyelid and attached to the mane. Medication is injected into the port and the medicine is carried through a tube directly onto the eye(s).
Veterinarians may also recommend immune-modulatory medications to suppress the immune reaction going into the eye. Cyclosporine is a common treatment and can also be administered through a sustained-release device. Because many of these treatments will suppress the eye’s immune response, it is important to confirm before treatment that the horse doesn’t have an infection or an ulcer made worse by the treatment. Steroidal treatments require regular checkups with the veterinarian. Unfortunately, use of these drugs is not ideal for long-term use, as they do not help to restore balance to the immune system and in fact, can create an even greater imbalance.
While these treatments are helpful in minimizing inflammation and pain, there are alternative treatments that can support a horse with ERU for the long haul. In Traditional Chinese Veterinary Medicine, the liver is the window to the eye. Your horse may not have liver disease, but ERU is caused when there is heat (inflammation) and stagnation in the liver. The goal of TCVM therapy is to clear the heat from the liver and eyes and remove the stagnation and pain.
This is accomplished with food therapy, acupuncture, and Chinese herbs. Herbs that help move liver qi include ginger, turmeric, and dandelion root. Artichokes can help clear liver heat. These methods provide comfort to the horse while maintaining the vision longer than with conventional medicine. There are a variety of herbal supplements on the market that can bring acute symptoms under control, as well as formulations that address constitutional deficiencies and prevent further ERU episodes.
In ERU, removal of the eye may be the best course of action. Eye removal may be indicated when pain cannot be controlled, or continual flare-ups of the disease make it difficult to administer medication. Eye removal is not a death sentence and doesn’t necessarily mean an end to a horse’s activity. Horses adapt very quickly, and they can live happy lives with only one eye.
Uveitis, particularly ERU, does not need to be a death sentence for a horse. Careful management and treatment can help preserve a horse’s sight longer, and even in cases where an affected eye must be removed, many horses can continue to live largely normal, active lives and even have successful competitive careers.
[1] https://www.horsejournals.com/horse-care/illness-injury/diseases/understanding-equine-vision-and-eye-disease
[2] https://ceh.vetmed.ucdavis.edu/health-topics/equine-recurrent-uveitis
