Equine recurrent uveitis (ERU) is the most common cause of
blindness in the horse. ERU is also known as "Moon blindness. Which is
due the recurring nature of the disease once thought to coincide with
the phases of the moon. Uveitis is the inflammation of the
vascular structures of the eye. These structures include the iris
(the colored part of the eye surrounding the pupil), ciliary body
(structures in the eye that help support the lens) and the
choroid (the vascular structure in the back of the eye that
supplies blood to the retina).
ERU is thought to be an immune-mediated disease process
that can be triggered by many different causes. Bacterial agents
such as Leptospira as been most commonly associated with
ERU, but some viral agents such as Influenza, Equine
Herpesvirus and Equine Viral Arteritis have also been
implicated as the initial cause of ERU. Blunt trauma or damage
to the eye itself has also been shown to start the
immune-mediated reaction that leads to ERU. Hypersensitivity to
certain parasites such as strongyles, and Onchocera cervicalis
are also known to cause ERU.
The initial episode of the uveitis may go unnoticed by the owner
and clear up on its own. Repeated episodes, due to the
immune-mediated aspect of the disease, can lead to damage to
other structures in the eye, such as the lens and cornea
resulting in blindness. Clinical signs associated with ERU vary
depending upon the severity and duration of the disease.
Episodes that appear suddenly are usually painful and many
horses become head shy. Excessive tearing from the eye, eyelid
spasms (blepharospasm), and light sensitivity (photophobia) are often
noticed. Within 1-3 days the affected eye will become cloudy and
red from underlying inflammation. A thick yellowish discharge
will also develop. Changes within the lens such as pigmentation
and the development of cataracts can occur weeks to months after
the uveitis begins. ERU is diagnosed by noting a history of
recurring and chronic eye disease characterized by the previously
mentioned signs and physical changes to the eye noted upon
As soon as signs are noticed, treatment for ERU should start
immediately to prevent further damage. Treatment includes
anti-inflammatory eye drops and drops that dilate the pupil.
Systemic anti-inflammatory drugs, such as fluxin, phenylbutazone
or ketoprofen are given to decrease inflammation and pain.
Anti-inflammatory drugs are the most important aspect of therapy
and may be necessary for several months. Dosage and frequency are
dependent upon the severity of the disease. Discontinuing drug
therapy too soon is often a cause for relapse. Stabling the horse
in a dark stall or covering the eye with a hood can also be
helpful in treating the pain associated with the disease. If a
specific cause of the uveitis is known or suspected, it should be
investigated and treated appropriately.
Serological testing for Leptospira and other bacterial agents
may help in confirming the cause, but a negative test result does
not rule out ERU. Biopsies of the conjunctiva (the outer lining
of the eye) can help diagnose the larval form of Onchocera
cervicalis as the underlying cause.
Clinical signs of the disease can come and go. Quiescent stages,
the stages when the eye seems normal, may last from weeks to
months to years before another episode occurs. Darkening of the
iris commonly occurs with ERU as it becomes more chronic.
Common ocular complications resulting from ERU include
glaucoma and limited pupil movement. The iris attaching itself to the back of
the lens or cornea causes limitation of pupil movement. Prompt
symptomatic treatment is helpful in prolonging the integrity and
vision in the affected eye. Unfortunately, each attack of ERU
leads to more damage to the eye and eventually blindness
develops, resulting in a poor long-term prognosis.