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Equine recurrent uveitis is the leading cause of blindness in adult horses. It appears
to be an immune-mediated disease. (See sidebar.)
Many agents have been implicated in equine recurrent uveitis, but the highest percentage
of moonblindness is blamed on leptospirosis, a bacteria commonly carried in the urine of
cattle and deer. The most likely method of transmission is contaminated drinking water.
Horses grazing pasture with access to streams or ponds where cattle or deer water often
show titers (antibiotic levels in the blood) to leptospirosis and some outbreaks have been
directly linked to these water sources. Other bacterial causes are possible, however,
because many affected horses do not have high leptospirosis titers. Among these are
brucella (more common in cattle), streptococcus, viruses such as equine influenza and
equine viral arteritis and some parasite larvae, notably onchocerca, strongylus and
toxoplasma.
Diagnosis is made on the basis of clinical appearance and lab diagnostics. These horses
initially show painful, tearing eyes with clear-to-slightly cloudy ocular drainage. The eyes
are held closed in bright light and the conjunctiva (the white tissue around the globe of
the eye) is red and irritated. Horses may be off feed and may behave abnormally, depending
on the degree of pain and their individual tolerance levels.
Why certain bacteria and viruses take up residence in the eyes is not known, but that
the eye is isolated from the rest of the body's systems explains the flare-ups and regression
of the disease. The fluid in the eye is not readily shared with the rest of the body;
therefore substances in the eyes are separate from the body and each eye is separated
from the other. Many horses can have advanced recurrent uveitis in one eye while the
other is unaffected.
Proteins are produced in the fluid of the eye as the immune system responds to ocular
irritation. This fibrin tends to make the eye cloudy and imparts a blue color to the globe.
The immune system will slow down or reduce the bacteria, virus or parasite and, after a
few days, the flare-up will resolve. The horse can then appear normal for a time until
some stress causes the foreign agent to become active again.
| Good guys gone bad
Immune-mediated diseases are caused by the body's overzealous response to a problem.
In these conditions, a parasite, bacteria or allergen enters the body and is recognized
as foreign by the body's defenses--the immune system. The system responds with specific
cells and internal reactions designed to destroy the invaders.
In most cases, the immune system kills the invader and removes it, and all is well.
In immune-mediated diseases, however, the body cannot kill and remove the problem, and
the defense response is so exaggerated that the very cells and toxins used to protect
begin to damage. Massive inflammatory responses occur and the body is literally destroyed
by the attempt to heal itself.
Moonblindness is a classic example, but immune-mediated diseases can occur in the skin,
lungs, intestines and kidneys.
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Eventually, these recurring periods of flare-up and damage will lead to blindness
in the affected eye. The invading organism can get into the bloodstream and into the
opposite eye, as well. For this reason, badly affected eyes with severely reduced vision
and recurrent painful episodes are removed to eliminate the source of the infection and
to preserve the remaining good eye.
The main goals of treatment are to preserve vision, reduce and control inflammation
and discomfort and minimize permanent damage. Owners must become aware of the early
signs of an impending flare-up because the earlier treatment is started, the quicker
the inflammation will resolve and the less immune-mediated damage will occur.
Treatment consists of topical and systemic corticosteroid application. Steroid and
antibiotic ointment is placed in the eye several times per day. It is important that
owners learn to differentiate between moonblindness flare-ups and other causes of eye
inflammation. Staining the eye to look for corneal damage is an important first step;
horses with corneal damage should never receive steroid ointment. The absence of stain
uptake on the globe of the eye confirms an inflammatory condition and uveitis treatment
is then begun.
Severely affected horses may need injections of corticosteroids in the tissues around
the eye. Phenylbutazone, aspirin and banamine are also used to help with inflammation and
pain. Antibiotics are only occasionally used because affected horses are rarely systemically
ill and, though bacteria may cause the disease, it is the immune response--not the
infection--that causes the damage.
A surgical procedure called vitrectomy is showing some promise. Surgeons remove tissue
lining parts of the eye and replace fluid in the eye during this operation.
There is some disagreement as to why this works and what benefit it provides to the
horse, but test cases have shown enough promise to make this a continuing area of research.
The implantation of small beads of a drug called cyclosporin has also shown some promise.
In this procedure, small beads containing a slow-release form of this anti-cancer drug are
imbedded in tissues of the eye. This drug inhibits the recurrent flare-ups and seems to
drastically slow the progression of the disease.
A vaccine for leptospirosis, recommended for horses at risk, is also somewhat
controversial--some horses have negative reactions to this attempt at protection.
Recognizing cases early on and treating them aggressively will help. Maintaining clean
water sources and closed pastures to limit contact with cattle and deer also makes sense.
Ken Marcella, D.V.M., is based at the Chattahoochee Equine Center in Canton, Ga.
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