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Eye Disease: ERU
(Scientific Name: Equine Recurrent Uveitis (ERU))


Equine Recurrent Uveitis (ERU – also called periodic ophthalmia or moon blindness) is one of the oldest and most common, recurrent form of eye inflammation that can have a devastating effect because it is the leading cause of blindness among horses. This eye disease occurs to varying degrees and intervals among equines and may affect one or both eyes. ERU is also sometimes known as ‘moon blindness’ (because it was once believed that the moon cycles had something to do with this disease – which isn’t true, although the disease tends to come and go periodically).



ERU refers to the inflammation of the uveal tract (middle layer of the eye) of the eye (uveitis). This tract consists of three components:


» the iris (the colored portion of the eye responsible for opening and closing the pupil);

» the ciliary body (which produces a watery fluid that maintains the shape of the eye); and

» the choroid (the back portion of the eye responsible for supplying blood and nutrients).


The inflammation of the uveal tract is from an autoimmune reaction by a horse to its own tissues in its eye. After a horse is initially affected, ERU recurs at varying intervals, each time causing additional damage that leads to permanent changes in the internal structures of the eye.


The two most common causes which start the autoimmune reaction of ERU are trauma and infectious diseases. Trauma to the eye may result from a blunt blow or by penetration of the eyeball (globe) by a sharp object. Other typical causes include:


» Bacterial organisms like leptospirosis;

» Viral agents like Equine Influenza,

» Parasites like Onchocerca cervicalis; and

» Miscellaneous sources like tooth root abscesses.


After the initial inflammation has been successfully treated and if the original condition was indeed recurrent uveitis, another set of circumstances may cause the inflammation to return. Possible circumstances could include wind, dust, pollen, stress of any kind or the premature cessation of the anti-inflammatory medicine. With ERU, the episodes of inflammation will continue to recur.

Some equine breeds have a predilection for developing ERU, including Appaloosas and Warmbloods.


» SYMPTOMS: The equine eye is a highly specialized structure and therefore responds to inflammation in a variety of ways:

» Common signs include cloudy, puffy, swollen and watery eyes.

» Your horse may also exhibit head shaking, a runny nose, white spots or even bleeding in their eye along with matter/pus collecting.

» An affected equine may also lose their balance, trip, run into things or constantly rub the inflamed eye.


» Other signs include eye discharge (which may be evident as tearing), a bluish (or even greenish) tinge to the cornea (corneal edema) and the presence of prominent red blood vessels in the whites of the eye (conjunctival hyperemia or scleral injection).

» Horses with this condition are typically hold the eyelids closed or squint (blepharospastic) and may be sensitive to light due to the painful nature of this disease.

» Another typical sign for an equine with ERU is a constricted pupil when the horse is in the barn or darkened stall. A constricted pupil indicates a painful spasm is occurring and immediate treatment is required to stop the spasm.

» Horse owners have also reported the following symptoms: little to no grazing, decreased appetite, spookiness, grumpiness, excessive blinking and the avoidance of sunlight.


Any of these signs may indicate a potential problem and the presence of ERU. Your veterinarian should be consulted immediately for a proper diagnosis and immediate, aggressive treatment to save your equine’s eyesight.




Short-term treatment consists of using prescription medication Atropine to dilate the eye and reduce the discomfort from spasms of the iris. A steroid or cyclosporine may also be recommended if the eye is not ulcerated. Using steroids in an ulcerated eye can make the condition worse and possibly lead to blindness. If the eye is already ulcerated, an antibiotic would be the recommended course of treatment. Medications can be in ointment or drop form.



In addition to eye ointments and drops, bute, banamine and aspirin (all non-steroidal, anti-inflammatories) are used to decrease the inflammation in the affected equine eye. In some cases, veterinarians will use the Atropine for a few days and then stop it so that the pupil can start to constrict again. The aim is to keep the pupil moving so that the little pieces of protein that form strands in the eye (synechiae) will not cause the pupil to fuse into one position.  Flunixin meglumine (a non-steroidal anti-inflammatory drug (NSAID)) is sometimes used in acute cases and can quickly bring relief and help facilitate a proper and pain-free examination of the affected eye.


Effective long-term management involves identifying and eliminating the specific triggers of the individual episodes. This is easier said than done. A daily log may help you figure out the possible patterns and causes.


Integrative Medicine


Not all animals can use all natural remedies; allergic reactions to oils
and/or herbs and digestive problems are possible.
A natural remedy is not a substitute for veterinary care.


The following nutraceuticals or natural/herbal formulas and alternative therapies can also provide effective treatment and relief for your equines dealing with ERU.


» Zinc: helps support the immune system in the eye.

» Vitamin A: can help decrease the inflammation in the eye.

» Yucca: helps decrease eye inflammation.

» Bee Pollen: may decrease allergic reactions.

» Flax, Hemp and Chia seeds: Omega 3 anti-inflammatory, immune enhancing.

» Hot/Cold Compresses: helps relieve pain.

» Eyebright herbal eye wash.

» Acupuncture: helps with pain and inflammation, normalizes a body’s immune responses.


Eye Disorders: Recurrent Uveitis (ERU)
– Equine Products –

(Reference: Veterinarians’ Desk Reference)

(Live Link)






1 scoop bid

Down-regulates the pro-inflammatory substances, tumor necrosis factor, alpha (TNF-α) and nuclear factor-kappaB (NF-κB), supports normal eye function.


3-4 scoops sid

Enhances the immune system, supplies bioactive proteins, immunoglobulins and peptides.

Haliotis Powder3

15 grams bid

Clears heat in the liver; detoxifies and brightens the eye.


8-12 capsules sid

Plant sterols and sterolins enhance immune function.


6-12 capsules sid

Has affinity for ocular tissue.

Zinc Picolinate (Double Strength)12

100-120 mg sid (30 mg per capsule)

Has immunologic action in eye.

Long Dan Xie Gan Tang (Gentiana Combination)14

4 tsp bid

Clears Damp Heat, inflammation, Liver Heat, eye inflammation.


To help you quickly find the right Integrative Medicine formulas and manufacturers to help treat your dogs, cats and horses, please refer to the Veterinarians’ Desk Reference
(Click Here)


(Always consult with your veterinarian to properly diagnose any health problems. Misdiagnosis and/or mistreatment -- including OTC and/or homeopathic products -- can lead to dangerous complications.)



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