Equine Recurrent Uveitis, also known as Moon BlindnessPeriodic Ophthalmia and Recurrent Iridocyclitis is a relatively common disease of the equine eye.  Despite its potential severity, little is understood of the disease and its causes, and treatment remains difficult.  It is characterised by recurrent episodes of moderate to severe ocular pain.

What Is Uveitis?

Uveitis means inflammation of the uveal tract, which comprises the iris, ciliary body and choroid.  The iris is the circular structure that forms the pupil, the ciliary body is the part of the eye that produces aqueous humour (the fluid inside the front of the eye) and the choroid is the vascular outer part of the eye that provides blood vessels to the internal structures.

Uveitis can occur as either a primary or secondary disease process.  Corneal ulcers, for example, will often be associated with significant secondary uveitis.  This is a completely different proposition to ERU, and will resolve with treatment of the primary condition.  Equine Recurrent Uveitis is a primary inflammation of the uvea, and as the name suggests is a recurring condition.

What Causes Uveitis?

The simple answer to this question is that nobody really knows!  Many theories have been put forward, including bacterial infection, vitamin deficiency and auto-immune disease.  In the past, it was thought to be related to the lunar cycle, hence the name “Moon Blindness”, although this theory has now been disproved.

The current leading theory is that uveitis occurs due to a hypersensitivity reaction to a certain type of bacterium (Leptospira spp.), although this is far from proven.  Despite this link with bacteria, antibiotics are ineffective against the disease as the bacteria themselves do not cause the problem, and active infection is rarely found in suffering horses. It is thought that the Leptospirosis causes an immune response that leads to problems with the iris which persists long after the Leptospira has been removed from the body.

There is also a genetic component to ERU, with Appaloosas having a far higher prevalence of the disease than any other breed.

Diagnosing Equine Recurrent Uveitis

The diagnosis of ERU relies upon finding signs of inflammation within the front of the eye with no other detectable primary cause.  Signs of active uveitis include pupil constriction, aqueous flare (protein in the aqueous humour) and hypopyon (pus in the aqueous humour).  Thorough examination of the entire eye to rule out any primary causes will result in a diagnosis of ERU.  A detailed history from the owner may reveal mild episodes in the past, but this is not always the case.

ERU is also one of the major causes of “blue eye”, or corneal oedema.  Inflammation inside the eye disrupts the endothelial water pumps that keep the cornea dehydrated, and therefore clear.  When these pumps stop working, water floods into the cornea and disrupts the collagen fibre arrangement, turning it a pale blue colour.

More advanced techniques include measuring the pressure inside the eye, the Intra-Ocular Pressure (IOP).  In the initial stage of a flare up, IOP will be reduced due to inflammation of the ciliary body, and decreased production of aqueous humour.  Chronically IOP will be increased due to inflammatory debris obstructing the drainage of aqueous humour.

In between active episodes of uveitis, ERU can still be diagnosed.  The characteristic changes are darkening of the iris, shrinking of the granula iridica or corpora nigra (the dark structures/lumps at the top of the iris) and butterfly lesions on the retina.  Adhesions between the iris and other structures (typically the cornea or the lens) can also occur, known as anterior and posterior synechiae respectively.   They can lead to scarring of the iris and if not treated, can lead to permanent papillary constriction (and therefore limited vision).

These changes are not always present, and a horse with ERU may have a normal appearing eye.

Treating ERU

The treatment of acute attacks of ERU can be challenging, and there are few options available.  Topical treatment is likely to be prolonged, and the combination of this with a painful eye can result in significant resentment from the horse.  Ocular lavage systems are commonly used to avoid this and enable treatment.

The aim of treatment is to reduce inflammation and pain within the eye, and this is usually accomplished with a three-pronged approach:

  • Topical Steroids
    • Topical steroids, usually prednisolone acetate, need to be applied frequently.
    • The frequency of treatment will start high but reduce.
    • Prolonged treatment is likely to be necessary, up to 6 weeks after clinical signs have resolved.
  • Topical Atropine
    • This is used to dilate the pupil, which will reduce pain and prevent adhesions (synechiae).
    • Atropine can cause impaction colic, so it is used strictly to effect.
  • System Anti-Inflammatories
    • Flunixin is most often used initially due to its good ocular penetration, anti-inflammatory and painkilling effects.
    • After 5 days an alternative may be introduced due to the side-effects of long term flunixin use.

It is also often useful to protect the eye from both trauma and bright light using a mask.  We have had good results with “Guardian” masks.

Much research is currently being performed to try and find a long-term treatment option that will prevent recurrence of the disease.  Some success has been found using Supra-Vitreal Ciclosporin Implants, and also with Pars Plana Vitrectomies.  These are extremely specialised surgical options performed at very few locations within the UK.


The prognosis for horse with Equine Recurrent Uveitis is unfortunately guarded at best.  If attacks are infrequent and easily managed, then they can be managed for many years with no problems.  However, in some cases the attacks become so frequent and severe that treatment becomes impossible, and surgical removal of the eye is the only humane option.  We have found that removing the eye in these cases can lead to a dramatic improvement in comfort and therefore the quality of life

ERU can also lead to blindness by triggering glaucoma.