Equine recurrent uveitis or ‘moon blindness’, as it is sometimes known, is an immune mediated condition whereby the body attacks itself causing inflammation within the eye.

Uveitis can occur following a traumatic event however, when there has been more than one bout and no obvious cause it is then known as recurrent uveitis, or ERU. The uvea is the iris, ciliary body and the choroid which makes up the middle layer of the eye (the layer between the white of the eye (sclera) and the retina).

If the inflammation affects the front part of the eye it is known as anterior uveitis; if the back, then posterior uveitis; or if the whole eye, panuveitis. Any acutely painful eye is an emergency and your vet should be called to attend.

Given how painful this condition is, the horse is likely going to need to be sedated and potentially a local anaesthetic block applied in order to facilitate examination. On examination, the horse may have a cloudy cornea (horses with uveitis may develop a transient or chronic glaucoma due to the increased pressure and inflammation) and there may be an anterior flare giving the eye an orange appearance which is due to an influx of inflammatory cells and fibrin into the anterior chamber (front of the eye).

These cells can then settle at the bottom of the anterior chamber which is known as a hypopyon, or they can form fibrin clots that look like small pieces of tissue paper within the eye. The pupil is constricted and small and thus it can make it difficult to see to the back of the eye with an ophthalmoscope.

An ultrasound examination may be used to facilitate examination of this part of the eye until treatment dilates the pupil and a normal examination can be performed or to aid detection of a retinal detachment.

Following a bout of uveitis, there may be scarring or permanent changes remaining within the eye. These include a darkened iris, evidence of synechiae (pigmented spots or strands on the lens where the iris has stuck to the lens when shut and inflammed), an irregular pupillary margin, evidence of cataracts, butterfly and bullet hole lesions on the retina, blindness or a shrunken eye.

These are the main signs that are looked for by a vet undertaking a pre-purchase examination.

Treatment involves both systemic and topical medication. Atropine is used to dilate the pupil and ease the ciliary spasm. Topical steroids (‘Pred-forte’ or ‘Maxitrol’) are potent anti-inflammatories and are used to decrease inflammation in the eye.

Sub-conjunctival, or systemic steroids can also be used but there are increased side effects associated with these for long term use. Non-steroid anti-inflammatory drugs such as flunixin meglumine or ‘bute’ are also given for their anti-inflammatory and analgesic effects.

If glaucoma persists, then a topical anti-glaucoma drug such as Cosopt may be added to the treatment regime. If a lot of fibrin is present, then an injection of a fibrinolytic known as tPA can be injected into the eye to help break up fibrin clots or stop clots from forming, however this is often a specialist procedure.

Treatment can be prolonged for weeks to months to get the inflammation under control. If administering medication is difficult then a small tube can be fitted underneath the eyelid known as a sub-palpebral lavage system.

This tube comes out of the upper eyelid, over the head and down the neck so that medications can be administered without the eye itself being touched. Occasionally, horses are hospitalised during the initial course of treatment and some may require lifelong treatment.

Surgery can be performed in severe cases where a cyclosporine A implant is placed under the white of the eye that slowly releases the immunosuppressive drug over a few years which can help to decrease the frequency and severity of uveitic flare ups. Vitrectomy and intravitreal injection of gentamicin are also reported treatments but not frequently performed in the UK.

When changes become chronic, or if the eye becomes blind then enucleation (eye removal) may be recommended as the best course of action – provided the remaining eye is unaffected.