Equine Recurrent Uveitis (ERU)

Equine Recurrent Uveitis (ERU) is a recurrent inflammation of the eye. ERU is the leading cause of blindness in horses. It can involve only one or both eyes. The disease can start at any age and in any breed. Horses with blue eyes and appaloosas are more often affected.



What is Equine Recurrent Uveitis? 

Uveitis is the medical term for inflammation of the eye. It can occur in just a segment of the eye (anterior uveitis in the front portion of the eye, or posterior uveitis behind the iris) or it can include all segments. If only one episode of uveitis occurs, it is termed ‘primary uveitis’. However, if two or more episodes of uveitis occur, it is defined as ‘Equine Recurrent Uveitis’.  

Some horses with ERU might only experience one episode of uveitis a year, while others can have episodes every month. This may also change, as ERU tends to be(come) progressive so that episodes might become more frequent over time. 

Signs may be subtle or very obvious. An untrained eye may easily confuse signs of ERU with an innocent conjunctivitis. The inflammation may also be restricted to deeper structures of the eye, behind the iris. Onset of ERU can therefore be very insidious. The eye is a fragile but vital structure. If your horse is experiencing any eye issues, we recommend contacting a vet dedicated to horses to assess the situation. 

Horses with ERU can become refractory to treatment. This means that the eye doesn’t respond to medical treatment as well as it should anymore, so that the inflammation and the damage that ERU does can’t be controlled medically anymore either. 


What does ERU look like?

An acute episode of ERU can present itself with all or some of the following symptoms:

  • Pain
  • Swelling or reddening of the conjunctiva
  • Squinting eye
  • Tear flow (lacrimation)
  • Constriction of the pupil (it’s smaller than in the (other) healthy eye, but if ERU manifests in both eyes they will both be constricted/smaller than they would otherwise be)
  • The eye capsule (cornea) might look dull or have a white-blue haze over it
  • There might be white-yellow deposits in the eye
  • The iris might have a different colour than usual

Other possible signs of past ERU episodes can be:

  • An irregular pupil
  • The “corpora nigra” (tiny black grape-shaped structures that cover the top of the pupil) might have shrunk
  • Cataract of the lens
  • The eyeball might shrink, which can create a bit of space between the eyelids and the eye, it can cause the eyelashes to point downward more or the third eyelid to become more visible
  • Diminished or absent pupillary light reflex
  • A darker or lighter colour of the iris (in certain areas or as a whole)
Fig 1 - Changes behind the iris are not easily seen without a proper eye exam
Fig.1: Eye exam in a horse with Equine Recurrent Uveitis. The pupil has been dilated. Changes in the clarity of the eye have led to severely reduced eyesight. The ‘corpora nigra’ are the black grape-like structures at the top of the pupil. They are of normal size and shape. These changes were not visible without a detailed eye exam.
Fig 2.1 - Change in iris and pupil shape due to ERU   Fig 2.2 - Change in iris and pupil shape due to ERU
Fig.2: Horse with Equine Recurrent Uveitis. The shape of the iris has changed. There is an adhesion of the iris bridging across the pupil on the right (notice the small pupil area to the right of the adhesion, yellow lines). The ‘corpora nigra’ have shrunken (notice only a small remnant is left, red line).

There are several treatment options for ERU.

Medical treatment of an episode

Treatment of an episode involves eyedrops and/or ointment that help keep the pupil open and reduce the inflammation locally, combined with an oral anti-inflammatory drug.

Constriction of the iris causes pain as the tiny muscles in the iris cramp up. We can treat this pain by making those muscles relax and opening the iris up again. This is done by an eye drop containing atropine. This drug is given to effect and then continued as required to maintain pupil dilation. Start by using this eyedrop two to four times daily, the effect on the pupil should be visible. Always check pupil size before you administer the eyedrops: once the pupil stays open treatment with atropine can be reduced to twice or once daily, or stopped completely. When you do reduce treatment, make sure the pupil stays nicely open. Note: keep in mind that this drug can cause the gastro-intestinal tract of the horse to slow down, which can potentially cause colic.

The inflammation of the eye can be controlled using a combination of oral medication and eye-ointment or eyedrops. The oral medication contains a non-steroidal anti-inflammatory drug, while the eye medication contains a corticosteroid.

The oral non-steroidal anti-inflammatory drug is given once daily until all signs of uveitis have stopped.

The corticosteroid eye medication is used for at least 1-2 weeks, after which the drug has to be tapered over a two-week period. The administration interval of the eye medication depends on the severity of the episode and can vary from every hour to four times daily. It is very important that this drug is administered as often as your vet recommends you, to get the inflammation under control. The longer the inflammation is active, the longer the risk of damage to the eye.

Tapering the corticosteroid eye medication is very important. If stopped too abruptly, the eye can experience withdrawal problems which can cause another episode of ERU. Your vet will tell you how to taper the eye medication, but an example could be going from 4x/day to 3x/day for 3 days, 2x/day for 3 days, 1x/day for 3 days and then every other day for another 5 days.


Pro-active / preventative treatment of ERU

There is a number of preventative treatments for ERU:

  • Vitrectomy: This treatment has been used in the management of chronic uveitis in humans for a few decades. This treatment can only be done on eyes that are currently not inflamed, as inflamed eyes are prone to post-surgical complications. By removing the vitreous (the gel-like substance inside the eyeball behind the iris and the lens), about 75% of horses show no further recurrence of ERU. Furthermore, in horses that have murky vitreous due to previous ERU episodes, vitrectomy may improve eyesight. However, with surgery there is always a risk. Post-surgical complications can diminish eyesight or cause blindness. This treatment is not recommended in patients that have pre-existing lens cataract, retinal detachment, a shrunken eye or glaucoma.
  • Cyclosporine implant: A slow release immuno-suppressive drug is implanted in the eye to prevent new recurrent episodes. This treatment can only be done on eyes that are currently not inflamed, as inflamed eyes are prone to post-surgical complications. This drug is not very effective at stopping inflammation. It works through preventing The implant can work for approximately 3 years, however this may be different in each case. About 75% of horses show no further recurrence of ERU. If a horse does flare up again, less medication is required to control the inflammation (treated would be as described under “medical treatment of an episode”).
  • Intra-vitreal gentamicin injection: This therapy has only been introduced in the last few years. For this reason, it has not been as widely studied as the previously described treatments. It is not clear how this treatment gets its positive effects, and there is not enough data yet for consensus on success rate or duration of effect.

However, specialists globally are using this treatment on a select patient base and have had positive results with fewer risks to the procedure and fewer post-operative complications. The procedure can be done on the standing horse, making general anaesthesia and the risks that come with it unnecessary. The data that is available from specialists at the moment shows that this treatment is associated with a high rate of success (about 95% of horses initially show no recurrence of ERU). Some specialists state that treatment lasts only for 4 months, where others say in some cases only a single injection may be necessary.

Regrettably, vitrectomy and cyclosporine implants are not available here in the Manawatu-Wanganui area. However, intra-vitreal gentamicin injections are. We work together with a specialist in ophthalmology to offer you this service.


If you would like to talk to us about ERU or book an appointment, please feel free to contact us.


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