Equine recurrent uveitis (moon blindness) in horses

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    This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.
  • Uveitis means inflammation within the eye. It should only be termed equine recurrent uveitis when two or more episodes occur. It is a complex condition, which is the most common cause of blindness in the horse worldwide and a very common cause of chronic eye pain in horses. Patient outcomes are improved by early diagnosis, appropriate therapy and clear understanding of the disease. Recent advances in treatment have helped.

    The condition was historically called moon blindness, as when the cause was unknown, the intermittent nature of the disease was suggested to be associated with the phases of the moon. We now know that recurrent equine uveitis is a recurring immune-mediated inflammation of the eye, which after a lifetime of inflammatory episodes is not surprisingly more common in older horses.

    ERU is fairly rare in the UK, with around half of those horses that have had uveitis suffering a recurring problem.

    ERU is more commonly seen in the USA and in central Europe, particularly in Germany and the Czech Republic. Much of the research into treatments of the disease has been carried out in these countries.

    It usually occurs in horses who are more than four years old and often becomes a noticeable problem in mid-adult prime performance years. The disease just affects one eye in about 50% of horse, except for the Appaloosa breed which has 80-87% bilateral uveitis.

    Equine recurrent uveitis [1,021 words]: Signs | Treatment | Causes | Risks | Prognosis | Prevention

    Signs of equine recurrent uveitis

    The clinical signs are highly variable depending on how chronic the condition is with many early cases showing relatively subtle signs of intermittent squinting, tearing or cloudiness in the eye, which may be dismissed as minor.In time these may progress to include a partially closed, inflamed eye with a watery discharge. Usually the eyelids will close over the painful eyeball, but if the eye can be seen, it may look cloudy with a constricted pupil. The horse will be sensitive to bright light and feel more comfortable in a dark stable.Summary of signs to look out for include:

    • Swollen eyelids
    • Sensitivity to light = keeping the eye closed
    • A cloudy cornea
    • Colour changes in the iris
    • A constricted pupil
    • Material within the front chamber of the eye such as blood, fibrin or pus
    • Reddening around the eye
    • A yellow-green discolouration deep within the eye
    • Pain, which shows as eye closure and increased tearing

    Treatment of equine recurrent uveitis

    With any signs of an eye problem, a vet’s advice should be sought urgently. With uveitis cases rapid, aggressive treatment is required to reduce the likelihood of the horse losing the sight in the eye and reduce the pain of the condition.

    The most critical component of the treatment is anti-inflammatory therapy, using medication, such as corticosteroids. These can be administered using drops or as an injection of a slow-release compound under the lining of the eyelids. In addition, an injection of corticosteroids can be given to deliver the medicine via the horse’s bloodstream. Complications can ensue if the painful eye develops a corneal ulcer, in which case steroid treatment will make it worse. For this reason, empirical treatment should never be started without checking with your vet first.

    When there is corneal ulceration present, other treatment options are required, including non-steroidal inflammatory medication. In all cases, it is important to give appropriate medication for pain relief and to reduce the spasm within the eye that is causing the pupil to constrict, as well protecting the delicate corneal surface of the eye.

    In Britain, we rely heavily on medical therapy whereas in other countries where the disease is more common and may have a different pattern of causes, surgery in the form of vitrectomy (removing the vitreous fluid from behind the lens), has a good record of preventing recurrence of the disease, but how successful it is at preserving vision is uncertain.

    Recent treatment innovations have become available that work on immunosuppressing the affected eye, using local implants of the immunosuppressive medication cyclosporine. This is used to help horses with the disease that cannot be managed with topical ointments, drops and other medication, particularly steroids.

    Another fairly new and not yet validated approach is injecting low dose of gentamycin or other medication directly into the eye. This can be performed with standing sedation and appropriate sterile preparation, but it not without risk.

    If the condition does not improve, the horse can be left with a permanently painful and unsightly eye. In these cases removing the eye completely is the best way forward, even in an older horse. This can be done in the standing, sedated horse using local nerve blocks and is relatively straightforward. This removes the source of discomfort and many one-eyed horses can return to full work, including jumping.

    Causes of equine recurrent uveitis

    The cause of the disease is both unclear and complex. Some cases of simple uveitis can be linked to an identifiable cause, such as blunt trauma, corneal infection or a disease that damages the eyeball. When there is no history of external trauma or injury to the eye, it is most likely to be an autoimmune type of disease: essentially, a disease caused by the horse’s immune system ‘attacking’ itself.

    Any inflammation of the structures within the eye can lead to severe and permanent damage to its function of providing sight. In horses with equine recurrent uveitis it appears that the eye has lost this immune privilege, which results in it giving a more generalised inflammatory response and subsequent permanent damage to the eye.

    In both Germany and the USA specific types of bacteria known as Leptospira species appear to be implicated. A recent study1 suggests that Leptospira does indeed cause some cases of uveitis in the UK, however this number is small compared to some other parts of the world.

    Which horses are most at risk?

    Appaloosa and Appaloosa-derived breeds appear to be susceptible to the disease, with clinical cases in these horses proving very difficult to manage. In addition, there is familial history of ERU in some breeding lines of other breeds.


    A US study2 found almost 30% of horses with ERU were blind in the affected eye by the time they were first seen by a vet. Around 30% of horses with uveitis had to be retired due to the condition, and another 30% performed at a reduced level than before the disease started. 15% of horses were put down due to bilateral blindness or chronic, uncontrollable pain.

    How to prevent equine recurrent uveitis

    Prevention is difficult as the trigger factors are not clearly understood. It is important to contact your vet promptly if your horse has a painful eye, rather than reaching for a tube of ‘eye cream’, since some treatments may make the condition worse.

    It is advisable to avoid breeding from affected horses, since genetic selection of individuals free of the disease may reduce its prevalence in the future.

    Owners often underestimate the severity of any visual defects, with little idea what their horse can or cannot see. Older horses often have a standard routine, rather like many older people, mostly in familiar surroundings with the same companions and few changes in their environment. This means these horses can cope with deterioration to their vision without people noticing.

    Recent research3 confirms that vets detect many more problems in older horses’ eyes than their owners/handlers knew were there. It therefore makes sense to have older horse’s eyes checked, possibly as part of a regular health check, so that any problems can be detected early.


    1: https://onlinelibrary.wiley.com/doi/full/10.1111/eve.12878 – Does leptospirosis cause uveitis in the UK? – December 2017

    2: https://www.ncbi.nlm.nih.gov/pubmed/25891653 – Prognosis and impact of equine recurrent uveitis – May 2016

    3: https://www.ncbi.nlm.nih.gov/pubmed/30080275 – Prevalence of owner-reported ocular problems and veterinary ocular findings in a population of horses aged ≥15 years – August 2018

    Also: https://onlinelibrary.wiley.com/doi/full/10.1111/eve.12548 – Equine recurrent uveitis: A review of clinical assessment and management – February 2016