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Equine Recurrent Uveitis (ERU), a leading cause of blindness in horses, is becoming all the more common, is difficult to treat, reoccurs easily, and is very painful.

ERU; recurrent inflammation of the inner eye (uvea), also known as Periodic Ophthalmia, Recurrent Iridocyclitis, or Moon Blindness, since it was originally thought that recurrence often appeared with phases of the moon.

Whatever we call it, the disease is unique to each horse and can affect horses of all shapes, sizes, breeds and colours, and doesn't discriminate between different lifestyles or environments. In short, any horse can get this nasty disease.

ERU is an immune-mediated disease. ‘Impairment of the normal blood-aqueous barrier in the iris and ciliary body vasculature owing to inflammation is the underlying cause of the clinical signs.’ (Kern, 1987). In simplified terms, ‘the blood is fighting the disease and the interior eye, while the eye is fighting the disease and trying to protect itself from being digested by the body's own blood.’ (Scott, 1993). In the resulting battle, the eye becomes inflamed and can also become ulcerated.

Unlike many diseases, it does not have one single cause and for this reason has baffled researchers. Once started, it may cause blindness if its progression cannot be halted. Fortunately, while the causes cannot be foreseen or eradicated, its progression can in many cases be slowed or stopped by fast, aggressive and consistent care.

It's said that possibilities of causes include anything and everything from trauma and bacterial, viral, fungal, parasitic, and other systemic diseases, although the absolute cause remains elusive to this day. However, according to the Merck Veterinary Manual, it is widely believed that ERU may be the result of immune-mediated responses to "persistent antigens and/or antigen mimicry (resemblance) of ocular tissues, or autoimmunity". If it's of any comfort, this is consistent with many other species, including man.

What is known is that the chief infectious cause of ERU is leptospirosis. The bad news is that clinical signs of ERU may not be evident for months or years after primary infection which is then too late. The most commonly implicated parasitic cause of ERU is migration of Onchocerca cervicalis microfilariae.

As the name ERU suggests, acute episodes can become a series of repeat attacks. Each subsequent attack will lead to increased residual damage leading ultimately to blindness. These episodes can be encountered in one or both eyes with an interval of weeks to years, and early support is essential.

It should therefore go without saying as to how important it is to maintain your horse’s health and immunity, to be naturally strong enough to provide a natural resistance to invasion of these parasites.


If you’re used to how your horse's eyes look like when they are normal, and you look at them daily, you'll notice very quickly if anything has changed. If your horse has a red or swollen or cloudy eye, if you come out and find your horse squinting or avoiding light, don't assume that it's ‘just dust’ or ‘just a windy day’, or that the problem will disappear if you ignore it. Eye problems are too serious and too dangerous, and the equine eye far too delicate, to take a chance.

Sudden clumsiness is often the result of a vision problem. If you suspect an episode of ERU, but don't see any swelling or redness in the eye, take your horse into their stable, make it as dark as possible, and look at the eyes. If you find a constricted pupil (it should dilate in a dark area), then call the vet immediately, because it is likely that your horse is now in severe pain and needs immediate treatment.

The sooner the condition is diagnosed, and the sooner you can begin an aggressive treatment, the greater your chance of saving the sight in at least one of your horse's eyes. If you get a diagnosis as soon as you see the first signs of eye trouble, it's possible to slow it down and even possible to stop further progression of the disease.

Be aware that ERU is a high-maintenance condition - you're going to be very involved in your horse's treatment, especially as the disease is recurrent. Even if you and your vet deal successfully with the initial episode, and your horse's eye seems to be normal again, you can't ever relax and stop watching for signs of trouble - at some point, there may be another episode. Each episode will cause a little more damage and take away a little more of the horse's sight. Typically, each episode will last a little longer and be a little more difficult to deal with than the previous one.

In the long term, you'll probably use topical medications only during acute episodes; the rest of the time, you'll be dealing with a maintenance regimen that may include daily aspirin or bute, and in some cases might even include antihistamines. Fly masks can help keep dust and debris out of a horse's eyes, and can also cut down on the amount of ultra-violet light coming into the eye. If you need to block all light from one eye, you can sew a soft pad into the fly-mask.

Anything that promotes inflammation in the eye can trigger an episode of ERU. Anything you can do to build up a horse's immune system may help the horse avoid any number of illnesses and diseases, and it's reasonable to think that keeping a horse's immune system strong might help protect it from developing ERU.

However, it's still a disease with a lot of ‘unknowns’. If your horse is diagnosed, plan ahead for eventual blindness, and change your management practices and training and riding practices accordingly - see Owners Tips below. Implementing these changes sooner than later puts you both at an enormous advantage - if your horse does become blind, they can make the difference between a terrified or potentially dangerous horse to a successfully managed, happy horse. Best case scenario is that if your horse does develop ERU and eventually becomes blind, you may still be able to continue riding.


A horse with untreated ERU can be a danger to themselves and to others - horses and humans - around them. As the eyesight becomes worse, they'll be more likely to bump into things, and more likely to react suddenly if they're surprised by a human coming up to them without speaking to them first. If the owner doesn’t realise that their horse is losing their eyesight, they won't handle them correctly and there's every likelihood that someone will get hurt.

Another reason that a horse with untreated ERU can be dangerous is that they won't just be dealing with a loss of eyesight - they'll very likely be in great pain. A horse that can't see you approach may be startled when you touch them; a horse that can't see you AND is in acute pain may kick out. If your horse is showing signs of pain, don't hesitate to call the vet right away.

Equine recurrent ERU is very painful to the horse. The most common signs are puffy, watering eyes, squinting, and red blood vessels at the sides of the eye and in the lids. Horses will often be photophobic (very sensitive to the sun and often to any light). In some eyes you can notice a white cloudiness or a blue or green tint. Another major sign to look for is a pupil that is constricted when the horse is in a darkened stable. A constricted pupil indicates that it is in spasm, and is very painful. Immediate support is needed to alleviate the spasm.

Other signs may include head shaking, a runny nose, white spots or bleeding in the eye, pus or other matter collecting, loss of balance, tripping, running into things or rubbing the eye, which is another concern for owners to keep their horse from rubbing the infected eye to prevent further damage. In some cases if you look across the surface of the eye you may even see ulcers although the untrained eye can still miss them.

If signs persist, even with treatment, your vet may want to consult a veterinary ophthalmologist since this disease can lead to blindness in one or both eyes if not treated aggressively.

Known causes

Causes can be generally classified as bacterial, viral, parasitic, and traumatic (Schwink, 1992). An additional possible cause is allergies.

The most commonly implicated bacterial infection is leptospirosis. The organism enters through the mucous membranes, and contamination comes from contacting the urine of the infected animal through water, mud, bedding or food. Cattle can shed leptospiroses in their urine for over a year and horses may shed for four months.

Leptospira are divided into six major serogroups which are then further divided into over 500 serovarities. Of these there are seven main ones that are found in horses. These are L. autumnalis, L. bratislava, L. canicola, L. grippotyphosa, L. hardjo, L. icterohemmorhagica and L. pomona.

Leptospiral bacteria like warm, moist conditions and are found in many animals. Horses can get it from drinking water that has been infected or they may pick it up from grass, hay or grain contaminated by the urine of infected animals. Horses can catch it from other horses, although it is thought that the majority have most likely contracted it from infected cattle, rats or wildlife.

One of the frustrations about leptospirosis and ERU in horses is that the ERU may not occur for 18-24 or more months after the leptospirosis infection. In addition, while some horses are very ill with the disease, others may show no obvious signs of infection. Testing for leptospirosis at the time ERU occurs may confirm that the disease was active at one time but it will be too late to do anything about the leptospirosis for that horse. However, understanding how leptospirosis exists and is spread can help you prevent the infection of other horses.

Leptospira can survive in water for up to 20 days and in manure up to 61 days. It is therefore very important to keep animals from drinking stagnant water, to have good management of manure, and to practice good sanitation. Human hands can spread infection from one animal to another or even to humans. Some precautions you can take include fencing water sources that wildlife can access, draining wet muddy areas where horses are pastured, thoroughly disinfecting stables and areas where animals who have active leptospirosis have been, and washing hands and clothes when in contact with the disease.

Another known cause is parasitic, the most common parasite connected with ERU being onchocerca. Culicoides, the biting midge and main culprit that causes Sweet Itch, is believed to be the primary transmitter. The adult lives in the connective tissue of the horse's neck and the microfilariae travel throughout the body.

ERU occurs when there are large quantities of dead microfilariae in the eye. Normally the eye can handle the live ones but the dead give off large amounts of antigens and these cause inflammation in the eye. Ironically for horse owners, it has been recorded that onchocerca can sometimes first be identified by the onset of symptoms following worming with Ivermectin. This is one drug that will effectively kill off the young microfilariae, but at the same time by doing its job it can initiate ERU if a large quantity of the microfilariae are in the eye at the time of worming. After ERU has started, some owners find that feeding a good liver tonic, several days before and after worming, can control the inflammation so that ERU does not flare up every time the horse is wormed.


Short term support usually consists of using Atropine to dilate the eye and reduce discomfort due to spasm of the iris. A steroid may also be indicated if the eye is not ulcerated or an antibiotic if the eye is ulcerated.

In addition to ointments and drops in the eye, bute (or similar) or even Aspirin are used to decrease the inflammation. In some cases, vets will use the Atropine for a few days and then stop it so that the pupil can start to constrict again. The idea is to keep the pupil moving so that synechiae, little pieces of protein that form strands in the eye, cannot cause the pupil to fuse in one position.

In addition to the standard treatments, many owners are now trying more natural methods such as acupuncture, MSM, vitamins, chiropracty, herbs, homeopathic remedies, hot and cold compresses and riboflavin.

In cases where nothing can be done to stop the constant bouts of pain for the horse, and the eye has degenerated, owners have opted to have the eye enucleated or removed. This stops the horse's pain and in many cases, since the sight was probably almost if not totally gone, the horse adjusts well to having one eye.

The main goal is to keep the inflammation from starting so permanently feeding a gentle, yet effective, anti-inflammatory and pain-reliever will be beneficial. Vets like to opt for a daily dose of Aspirin, although this comes with its own toxicity residue which has to be metabolised by the immune system, and is also well known for causing gut issues, as well as a possible cause of ulcers. A useful herb alternative in this instance would be Meadowsweet, as it contains the very constituent that aspirin was synthesised from, as well as all the natural buffers for any side effects, which Aspirin doesn't have. Bear in mind as well that if you compete, many vet meds by way of pain-relief/anti-inflams and so on, are now on the FEI banned substances register, so you may need to discontinue feeding several days prior to the competition.

In essence, with ERU, each case needs to be evaluated and then handled individually - no two cases are exactly the same. It will be very time consuming, but by being diligent you have a chance of saving your horse's vision if the disease is successfully halted or at least slowed down.

Top Tips

  • Face masks - bright light irritates the eye, so a face mask will go a long way to protect against both daylight and dust. Be sure to check that the guard hairs around the horse's eye are not being curled around back into the eye - you may need to trim them. If your horse will tolerate it, a mask with a black plastic bubble over the eye or a blackout eye-patch means your horse can be outside in daylight hours while the eye is still dilated. Otherwise you may have to bring in during the day. This obviously only works if it is the first eye that is involved. If possible, turn out at night in the summer, and leave the horse in during the day. And let's not forget winter; if there's snow, snow gives off a searingly bright glare, so again, a mask will also help here. It will also cut down the effect of wind directly hitting the eye.
  • Long term management involves getting a handle on what triggers individual episodes. Unfortunately this can vary at different times but some of the big offenders are: wind, dust, getting chilled, competition stress, a new horse on the yard, ammonia build up in the stable, change of feed, seasons changing, strong sun, severe cold, worming, flies/mozzies, injections and meds. Many people find keeping a daily log is useful to track the triggers, ie. wind and weather conditions that day, whether their horse was in/out, medications given, any exercise, and so on ... basically, anything out of the ordinary. A month at a glance is great, and you'll soon be developing your own abbreviations so it all fits. You'll soon be able to discern patterns which might then help you to prevent future episodes, i.e., if it's forecast to be is windy, the bubble hood to keep the wind off the eye; if it's going to be wet and rainy, bring in, and so on.
  • Depending on the extent of the blindness, the horse may have blind spots, or a blind side. If they're blind or almost blind in one eye, they'll lose their depth perception. They may trip on rises or dips in the path. At the onset of the disease, if you haven't already got them adjusted to it, start doing everything from both sides; leading, grooming, saddling, and mounting if you can.
  • Get in the habit of talking to them constantly so they knows where you are. Keep a hand on them while working around them so they can hear and feel where you are. When leading a horse that is blind in one eye, stay on the good side, despite your natural reaction that you should replace the horse's bad eye. The first time they jump left because of something scary they see with their good right eye, you'll realise the importance of staying on the good side. Most horses will not willingly jump into us, but if they can't see where we are, in their fright we may get stepped on.
  • Owners who are riding semi or totally blind horses, say that trust and confidence from their horse is paramount. Going blind can be very stressful for the horse, especially if it happens quickly. Horses that go blind over a period of years tend to get used to it gradually and may be less bothered. How the horse reacts will depend on their makeup and you must be tuned into their reactions. Dressage lessons have helped a number of riders with cueing and also can help you to straighten the horse if they have become crooked to compensate while losing sight. Changes in how they deal with you or other horses, their food or the work you are asking of them, can indicate that stress is building and needs to be dealt with before it gets worse. A horse can be fine with blindness for months and then for some reason it becomes poorly tolerated. Watch for signs and get professional help from a trainer to work through the problem before it gets out of control.
  • Turnout - some horses are happy in a field with with a small established group of calm horses - the other horses often protect and guide the blind horse. In other cases, a horse who is low in the social order may be terrified of being in a field with others and may need a paddock or field of their own. If you are putting your horse in a new field, walk them around the perimeter of the field so they have an idea of their boundaries. Letting their whiskers grow will help them sense when they're near a fence or other obstacle. Owners have also put bells on other horses so that the uveitus horse can keep track of their companions.
  • Watch for signs that the horse may not be tolerant of strangers/children so take steps to protect them from too close contact with the horse.
  • At the first sign of ERU, start teaching voice commands to your horse if they don't know them already: up, down and step (for over logs on a trail) are a few that come in very handy. The added benefit is that if you're trail riding, your horse will become much safer. In a tough situation your horse will be accustomed to listening to you when it's essential. If the horse is blind on one side you must always be ready with the leg on that side. Be aware of what the horse will see on their good side since something scary will cause them to move away regardless of what's on the bad side. On trail rides experiment to discover the number of horses your horse is comfortable with and the position among them he prefers; some are only comfortable leading or following. Also when riding, especially on trails, remember to relax the reins so that they can look around.
  • Beware lunging! Lunging the blind or partially blind horse is fine if they're used to being lunged consistently, but if they're not, they may want to let off a bit of steam first. A Top Tip - don't start with the bad eye on the inside toward you - this is a potentially dangerous situation. If your horse is hooning around, they can't see you, so will have no concept of where you are. Also, especially if they're newly blind, they may be off balance on a small circle which could bring them towards you. Always start with the good eye toward you, keep the sessions short and only do walk/trot until you know they're listening. When they're calm and listening with the good eye in, then switch sides, still keeping to the walk/trot routine. Voice commands are so important to learn at the very beginning and not rely on a lunge whip for signalling, since you will have a blind side towards you for part of the time.
  • In some respects, the horse is like a recovering human invalid - they'll come to a point where they no longer want to be babied. ‘Consistency’ is the crucial word here - how you handle them and what you expect is very important. As much as possible, approach the horse as you normally would - if you are going to work or ride, expect them to do what they're supposed to do. If you expect less than they can do, or let them get away with things, you could potentially have a spoilt and/or dangerous horse on your hands.
  • Accidents! There will be times when they'll accidentally run into things or trip over things because you didn't think ahead, but you will learn, and horses are very forgiving. You in turn need to learn how to ask, and how to anticipate what to tell them, so that all goes well.

I can take no credit for any of the above - all acquired from lots of research via the fabulous world wide web, and all courtesy of other generous people who have shared their own experiences.