The periodic eye inflammation - you should know that!

Perhaps you have heard the term moon blindness before? This term actually stands for Periodic Eye Inflammation (ERU for short, Equine Recurrent Uevitis for long). In fact, almost 12 % of the horse population worldwide suffer from this recurring disease with unpleasant consequences. An equine disease that sometimes causes uneasy thoughts in many horse owners.

How do I recognize the disease at all?
What if?
Can I prophylactically prevent my partner from being part of the target group in the first place?

Maybe our magazine article gives you information on the topic, because as a horse owner or rider dealing with this is important.

Periodic Equine Eye Inflammation - Definition:

This equine disease, also called moon blindness, is a recurring inflammation of the inner structures of the eye up to a detachment of the retina. The latter unfortunately results in blindness from the horse's eye. These so-called inflammatory episodes are indeterminate and can have different symptoms and strengths depending on the "infestation".

If, for example, the front part of the horse's eye is affected, the animal shows clear symptoms and has visible pain.

If the rear part is inflamed, there is hardly any pain and the inflammatory episodes often go unnoticed. Understandably, the diagnosis is often difficult to determine here. Unfortunately, this undetected damage can then no longer be reversed.

And moon blindness because it used to be thought that the periodic lunar cycle had an influence on the inflammatory events in the eye.

In the case of periodic eye inflammation in horses, a distinction is also made between 3 phases of progression:


  1. The first inflammatory thrust (acute)
  2. The inflammatory interval. Signs of thrusts are recognizable.
  3. One or more, recurrent (recurrent) inflammatory thrusts


Clinical signs may vary. It means that the horse's disease is not always present itself in the same way. Depending on the severity, there may be differences. Symptoms can appear gradually or suddenly.

In up to 30% of cases, the horse gets sick in both eyes at the same time.

The most important identifiers are: 

- Strong clamping of the eyelids, winks

- Tear flow

- Eye swelling

- Lid can be heated

- light-shy

- Conjunctiva is red

- Cornea is cloudy (smoky-flaky)

- Constriction of the pupil

- Pain

- Horse can not be caressed near the eyes anymore

- Apathy

- Fever

- Possibly pain-face (ears backwards, crosletric, mouth, knew)


The following symptoms are recognizable after one or more thrusts:


- The cornea is permanently clouded

- Also the lens gets cloudy

- shape and / or color of the iris changes 

- Detachment of the retina

- Turbidity of the glass body



Any other equine disease affecting the horse's eye must be excluded by the veterinarian or the equine clinic! These are, for example, corneal inflammations, glaucoma or abcesses.

If leptospires (bacteria) can be detected in the eye, the horse can still be helped. A cure would even be possible if the horse's eye has not yet been damaged too much. However, there is no way around a vitrectomy (see below for explanation).

In this case, the moon blindness will be called IERU (leptospiral ERU).

Periodic eye inflammation in horses ERU - treatment and medicinal approach

If the stage of recognition was detected early, anti-inflammatory ointments/drops (containing cortisone) or antibiotics (for IERU) are instructed.

However, if the periodic eye inflammation in the horse is in full swing, muscle paralysing, pupil dilating drugs are used. These reduce the pain and prevent adhesions.

Corticosteroids (intravenous) can also help to combat inflammation. In very severe attacks, these are even injected into the conjunctiva. In addition, there is a catheter that can be used to rinse the eye and to administer medication.

Today, it is also possible to place an implant (virus inhibitor) with active substances in the animal's eye. Over a longer period of time, this implant delivers the appropriate amount of active ingredient of a medicine. Drops or ointments are then no longer needed during this time.

Another surgical measure would be a vitrectomy. In this operation, the vitreous body in the horse's eye is crushed and removed. All inflammations and opacities are removed. The visual acuity is significantly improved.

A horse that is already blind also needs help. This means that a diseased, severely painful eye should be removed under general anaesthesia. The pain centre disappears and the animal can find joy in life again.

A thus partially blind horse may be handicapped for us, but in fact they cope well with it. The "restriction" is hardly noticeable to most horses after a very short time.

These procedures are of course carried out sterilely in an equine clinic.

Are there preventive measures that can prevent an outbreak of the disease?

Since the cause of the disease is not yet fully understood, it is difficult. There is talk of leptospires, steptococci, allergic reactions, viral infections, autoimmune diseases, autointoxications and even heredity.

Well, we already had the leptospires above - these can be treated initially with antibiotics via the vet/horse clinic. However, this strain of bacteria and the resulting infection can already be "contained" preventively. The main hosts of these leptospires are rodents. Have you ever seen a little mouse scurrying around in the stable? If not, good. If so, a targeted control should be carried out in any case. Especially horse feed that is standing around is a land of milk and honey for the little rodents. And it is precisely in their urine (in sick mice!) that the leptospires are found. Not every horse gets sick from this, but the periodic eye inflammation in horses (IREU) is no coincidence.

So what do?


- Do not leave horse food open (feed tonnes are great!)

- Regular cleaning of the chamber as well as all containers for feeding

- Keep order (so the mice / rats do not have a chance to swap or even to settle)

- avoid wet places in the premises

- Always check and clean soaks regularly

- Make sure that the hay stays clean (here mice really like to shake around!)

The moon blindness is the most common cause of blindness. Once the eye is damaged, there is no healing anymore. If there is suspicion, a timely appointment at the vet should be organized immediately!