ERU, aka Moonblindness in Horses

ERU stands for equine recurrent uveitis, a painful condition where the horse experiences cycles of inflammation in the eye.

The Role of Light

Inflamed eyes are very sensitive to light. Light makes the pupil contract, which can be painful and may also actually worsen the inflammation. Even low-level light can be painful.

To avoid having to keep their horses confined and in the dark, many people rely on the Guardian Mask with 95% sunblock filter,, to keep their horses comfortable outside. Others choose to adapt a regular fly mask by either sewing a dark fabric patch onto the mask to block light or using a patch of duct tape.

If you have a horse with ERU, most experts recommend keeping the horse indoors for the first few days of treatment. Treatments typically have to be given every few hours anyway. If you must keep the horse outside, definitely use a light-blocking mask. Many experts advise keeping a mask on an ERU horse at all times when he is out in the sun.

ERU is sometimes called periodic ophthalmia or moonblindness, since it was thought at one time that the cycle of the moon might trigger the attacks of this eye problem.

Early signs may include excessive tear production, holding the eye shut or partially shut and rubbing of the eye. With a more severe inflammation, the horse will hold the eye tightly shut, avoid light, and the cornea will be cloudy/whitish. A vet exam will show inflammation of the inner structures in the eye, and corneal ulcers may develop.

Individual horses vary a great deal in how severe their attacks are, but in general the attacks do tend to become progressively worse each time. Some horses will go months between attacks of eye inflammation, while others will go weeks. Still others never really have the inflammation under good control. None of the currently used treatments can cure this condition, but they may control the inflammation and slow the progression.

The current thinking is that ERU begins as an infection, usually with a Leptospira organism. Parasite larvae of the Onchocerca family have also been implicated, but the odds of this happening have dropped considerably since widespread use of ivermectin as a dewormer. (Ivermectin kills these larvae.)

In humans, similar eye problems may develop after infection with the Lyme disease organism or other bacteria. How often this might happen with horses is unknown, but horses that don’t test positive for Leptospira might have another organism involved. Another theory holds that a severe eye injury may also start the ERU process.

Antibodies to Leptospira have been found both in the blood and inside the eyes of horses with ERU. Sophisticated testing has been able to confirm that immune-system tissue inside the eyes is actually producing these antibodies, so the organism has definitely been in there. There is considerable debate, though, as to whether horses with ERU still have an active infection. We do know they have antibodies in their eyes that are directed against both the organism and the eye tissue itself (an autoimmune reaction).

As with any acute, painful problem or illness, a horse should not be exercised, shipped or otherwise stressed when having an acute ERU flare-up. Current strategies for treatment include:

  • Oral or injected anti-inflammatory drugs like flunixin (Banamine) or phenylbutazone (Bute) to control inflammation and pain. Some vets use these only during flare-ups, while others recommend low doses on a daily basis in hope of minimizing attacks.
  • Topical anti-inflammatory eye drops (flubiprofen), as above.
  • Topical antibiotics during flare-ups to protect the cornea from surface infections.
  • Oral or injected corticosteroids during flare-ups to fight inflammation and the autoimmune reactions.
  • Topical corticosteroids during flare-ups, but only if the cornea is intact because topical steroids greatly weaken a horse’s ability to fight off surface infections.
  • Topical atropine on the eyes. This dilates the pupil by relaxing spasm in the eyes and also helps prevent the pupil from scarring down in a tightly closed position.
  • Topical immunosuppressant drops of cyclosporine. These don’t penetrate deeply in the eye to control reactions there, but can help calm corneal autoimmune reactions.
  • Topical serum. A fairly new approach is to collect some of the horse’s own blood, spin it down to separate the cells from the serum, and use the serum as drops to be put on the eye several times a day. Serum contains substances termed “growth factors” that help corneal ulcers heal. The serum may also help control inflammation.
  • Implantation into the eye of a tiny pump for timed releases of cyclosporine (experimental surgery, available at North Carolina State University).
  • Surgical vitrectomy – removal of the gelatinous sac in the back of the eye. This can improve vision, and some studies have actually found Leptospira organisms hiding inside this structure.
  • Surgical enucleation – removing an eye with advanced disease that is not adequately controlled with the above treatments and is causing the horse constant pain.

Because of ERU’s frequent connection with infection, especially Leptospira, you’d think that treating these horses with antibiotics would be important. However, antibiotic treatment is controversial.

One reason is that many believe there really is no active infection in many of these eyes. Another reason is that when live organisms are present, using antibiotics to kill them can trigger or worsen inflammation in the eye. The risk of worsening the inflammation, but possibly curing the problem, has to be weighed against a lifetime of ERU painful flare-ups and the possibility of the horse going blind even with aggressive treatment. Poor response to antibiotics is also sometimes given as a reason not to use them, but no trials with long-term antibiotics have been done.

Similar conditions in humans take many weeks to months of antibiotics to get rid of the organisms, and the eyes may be the last symptom to clear up. If you’re going to try antibiotics, you need to work very closely with a vet, count on a long treatment course and be prepared to fight any flare-ups in the eye that may occur.

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