Periodic Ophthalmia

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Also Known As

Equine recurrent uveitis, Moon blindness


Equine periodic ophthalmia or moon blindness is an immune disease with a number of potential causes. Redness, swelling, pus, pupil constriction in the dark, cloudiness, squinting, and sensitivity to light are common symptoms. Treatment by a veterinarian is critical because the wrong treatment or no treatment may lead to blindness. While the causes cannot be eradicated, the progression of periodic ophthalmia can often be slowed or stopped by fast, aggressive, and consistent care.


  • Redness
  • Swelling
  • Watery eyes
  • Pus
  • Constriction of pupil in the dark
  • Cloudiness
  • Squinting
  • Sensitivity to light


A multitude of causes can lead to this painful disease. According to Kern (1987), "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.".

The most common cause of periodic ophthalmia is a bacterial infection known as leptospirosis which enters through the mucous membranes. The contamination with the bacteria comes from contact with the urine of an infected animal through water, mud, bedding, or food.

Other bacteria that cause infections that may lead to periodic opthalmia include Streptococcus equi, Escherichia coli, Rhodococcus equi, and Brucella.

Viral infections linked to equine periodic opthalmia are respiratory equine herpes virus and influenza virus.

A parasite connected with the disease is Onchocerca, which is often spread by culicoides, a biting midge of the Ceratopogonidae family. The adult lives in the connective tissue, and the microfilariae travel throughout the body, and, when an infestation occurs in the horse's eye, opthalmia may occur.

Trauma to the eye allows infection to become established and can lead to cases of opthalmia. Allergies have also been implicated. The months during which pollen and ragweed type allergies are known to occur have been noted as the highest months for the onset of the disease in horses in some areas.


Because of the number of potential causes for the disease, good stable and horse hygiene is very important. Leptospiria, the bacteria that is most commonly implicated in causing the disease, likes warm, moist conditions. Horses are exposed through stagnant water, grass, hay, or grain contaminated with the urine of infected animals. Keeping animals from drinking stagnant water, good management of manure, and good sanitation, including washing of hands when caring for horses, are important.

Some veterinarians recommend vaccination in areas where leptospirosis is prevalent, but there is no approved vaccine for horses at this time.

Effective fly control, frequent changes of bedding, routine worming, minimizing contact with cattle or wildlife, and restricting access to swampy areas are important in minimizing the risk.


Given the serious nature of the disease, a veterinarian experienced in equine opthamology should examine the eye and prescribe treatment. If a steroid is used when the eye is ulcerated, blindness can result. Treatment includes atropine to dilate the eye, followed by either a steroid or antibiotic, as prescribed by a veterinarian.

Bute, banamine, or aspirin may be used as anti-inflammatory agents along with other medications. After the initial episode has been treated, the eye may appear to return to normal.

Experience with this disease shows that future episodes will occur periodically, with each one lasting a little longer. Each time an episode occurs, the eye loses more sight. Long-term maintenance treatment is necessary on a regular basis to keep the inflammation from starting again. Aspirin, bute, or banamine is usually prescribed by the veterinarian for this purpose.

Occasionally, it is necessary to have the eye removed. This stops the horse's pain, and in many cases, the horse adjusts well to having only one eye.

Wearing fly masks to cut down on light provides relief for many horses with opthalmia, and some veterinarians suggest using dietary supplements to help improve the physical condition of the horse as a preventive measure against opthalmia.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.