Uveitis in horses: causes, symptoms, and treatments

Uveitis is the leading cause of blindness in horses. It is a very common eye condition in horses, but one that is often overlooked by owners. Its recurrent nature can make it a real nightmare.

1. 1. Anatomy of the eye and causes of uveitis

Uveitis refers to inflammation of the uvea. The uvea is the vascular part of the eye, comprising the iris, which everyone knows and can see, but also the choroid, which envelops the entire eye, and the ciliary body (see diagram below).

There are many factors that can trigger this inflammation and therefore an uveitis attack:

- trauma (blow to the eye)

- photosensitivity (reaction to sunlight) usually caused by food poisoning

- an individual predisposition (Appaloosa breed, blue iris eyes, etc.)

- an autoimmune reaction (the horse's antibodies turn against itself and attack its own eyes), particularly when the horse has contracted leptospirosis at some point.

2. Symptoms of uveitis

It is essential to know how to recognize uveitis so that you can contact a veterinarian quickly.

During a uveitis flare-up, horses often exhibit several of the following symptoms, though not necessarily all of them:

- severe pain, with the eye closed, avoiding light

- generally clear discharge

- miosis (constricted pupil forming a horizontal slit)

- corneal edema (blue veil over the eye)

- a change in the color of the iris

- swollen eyelids

The main problem with uveitis is its recurrent nature. Painful attacks generally improve with prescribed treatments, or sometimes even on their own at first, but they leave sequelae and recur at increasingly shorter intervals.

3. Medication, surgery, and long-term management

Medical treatment, usually prescribed as a first-line treatment by the veterinarian, involves systemic and local anti-inflammatory drugs, combined with a mydriatic to dilate the pupil and limit sequelae.

However, this treatment only manages the acute crisis and does not prevent recurrence. In cases of recurrent uveitis, this will generally not be sufficient. Each crisis leaves sequelae, the main ones being cataracts and synechiae (adhesions of the iris preventing the pupil from opening properly).

There are surgical options to prevent uveitis recurrence:

• Cyclosporine implant: placed in the eye, it blocks the immune response responsible for uveitis. Its effectiveness is recognized but unfortunately limited in time, approximately 2 to 3 years.
• Vitrectomy: this involves removing the inflamed vitreous body from the eye. The aim is to remove the antibodies and molecules causing the inflammation. When performed early enough, before the inflammation has become too established and the eye has suffered any damage, it can prevent 90% of recurrences and preserve vision.
• Enucleation: as a last resort, this prevents horses that have lost their sight from suffering during each seizure.

Eye surgery is constantly evolving and advancing, offering hope for horses suffering from conditions such as cataracts.

In addition to treatment, managing recurrent uveitis requires lifestyle changes and preventive measures that are essential for limiting new flare-ups and keeping the horse comfortable.

The main factors that cause inflammation and therefore trigger flare-ups are the sun, specifically UV rays, and wind.

Wearing a UV protective mask therefore significantly reduces the risk of recurrence. The horse can return to normal life and go out into the paddock.

It is also recommended to minimize dust in the environment, which can also cause irritation. On windy days, it is best to keep the horse in its stall, if possible in a closed stable.

Pauline Cantet, equine veterinarian.