Uveitis in horses: causes, symptoms and treatment

Uveitis is the leading cause of blindness in horses. A very common ocular pathology in horses, it 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 means inflammation of the uvea. The uvea is the vascular part of the eye, comprising the iris, which everyone knows and can see, as well as the choroid, which envelops the entire eye, and the ciliary body (see diagram below).

There are many factors that can trigger this inflammation and thus the uveitis attack:

- trauma (impact on the eye)

- photosensitization (reaction to sunlight) generally caused by food poisoning

- individual predisposition (appaloosa breed, blue iris eye, etc.)

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

2. Symptoms of uveitis

It's vital to know how to recognize uveitis in order to contact a veterinarian quickly.

In a uveitis crisis, the horse often presents several of the following symptoms, but not necessarily all of them:

- severe pain, with the eye closed, avoiding light

- a generally clear flow

- miosis (constricted pupil forming a horizontal slit)

- corneal edema (blue haze over the eye)

- a change in iris color

- swollen eyelids

The main problem with uveitis is its recurrent nature. Painful attacks generally improve with prescribed treatments, and sometimes even on their own at first, but leave after-effects and recur within an increasingly short space of time.

3. Medication, surgery and long-term management

Medical treatment, generally prescribed by the vet as a first-line treatment, involves general and local anti-inflammatories, combined with a mydriatic to dilate the pupil and limit after-effects.

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

Surgical options are available to prevent recurrence of uveitis:

• cyclosporine implant: placed in the eye, it blocks the immune reaction responsible for uveitis. Its efficacy is recognized, but unfortunately limited in time, approximately 2 to 3 years.
• Vitrectomy: this involves aspirating the inflamed vitreous body from the eye. The aim is to remove the antibodies and inflammation molecules it contains. Performed early enough to prevent the inflammation from becoming too deep-rooted, and to avoid after-effects on the eye, vitrectomy prevents 90% of recurrences and preserves vision.
• Enucleation: as a last resort, enucleation saves a horse that has lost its vision from suffering each time it has an attack.

Ocular surgery is constantly evolving and progressing, giving hope to horses suffering from sequelae such as cataracts.

In addition to treatment, the management of recurrent uveitis requires a healthy lifestyle and preventive measures to limit new attacks and keep the horse comfortable.

The main factors contributing to inflammation and therefore to attacks are the sun, more specifically UV rays and wind.

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

It's also advisable to keep dust to a minimum, as it can also cause irritation. On windy days, it's best to keep your horse in the stall, if possible in a closed stable.

Pauline Cantet, equine veterinarian.