Corneal ulcers, also known as ulcerative keratitis, represent one of the most common and serious ocular conditions in horses. [1][2] This condition involves the erosion of the corneal epithelium, the outermost layer of the cornea. [1]

The severity of corneal ulcers in horses can vary widely, ranging from superficial cuts or abrasions to full thickness corneal punctures (perforations) accompanied by prolapse (dislocation) of the iris. [2]

Since the cornea contains a large number of nerves, ulceration to this structure is very painful for the horse. [3] Additional signs of keratitis include redness, excessive tearing, eye discharge, and a cloudy appearance to the eye. [3]

If not treated promptly and appropriately, a corneal ulcer can lead to bacterial or fungal infection and potential vision loss. [2] Other complications include globe rupture and phthisis bulbi (end-stage eye). [2]

Immediate veterinary intervention is critical to preserve vision. With proper treatment, most horses can recover fully, maintaining both their health and quality of life.

Corneal Ulcer in Horses

Horse eyes are prominent, making them prone to injury. This is especially true of the cornea, which is the outermost layer of the eye.

A corneal ulcer or keratitis occurs when there is a loss of the corneal epithelium, the outermost layer of the eye, accompanied by inflammation of the cornea. In horses, this is often due to trauma, foreign bodies, or infections. [1][2]

This injury irritates or damages the epithelium, leaving the underlying structures in the cornea unprotected. [2] This condition is highly painful and can lead to a deeper infection. [2]

Equine Eye Anatomy

To better understand the different types of corneal ulcers, it’s helpful to familiarize yourself with equine eye anatomy.

The cornea is the transparent front part of the horse’s eye that covers the iris, pupil, and anterior chamber. It is a clear dome on the front surface of the eye, measuring 0.8 to 1.0 mm (0.03 to 0.04 in) thick.

The cornea protects the front of the eye, allows light in, and helps focus light on the retina at the back of the eye. [3][4] It is made of three primary layers: [3][5]

  • Epithelium: The epithelium is the outermost layer which repels tears and keeps organisms from entering the deeper layers of the cornea.
  • Stroma Layer: The stroma layer makes up approximately 90% of the cornea’s thickness, lies under the epithelium and is mainly composed of water.
  • Deep Endothelium: The deep endothelium lies beneath the stroma layer. Descemet’s membrane is the basement membrane of the deep endothelium.

In addition to the cornea, it’s important to understand the significance of several other parts of the eye anatomy in horses:

horse-eye-anatomy | Mad Barn CanadaIllustration:
  • Iris: The iris is the circular, colored area in the center of the eye. It controls the amount of light entering the eye by making the pupil larger or smaller. [4]
  • Pupil: The pupil is the black area in the middle of the iris, and is a small hole in the eyeball that allows light in. In horses, the pupil is shaped like a horizontal oval. [4]
  • Lens: The lens sits behind the iris and changes its shape to focus light onto the retina. [4]
  • Retina: The retina contains photoreceptors, which are cells that sense light. Each photoreceptor is attached to a nerve fiber. When light hits the retina, the photoreceptors are stimulated, which creates electrical impulses in the nerve fibers. [4]
  • Optic Nerve: All of the nerve fibers are bundled together to form the optic nerve, which carries electric impulses from the retina to the brain, where images are translated from light signals into the sense of vision. [4]

Types of Corneal Ulcers

There are three types of corneal ulcers in horses: simple, recurrent and complex.

Simple Corneal Ulcers

Simple corneal ulcers are superficial, involving the outer layer of the cornea and up to one-third of the stroma layer.

This type of ulcer may not be noticeable to the naked eye, but does appear under fluorescein stain during ophthalmic evaluation. [3]

With simple ulcers, deep infection is not present, but the eye could still be contaminated with bacteria or fungi. [3] Simple ulcers usually heal quickly without complication. [7]

Recurrent or Refractory Corneal Ulcers

Recurring ulcers, also known as indolent ulcers, persist because the corneal epithelium is not able to adhere to the underlying stroma, preventing it from healing properly. [3]

Recurrent ulcers are visible under fluorescein stain. They are distinct from simple ulcers as staining is also seen beneath the periphery of the corneal epithelium, indicating detachment of the cornea from the stroma. [3]

Complex Ulcers

This category of corneal ulcers involves deep ulcerations that reach the stromal layers of the cornea. In these cases, bacterial or fungal infections are typically well-established, leading to moderate to severe swelling of the cornea. [3]

Complex ulcers are known for their prolonged healing times. [7]

Causes

Corneal ulcers in horses typically begin with a micro-puncture injury to the eye, often caused by small, sharp pieces of plant material like hay. Horses that are active or kept in environments where branches, dust, or sand are prevalent are particularly at risk for such injuries. [3]

When the corneal epithelium is punctured, bacteria or fungal organisms can become implanted in the tear film (fluid layer on the surface of the eye), complicating the healing process. [3]

The corneal epithelium