The cornea is the clear part of the eye. A thin sheet of tissue called epithelium (“epi” means “on top of”) covers the outer surface of the normal cornea. The inner surface is called the endothelium.  A corneal abscess occurs when bacteria or fungus along with the animal’s own white blood cells enter the cornea through an injury on either side of the cornea. The epithelium and the endothelium may heal too quickly and essentially “seal” the bacteria or fungus inside the cornea. The bacteria or fungus then continues to grow within the cornea, resulting in the formation of an abscess. Some abscesses are sterile, with only the horse’s white blood cells present.

A cataract is a dense opacity of the lens. Horses may be born with cataracts (congentital cataracts) or cataracts may develop after birth in the foal (juvenile cataracts) or the adult horse (adult or senile cataracts).

There are multiple causes of equine cataracts. Cataracts may be inherited, but this is not very common in the horse. Cataracts may be caused by an injury to the eye, but most cataracts in adult horses are caused by diseases that inflame the inside of the eye. One such disease is equine recurrent uveitis (also called moon blindness or periodic ophthalmia).

The cornea protects the delicate structures inside the eye while allowing for vision. Injuries to the cornea of the horse usually results from trauma. The traumatic incident may be anything from running into a fence post to getting sand, straw, or insect repellents in the eye. Other causes of corneal injuries in horses include eyelid problems, viral diseases, or low tear production. A horse with a corneal ulcer may tear, squint, and have a cloudy, red eye.

Equine recurrent uveitis (ERU) is also known as “periodic ophthalmia” and “moon blindness”. Uveitis means inflammation of the iris and deeper structures inside the eye. ERU is the most common cause of defective vision or blindness in the horse.

ERU is characterized by multiple episodes of “active” uveitis alternating with periods when the eye is “quiet” or not inflamed. One or both eyes may be involved in the disease. Chronic ERU may result in partial or complete loss of vision.

Squamous cell carcinoma is a common tumor of horses in areas of intense sunlight or high altitude. It is one of the most common tumors of the eye of the horse. Non-pigmented skin and membranes are especially susceptible to the development of this tumor. The tumor may involve the eyelids, third eyelid or the globe. Appaloosas, color-dilute breeds and draft breeds are particularly susceptible to development of this tumor. A horse with an early squamous cell carcinoma shows reddening, roughening or ulceration of the affected area. There is usually increased tearing of the eye.