The cornea is the clear front part of the eye. A thin layer of tissue called epithelium is the cornea’s protective outer layer. The inner surface is called the endothelium and the portion in between is called the stroma. A corneal abscess occurs when bacteria or fungus along with the animal’s own white blood cells enter the cornea through an injury. The injury may heal too quickly and essentially “seal” the bacteria or fungus inside the cornea creating a pocket of infection or abscess. Abscesses may also be sterile, with only white blood cells present.

A cataract is an opacity or cloudiness of the lens. A horse may be born with cataracts (congenital cataracts), develop them at a young age (juvenile cataracts), or develop them as an adult.

There are multiple causes of equine cataracts. Congenital and juvenile cataracts are probably due to either heredity or accident of development inside the mare. These are fortunately rare in horses. Consideration should be taken as to the wisdom of breeding these horses and their close relatives. Most cataracts in adult horses develop secondary to diseases that cause intraocular inflammation like Equine Recurrent Uveitis or ERU. Trauma is another possible cause.

Corneal ulceration or ulcerative keratitis is the most common ocular problem we see in horses. This is due to the fact that equine eye especially the cornea are so prominent and therefore prone to traumatic injury. 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 present with any combination of the following clinical signs: tearing or ocular discharge, squinting, cloudy cornea and/or a red eye.

Uveitis is inflammation of the iris and deeper structures inside the eye. Uveitis may occur as a result of a number of ocular or systemic conditions. Equine recurrent uveitis or ERU is a syndrome of recurrent or chronic episodes of uveitis. It is one of the most common causes of blindness in the horse. It is characterized by repeated episodes of uveitis alternating with periods of quiescence, when the eye is “quiet” or not inflamed. One or both eyes may be involved in the disease. Although the initiating factors are still not completely understood, the overwhelming force causing the ocular damage is clearly immune-mediated.

Squamous cell carcinoma is a tumor that originates from squamous cells. Squamous cells form the surface layer of many tissues throughout the body, including the cornea, conjunctiva and eyelid. Because squamous cells are located on the surface of these tissues, they are exposed to UV light. This exposure may result in damage, mutation, and eventually cancerous transformation of these cells.