Ocular squamous cell carcinoma (SCC) is the most common cancer that affects horses’ eyes. These tumors can spread rapidly, resulting in vision impairment, blindness, and in rare cases, death. In the initial stages, these lesions can appear benign, so detection can be a challenge for owners, unless they know the concerning signs. Our team at Veterinary Vision Center would like to provide information about this dangerous condition, to ensure you can detect the issue in your horse as early as possible.

What causes equine ocular squamous cell carcinoma?

Squamous cells are the thin, flat cells that make up the epidermis (i.e., the outermost skin layer). Squamous cells are also found in mucous membranes, including the ocular tissues. SCC occurs when the squamous cell’s deoxyribonucleic acid (DNA) is disrupted, causing the cells to multiply uncontrollably. Exposure to ultraviolet (UV) light is a known risk factor for this disruption. Genetic risk factors have also been identified and affect breeds including Belgians, Clydesdales, Appaloosas, and paint horses. White, grey, and palomino horses are also at higher risk for developing periocular SCC.

What signs indicate equine ocular squamous cell carcinoma?

SCC can target multiple ocular tissues and presents differently, depending on the affected structure.

  • Conjunctiva — When SCC affects the tissue that lines the horse’s eyelids, the initial signs include mild redness, and possibly mild tearing. Lesions start as small white or light pink tumors on the eyelid margin.
  • Limbus — When SCC affects the tissue where the cornea meets the sclera, they often have a cauliflower-like appearance. The typically light pink lesions begin at the eye’s outer edge, and then progress, extending across the cornea, and appearing flat and rough.
  • Cornea — When SCC affects the cornea, the lesion can mimic scar tissue or other conditions, such as eosinophilic keratitis (i.e., a chronic inflammatory corneal disease).
  • Third eyelid — When SCC affects the third eyelid, lesions typically begin at the eyelid margin and are usually proliferative or ulcerative. Proliferative lesions present as a thickening of the third eyelid, and ulcerative lesions cause tissue loss in the area.
  • Eyelid — When SCC affects the eyelid, lesions are usually ulcerative. Lightly pigmented eyelids are more commonly affected.

How is equine ocular squamous cell carcinoma diagnosed and treated?

A  veterinary professional will likely suspect SCC based on appearance, but definitive diagnosis is made by biopsy and histologic evaluation. The treatment approach is determined by the tumor’s size and location. Early diagnosis is important for successful treatment, which can include:

  • Surgery — Surgical repair can be problematic, because a horse does not have much extra skin around their eye that can be harvested for reconstruction. Upper eyelid SCC is more challenging than lower eyelid SCC, because the upper eyelid is more mobile, and protects the globe more. When excising a lesion, care must be taken to ensure enough eyelid is left to protect the globe. If the lesion affects the third eyelid, the entire third eyelid can be removed, effectively removing the tumor. If SCC has progressed to the point that the horse is blind, enucleation can be curative. Small lesions in the conjunctiva can also be removed, but if any cancerous cells are left, the lesion will return quickly.
  • Cryotherapy — For some lesions affecting the eyelid, cryotherapy can be effective. This method can be used after surgical removal, if the entire mass could not be removed. A probe is used to deliver liquid nitrogen to the area.
  • Radiation — Radiation therapy can be effective for lesions affecting the cornea, limbus, and conjunctiva.
  • Chemotherapy — Injecting a chemotherapeutic agent in an eyelid SCC can slow the progression, but is usually not curative.

What is the prognosis for equine ocular squamous cell carcinoma?

Prognosis depends on the lesion’s size and location. The earlier SCC is diagnosed, the better the prognosis, because smaller lesions are easier to treat and manage. Recurrence is common, especially when surgical removal is the sole treatment modality. In 10 to 15 percent of affected horses, the SCC invades the bone or metastasizes to other organs, including the local lymph nodes, salivary glands, nose, and lungs. If this occurs, prognosis for survival is poor. 

How is equine ocular squamous cell carcinoma prevented?

Since SCC has a genetic component, completely preventing the condition is not possible. However, since exposure to UV light puts horses at greater risk, ensuring they wear a UV-protective fly mask, and stabling them during peak sunlight hours, can help decrease their risk. Also, carefully monitoring their eyes for any changes is important, to allow for early diagnosis, when the condition is more easily treated.

SCC is a concerning disease in horses that can lead to blindness and possibly death. However, if caught early, the lesion can usually be successfully treated, to save your horse’s vision. If you are concerned your horse may be affected by SCC, do not hesitate to contact our team at Veterinary Vision Center to schedule an appointment.