Ocular Neoplasia in Horses

In horses, tumors of the skin, eye, and genital system are the most frequent, and malignancy is quite variable based on tumor location and type.1 Neoplasms of the eyelids and conjunctivae are the most frequent ophthalmic tumors in horses; most are either SCC or sarcoid. Orbital neoplasms are rare and are usually local extensions of eyelid, conjunctiva, or sinus tumors or systemic neoplasms, including lymphosarcoma. Intraocular neoplasms, usually malignant melanomas, are rare.

Squamous cell carcinoma occurs most frequently in horses as they age1 and may occur more frequently in those with nonpigmented or lightly pigmented eyelids. The Appaloosa and draft breeds are affected most frequently. Ultraviolet radiation may be an important risk factor, because the incidence in North America is higher in southern and western mountainous areas and in areas of increased altitude or mean solar radiation.

The eyelids (~23% of cases), limbus (~28%), nictitating membrane (~28%) and other locations such as the cornea, conjunctiva and orbit (~21%) can be affected with ulcerative and/or proliferative masses.1 The bony orbital rim and anterior orbital tissues should be carefully examined, and orbital imaging (CT or radiography) may be necessary to search for local invasion.

Treatment of ocular SCC in horses is similar to that in cattle, although initial evaluation is usually earlier, and greater emphasis is placed on cosmetic appearance after treatment. The affected area should be closely examined because this mass often can be removed by keratectomy, along with adjunctive beta-irradiation or cryotherapy, and covered by a permanent advancing bulbar conjunctival or amniotic membrane graft. Surgical excision for equine SCC yields ~50% success when used alone. When surgery is combined with beta-irradiation, cryotherapy, hyperthermia, or local chemotherapy, the success rate is markedly increased to 75%–100%.1 For lesions affecting the eyelids, surgical excision along with photodynamic therapy has good success. As with cattle, smaller lesions respond better to treatment.

Tumors on the nictitating membrane are usually treated by complete excision of the nictitans and adjunctive beta-irradiation or cryotherapy. The surgical site should be closely inspected and probed for deeper infiltration of the tumor into the medial orbit and monitored carefully after surgery for any evidence of recurrence. Small SCC lesions can be excised more widely, and large, reconstructive blepharoplasties or corneoconjunctival surgeries may not be required. Often, early referral to specialty services that routinely treat these types of neoplasms will yield the best possible outcomes. The preventive role of face shields or masks with the goal of reduced ultraviolet exposure to the external eye is not known; however, the reduction of exposure to ultraviolet radiation should reduce risk.

Equine sarcoids generally affect younger horses between 3 and 6 years of age.1 Because sarcoids are locally destructive and have a high recurrence rate after surgery, effective treatment when the periocular tissues are involved presents cosmetic and functional problems. As with any neoplasm, early intervention improves success.

Sarcoids are grouped into occult, verrucose, nodular, fibroblastic, mixed, and malignant types. They appear initially as subcutaneous masses in the eyelids or canthi; they usually enlarge rapidly and may invade the skin, appearing as red, fleshy masses. Treatment is surgery, hyperthermia, cryotherapy, chemotherapy, radiation, or a combination of these therapies. Excision with photodynamic therapy has also shown some success. After attempts to surgically remove the sarcoid, recurrence may be rapid and precede wound healing, particularly if no adjunctive therapy is added. Immunotherapy using bacille Calmette-Guérin (BCG) as a potentiator of the cellular immune system is reported to have a high success rate; however, availability is becoming limited. Gamma radiation therapy using platinum-sheathed iridium-192 (¹⁹²Ir) is highly successful; however, it is less convenient and less readily available, and it usually requires a total dose of 7,000–9,000 rads. As with SCC, early referral to specialty services that routinely treat these types of neoplasms will yield the best possible outcomes.