THE MERCK VETERINARY MANUAL
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Ocular Neoplasia in Horses

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In horses, tumors of the skin, eye, and genital system are the most frequent, and ~80% of eye neoplasms are malignant. Neoplasms of the eyelids and conjunctivae are the most frequent ophthalmic tumors in horses; most are either squamous cell carcinoma 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 8–10 yr old and may occur more frequently in those with non- or lightly pigmented eyelids. The Appaloosa and draft breeds are affected most frequently. Ultraviolet radiation may be important, 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 (~15%), conjunctivae and limbus (~25%), and nictitating membrane (~30%) can be affected with ulcerative or proliferative masses. Bilateral involvement is infrequent (~15%). Squamous cell carcinoma of the nictitans is more likely to invade the orbit than are those from other sites. Treatment of ocular squamous cell carcinoma in horses is similar to that in cattle, although presentation for treatment is usually earlier, and greater emphasis is placed on cosmetic appearance after therapy. Surgical excision for equine squamous cell carcinoma yields ~50% success when used alone. When surgery is combined with cryotherapy, hyperthermia, or local chemotherapy, the success rate is markedly increased. As expected, small squamous cell carcinomas can be excised more widely, and the need for later reconstructive blepharoplasties or corneoconjunctival surgeries is more limited. Often, referral to specialty services that routinely treat these types of neoplasms can 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 unknown, but their use should start at a very young age.

The equine sarcoid (see Equine Sarcoids) generally affects young horses (average 3.8 yr old) and represents ~40% of all neoplasms in horses. 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. Sarcoids are grouped into occult, verrucose, nodular, fibroblastic, mixed, and malignant types, and they are divided histologically into neurofibroma, neurofibrosarcoma, myxosarcoma, and fibromyxosarcoma. 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. After attempts to surgically remove the sarcoid, recurrence may be rapid and precede wound healing. Immunotherapy using BCG (bacille Calmette-Guérin) as a potentiator of the cellular immune system is often successful (~70%). Injections should be repeated at 2- to 4-wk intervals until the mass disappears. Systemic corticosteroids and antiprostaglandins before and after treatment may decrease the likelihood of systemic anaphylactic reactions. Gamma radiation therapy using platinum-sheathed iridium192 is highly successful (~95%) but less convenient and available and usually requires a total dose of 7,000–9,000 rads.

Last full review/revision August 2013 by Kirk N. Gelatt, VMD, DACVO

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