For the Health of It
Eye Neoplasia
Reprinted with permission from the April 2018 issue of Equine Disease Quarterly
Neoplasia is the abnormal growth of tissue which , if it forms a mass , is commonly known as a tumor . Neoplasia may be benign ( tends to be less destructive ) or malignant ( tends to be invasive and may spread to other tissues ). The spread of a tumor is known as metastasis . Tumors of the eye itself are very rare in horses , but tumors of the structures around the eye are surprisingly common . The most commonly diagnosed types of neoplasia in this area are sarcoids , melanomas , and squamous cell carcinomas . Other tumors such as lymphoma and mast cell tumors may also be found . Most tumors around the eye of the horse do not metastasize , but they can be locally aggressive and have serious effects on the welfare and use of the horse . Early treatment of eye neoplasia is therefore strongly recommended .
Sarcoids are believed to be the most common skin tumor of the horse and can be seen in various forms . Sarcoids that occur around the eye are frequently more agressive in nature , invading into the eyelid musculature , especially those located on the upper eyelid . Horses will usually have additional
8 • Walking On sarcoids in other locations , so a thorough examination of the horse is recom- mended to identify any other lesions that need to be treated . Traditionally , sarcoids were often left without treatment , but as they almost invariably become larger and more difficult to treat , early intervention is strongly recommended to avoid long-term sequelae . Treatment may involve topical or intralesional chemotherapy , surgical procedures ( most successfully by laser surgical removal ), or radiotherapy . No single treatment is 100 % successful for these difficult lesions . Radiation has the highest reported chance of success , but is only available in a small number of specialty centers .
Melanomas are most commonly found in grey horses and may be within the skin or affecting the structures of the eye itself . Athough a majority are considered benign tumors , in horses that are not grey melanomas are likely to be malignant , and even in grey horses they often metastasize . Skin melanomas are usually successfully treated via surgical excision , but melanomas within the eye itself often require enucleation ( removal of the eye ). Occasionally , surgical removal of an intraocular melanoma may be successfully attempted , but this is a highly specialized surgery and is rarely a suitable option .
Squamous cell carcinomas commonly are diagnosed in the structures around the eye , and are especially likely to occur in horses with limited pigmentation of these structures . They will eventually metastasize , and early complete surgical removal is recommended as it is frequently curative , although horses with no pigmentation will always be at risk of developing further lesions . Radiation treatment and topical or intralesional chemotherapy can also be useful in selected cases .
Lymphoma may occur as a mass in , on , or around the eye , and the clinical signs associated with its development will depend on its location . Although occasionally this may be a solitary mass , lymphoma has a high likelihood of involving distant structures , and without very early recognition and treatment is often fatal . Complete surgical removal of the eye and surrounding structures may be curative in some cases .
Mast cell tumors are occasionally seen affecting the structures around the eye , and can usually be successfully surgically removed . In horses ( unlike in dogs ) they are rarely a malignant lesion .
Other tumor types are very rare , but may present in a variety of forms . It is always prudent to seek early veterinary intervention when masses arise to maximize the chance of a good result .
CONTACT : Anna Hollis , BVetMed , DACVIM ,
DipECEIM , MRCVS anna hollis @ aht org uk
Animal Health Trust Newmarket , United Kingdom