Background: Squamous cell carcinoma (SCC) is a locally invasive malignant tumor of the squamous epithelial cells. This
neoplasm affects the eyelid, conjunctiva and third eyelid as well as the cornea. The SCC is the most common neoplasm of
equine eye and ocular adnexa. Breeds, Appaloosas, American Paint Horses, Thoroughbreds, and Quarter Horses are reported in
literature as being the most predisposed ones to SCC. Although presenting a common occurrence, SCC in the third eyelid of
horses is rarely reported in literature. This paper aims to describe clinical and pathological signs, and a surgical intervention
of a squamous cell carcinoma case in the third eyelid of a horse.
Case: A 15-year-old, gray, stallion, Criollo horse presented a third eyelid mass in its left eye. The owner of the horse reported
the growth of a red mass in the third eyelid of the bulb of its left eye with the presence of intense bloody discharge for two years.
During anamnesis, a history of a previous ocular perforation was reported. The owner of the horse also reported that the outdoor
housing of the horse is located in a sunny area, resulting in its relatively high exposure to solar radiation. The mass was
approximately 3 cm in diameter, ulcerated and located at the free edge of the third left eyelid, but no change was observed in
the right eye. We opted for the exenteration of the left eyeball. The CBC and biochemical profile of the patient showed no
changes. The radiographic evaluation did not demonstrate any evidence of metastasis. The eye, orbital tissues, eyelids, and
nictitating membrane were surgically removed. After establishing a sterile field, the exenteration was performed, and a continuous
suture pattern of 2-0 polyglactin was used to close the subcutaneous tissue; the skin was closed with single interrupted sutures
with 3-0 nylon. The third eyelid was placed in buffered 10% formalin, processed for light microscopy and sections were
stained with hematoxylin eosin. The horse was discharged 10 days after the procedure.
Discussion: The squamous cell carcinoma is a locally invasive malignant neoplasm. It is the most common ocular neoplasm
in the horse, affecting eyelids, nictitans, conjunctiva and cornea. It may raise anywhere in the body with a multifactorial
etiology, and, the exposure to ultraviolet light and lack of periocular pigmentation are considered the main predisposing
factors. The horse observed in this study is Criollo, dapple coat clear, with 15 years of age, set up outdoors and exposed to
constant solar radiation. The preconized treatment for squamous cell carcinoma cases is tumor excision through a surgical
margin, and, in this case, we opted for the exenteration. The third eyelid was sent for histopathological analysis as revealed
islands and nests of pleomorphic epithelial cells and keratin pearls, and the diagnosis of the third eyelid was squamous cell
carcinoma. Squamous cell carcinoma is considered malignant, highly invasive with high metastasis. But after 12 months of
postoperative follow-up, there was no sign of recurrence or metastasizing. We conclude that the surgical removal of the tumor,
with a margin of safety, was effective for the treatment of SCC in the third eyelid of a horse.
