Third Eyelid Tumors Require Prompt Vet Attention

Squamous cell carcinoma (SCC) is one of the most common cancers in horses. Commonly appearing as small, wartlike bumps on the eyelid or surface of the eye, they require early treatment. While a conscientious owner might quickly notice a new lump on the shoulder of his or her equine friend, even the most watchful horseperson will miss a concealed third eyelid tumor.

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Squamous cell carcinoma (SCC) is one of the most common cancers in horses. Commonly appearing as small, wartlike bumps on the eyelid or surface of the eye, they require early treatment. While a conscientious owner might quickly notice a new lump on the shoulder of his or her equine friend, even the most watchful horseperson will miss a concealed third eyelid tumor.

The nictitating membrane, or third eyelid, is an interesting anatomical component of most mammals’ and birds’ eyes. It is a white to pinkish membrane that usually remains in hiding in the corner of the eye until debris or trauma is detected, in which case it swiftly slides across the eyeball in a horizontal motion. Interestingly, humans do not have a third eyelid, but we do have a remnant called the plica semilunaris. It is visible in the corner of your eye as a small white membrane where “sleep” or eye discharge can accumulate.

In contrast to humans, horses have a substantial nictitating membrane. The trouble is most horse owners don’t know to look at it because they never see it. “Squamous cell carcinoma of the third eyelid can be a very easy lesion to miss,” said Amber Labelle, DVM, an ophthalmology resident at the University of Illinois Veterinary Teaching Hospital in Urbana. It’s one of the many reasons why horses should have a thorough eye exam once a year, often when they receive their annual vaccines.

Just like certain skin tumors in humans, SCC is linked to UV radiation from the sun. Labelle said is especially common in horses with less pigment in their eyes, such as Paints, Appaloosas, and some draft breeds. SCC is also commonly found in the genital region of older male horses with light skin. In short, the less pigment a horse has in a certain area, the more cancer-causing UV radiation it can absorb

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