Eye Cancer in Horses: Ocular Squamous Cell Carcinoma
Squamous cell carcinoma (SCC) is one of the most common kinds of cancer in cattle, humans and horses, affecting skin and other epithelial tissues (such as the surface of the eyeball). On horses with unpigmented (pink) skin, SCC may appear anywhere the skin is thinly haired (less protection from UV rays), such as around the vulva or sheath, or on the eyelids.
SCC is the most common primary tumor of the equine eye, according to Dr. Elizabeth Giuliano, Department of Medicine and Surgery, University of Missouri. “It can affect the eyelids, third eyelid, conjunctiva or cornea. On the eyelid it usually appears as a raised lump or proliferating mass of tissue that is redder than the surrounding tissue. It may be ulcerated or bloody,” says Giuliano.
Dr. Amber Domino Labelle, assistant professor and veterinary ophthalmologist, University of Illinois, says cancerous growths may also occur at the junction between the cornea—the clear surface of the eyeball–and the conjunctiva that covers the white of the eye. “It can distort the tissues or create a mass on the surface of the eye,” she says. Often the affected eye will be squinting, watering, or have a yellow/white discharge. This might be what the owner notices first, and then a closer look at the eye may reveal the early stages of cancerous growth.
Some horses are more vulnerable to SCC than others. “Risk factors include intense sunlight and lack of pigmentation around the eye. Horses with pink eyelids (rather than dark skin) are more likely to develop SCC. Horses exposed to high levels of UV light are especially at risk, in regions with a lot of sunshine,” says Labelle.
Horses with a lot of “white” (sclera) showing around the cornea—such as at the rear corner of the eye–may be at risk, since that area is then exposed to more sun damage. Horse owners should not assume a horse is safe from SCC just because it has dark skin around the eyes. Both of our ranch mares that developed cancer have dark skin.
Some breeds tend to have more incidence of SCC. “Haflingers and some draft breeds have more cases,” Labelle says. Appaloosas and Paints/Pintos also have more eye cancers–probably because many of them have white areas around the eyes, with lack of pigmentation to give protection from UV rays.
“Recent studies suggest that equine papilloma virus may be a factor in development of some lesions. There is research evidence that SCC in other locations, particularly the penis, may be due to equine papilloma virus, but we are not yet sure if this is the case in lesions of the eye,” Labelle says.
The best way to know if a suspicious lump is cancerous is to have your vet perform a biopsy and have the tissue sample checked for cancerous cells. Once a diagnosis is established, options for treatment may depend on how early the lesion was discovered—whether it is still very small or has become large, and where it is located.
“Treatment is always more effective if begun early, when the tumor is small,” says Giuliano. “The choice of treatment may vary, depending on cost, location and size of the tumor, frequency of treatment needed, etc. Treatments include surgery to remove the tumor, radiation therapy, chemotherapy, immunotherapy, radiofrequency hyperthermia, COX inhibition with piroxicam, cryotherapy (freezing) and local photodynamic therapy,” says Giuliano.
She has been researching the use of local photodynamic therapy (PDT) to treat growths on eyelids. “Eyelids present a challenge in horses, because if you can’t save the eyelid, you can’t save the eye,” she says. If part of the eyelid has to be removed, it is nearly impossible to do reconstructive surgery. The horse’s eyelid skin is tightly adhered to the bone beneath it; there is no extra, loose skin like that of a dog.
During the past 10 years she has successfully treated more than 30 horses using local PDT. This involves giving the patient a photodynamic agent, then using a laser to direct a focused light to destroy the tumor cells and associated blood vessels. She perfected this method for equine eyelid tumors–injecting the photoactive agent directly into the tumor bed (after debulking the tumor with surgery), and using a laser to stimulate the photodynamic agent in those tissues to kill any residual tumor cells. She prefers this method over chemotherapy agents, which can be toxic to the people handling the drug.
“Lesions on the third eyelid or the eyelid itself can often be treated on-farm, without bringing the horse to a veterinary hospital,” says Labelle. “Growths on the eyeball itself, by contrast, usually need more meticulous surgical intervention to try to save the eye. Surgical removal is followed by chemotherapy or cryotherapy (freezing) to make sure there are no stray cancer cells left in the area,” Labelle says.
If the growth on the eyeball is neglected too long, however, the eye itself may need to be removed. If the cancer spreads it will destroy the eye and eventually kill the horse. One way to tell if it has spread is to check the lymph nodes in back of the jaw, to see if they are enlarged.
Thus it is important to deal with SCC in the early stages. Horse owners should always check eyes when handling their horses, to become aware of any abnormalities. Yet some lesions may be well started before they are noticed, especially those on the third eyelid, since most of that structure is tucked underneath the outer surface.
“The easiest way to reduce the risk of SCC is to use fly masks (especially the type with UV blocking material), to reduce exposure to UV light,” says Labelle. Keeping an at-risk horse out of intense sunlight is best, but she doesn’t recommend keeping horses in stalls.