Squamous cell carcinoma (SCC) is one of the most frequently diagnosed cancers in horses and the most common cancer of the equine eye. Although it is typically slow to metastasize, it can be locally invasive in areas where complete surgical removal is challenging.

Here’s what to watch for to catch this cancer early and what to do if your horse has it.

What is Squamous Cell Carcinoma?

The name itself—squamous cell carcinoma—can sound confusing and even intimidating, so let’s start with the basics.

What is a squamous cell, anyway? These are normal cells in the body. They are primarily found in the skin, particularly at mucocutaneous borders. These borders are where dry skin intersects with a mucous membrane, such as the line between skin and the lips, nostrils, anus, genitals, and eyelids.

Understanding the normal location of this cell type then provides context for where SCC is commonly found: in or around the eye, the genitalia and anus, and lips/nose.

The term “carcinoma” refers to a type of cancer that starts within skin cells or the lining of internal organs. In addition to SCC, there are a few other types of carcinomas, such as basal cell carcinoma, which is rare in horses but common in humans.

SCC development is related to sun and subsequent UV exposure. It’s no surprise, then, that SCC is common in horses who can spend most of their life outside, accumulating UV skin damage.

However, horses with lots of non-pigmented (pink) skin on their faces, such as horses with wide blazes or bald faces where eyes and muzzles have pink skin, have a higher rate of SCC.

Breeds that tend to have extended white on their faces such as Clydesdales, Belgians, Appaloosas, and some Paints have a higher risk of developing SCC than other breeds. Lighter-colored horses, such as palominos and cremellos, are also at higher risk for SCC.

What To Look For

So what does SCC look like in a horse? Typically, this tumor initially appears as a small, raised lump or nodule under the skin. SCC within the eye may give the cornea a flat or roughened opaque appearance.

Early SCC can be easily missed due to its small size and sometimes hidden location. As the tumor grows, it can erupt, become ulcerated, and start to impinge on neighboring tissues. SCC of the eyelid can be particularly problematic. As the tumor enlarges, it may start to pull at the lid. This can either rub the cornea or pull outward, resulting in the inability of the horse to fully close his eye.

Because it is easier to remove when it is small, the best course of action with SCC is early detection. When you groom your horse, keep an eye out for odd bumps or lesions. Owners of white-faced horses should be especially observant of their horse’s facial features on a regular basis, including a close look at the eye itself.

SCC on the eyelid typically starts off looking like a small scab, while SCC at the corner of the eye or within the third eyelid is typically a raised, pink mass. Horses with ocular SCC may also have eye discharge.

Regular checks should also include the genitals and underneath your horse’s tail. For male horses, make the most of sheath cleaning: this is the perfect time for an inspection.

SCC Treatment Options for Horses

What should you do if you find something suspicious on your horse? The first step is to have your vet out. Although you may have a very high suspicion of SCC based on the lump’s location or your horse’s history or pigment pattern, you can’t diagnose SCC based on visual examination alone.

Sometimes SCC can be confused with scar tissue, other types of tumors (such as papillomas or sarcoids), or even a skin allergy. A biopsy, or tissue sample, is the best way to definitively diagnose this tumor.

Once your vet confirms the diagnosis, it’s time to talk treatment options. As with most tumors, its location, size, and involvement of neighboring tissues heavily influence treatment options and prognosis.

Surgical removal is a common choice, but location of the tumor dictates treatment success rate. For example, SCC at the corner of the eyelid can be very challenging to reconstruct after removal of the diseased tissue. Other times, SCC may grow into deeper structures around the eye. In these circumstances, total enucleation may be the best option for the horse’s future.

Adjunct treatments along with surgical removal are the most common choices of treatment and management of SCC. These adjunct therapies occur at the time of surgery and may need to be repeated.

Cryotherapy (freezing) is a common adjunct therapy that can be done on site at your barn. Your vet will apply liquid nitrogen, which is -320° Fahrenheit, to the edges where the tumor was removed. This helps destroy any leftover cancer cells.

Some radiation and chemotherapy options are also available. Depending on the regimen, they may require a visit or two to a referral hospital for application.

Photodynamic dye therapy (PDT) is a relatively newer option for SCC treatment. This therapy involves injecting a dye containing photosensitive compounds into the affected area.

This dye is taken up selectively by the cancer cells. Next, a focused beam of light using a specific wavelength is directed at the diseased tissue. The light reacts with the dye, creating a skin reaction that kills the tissue.

Because only the cancer cells take up the dye, other nearby healthy tissue is relatively unaffected, providing a much more focused treatment compared to radiation and chemotherapy.

Prognosis after surgery and adjunct therapy for SCC depends heavily on the size and location of the tumor and how locally invasive it is. Horse owners should note, too, that once a horse has had SCC, he is at higher risk of developing it again.

Prevention Tips

Along with regular close monitoring of your horse’s skin, reducing your horse’s exposure to UV radiation is another tool for SCC prevention. Regularly applying sunblock to your horse’s non-pigmented facial features can help, as can putting a fly mask on your horse in the summer months to shade his eyes.

Interestingly, researchers at the University of California-Davis School of Veterinary Medicine have identified an equine gene variant that is a risk factor for ocular SCC in certain breeds, including Haflingers, Belgians, Rocky Mountain Horses, Connemaras, Holsteiners, and Belgian Warmbloods.

Horse owners can send plucked hairs from the mane that include the roots to the UC Davis Veterinary Genetics Laboratory to identify whether their horses are at higher genetic risk of developing ocular SCC.

Treatment options continue to develop for this equine cancer, and horse owners have more options than they did just a decade ago. While this is encouraging, the best course of action is still vigilance to catch this tumor early for a full recovery.

Key Takeaway

As with any cancer, squamous cell carcinoma is intimidating. But remember—prevention and early detection are your best weapons.

This article about squamous cell carcinoma in horses appeared in the June 2024 issue of Horse Illustrated magazine. Click here to subscribe!