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Skin Cancers

Cancer awareness programs abound in human medicine: virtually everyone gets screened for at least one type such as breast, cervical, colon or prostate cancer. Cancers of the inside of the body are rare in horses, but cancers of the skin are fairly common. Fortunately screening for these tumors is easy—you can see them! Horse owners should be on the lookout for three common skin tumors: sarcoids, squamous cell carcinomas and melanomas.


Sarcoids are the most common skin tumor. They show up in several different forms: some are hairless circles, others are wart like thickened bumps, some are smooth firm lumps and still others are fleshy, ulcerated masses. Common sites for growth are on the ear, neck or around the eyelids. They are caused by a virus that is spread by flies. No breed, age or sex predilection has been found.


Sarcoids are sometimes just nuisances. If they are small (quarter size) and are not located near the eye or a place where the bridle or saddle would rest, they may never bother the horse. However if they are large, or fast growing, or located near the eye or tack sites, they may become a big problem.

Treatment of sarcoids is tricky. Simple removal may result in the mass coming back with a vengeance. If your veterinarian decides to operate on a sarcoid, usually another type treatment will be paired with the surgery. Treatments choices include freezing the tissue (cryotherapy), heating the tissue (laser treatment), injecting or implanting chemotherapy type drugs in the area and/or applying a topical paste that may make the body “reject” the sarcoid cells (Xterra).

Sarcoids have never been reported to spread inside the body. However, certain forms are very aggressive locally and hard to control. You should always ask your vet to examine any odd skin region that becomes hairless or turns into a lump.

Squamous cell carcinoma is an invasive tumor that can affect areas of skin, skin mucosa (lining) and sinuses. It is most commonly found on the external genitalia and around the eyes, including the cornea and third eyelid. Some of these tumors look like tiny pink cauliflowers while others are ulcerated pits in the tissue. They can be very aggressive locally and may harm the function of nearby structures. Horses with light skin of the eyes or genitalia are at risk. Ultraviolet light exposure is thought to be a frequent trigger for the cancer growth.

Squamous cell carcinomas are always serious and require prompt veterinary treatment. Usually the first thing that is done is surgery to remove as much abnormal tissue as possible. The goal is remove enough tissue that the tumor cells are surrounded by a “margin” of normal cells that is about as wide as your thumb. However sometimes it is not possible to remove so much tissue without harming the horse. In these cases supplemental therapy is advised.

These photos show a horse with squamous cell carcinoma of the eyelid. Treatment included surgery to remove the largest mass, cryotherapy (freezing) of the smaller mass, and local injection of a chemotherapy drug. The tumor responded well but there is always a risk it could come back.

Supplemental therapy can include injecting or implanting chemotherapy type drugs around the tumor region, freezing the tissue (cryotherapy), radiation therapy, or photodynamic therapy. In some cases a topical chemotherapeutic drug will be prescribed in the form of drops (for tumors near the eye) or cream (for tumors in other parts of the body). Some horses are put on an oral drug called piroxicam that may slow the return of cancerous cells.

Squamous cell carcinomas occasionally spread inside the body, usually to the stomach. The major risk, however, is that they will expand rapidly at the original site. As they have a predilection for areas of the body near vital openings (eyes, genitalia, mouth, sinuses), local spread can be devastating for the horse. If you see any region on your horse’s eye, genitalia or face that becomes pink, irritated and raised, have it checked out without delay.


Melanomas are a common tumor of gray horses: over 80% of gray horses over 10 years have at least one. Ranging in size from a blueberry to an orange, melanomas are firm dark spherical lumps that often occur in clusters. The most common locations are around the anus, vulva, udder, tail and sheath. They also can grow in the lips, near the salivary glands in the throat and around the eyelids.

Most melanomas grow slowly. Although single tumors can be removed surgically, most often the horse presents with multiple masses. In these cases the tumors are usually just monitored, unless they are growing quickly near some part of the horse where a large mass will cause a problem. Rarely, they can occur in an aggressive fast growing form that may spread internally.

Almost all melanomas occur in gray horses. If a melanoma occurs in a horse of a different color, the tumor may grow rapidly or spread internally. Sometimes an oral medication called cimetidine is prescribed to try to slow the growth of melanomas.


Biopsy of tumors: Sarcoids, squamous cell carcinomas and melanomas are the most common skin tumors of the horse. However, there are many other rarer skin tumors, and a few non-cancerous conditions that can mimic “the big three”. Tumors that may look like one of the common growths include mast cell tumors, fibromas, papillomas, carcinomas in situ, lymphomas, schwannomas and a whole list of other rarer cancers. Non-cancerous look-a-like conditions include a parasite infection called habronemiasis, an inflammatory condition called a collagen granuloma, and some other miscellaneous cyst- like growths.

Veterinarians may advise a procedure called a biopsy to figure out the make- up of a skin lump or abnormal section. A biopsy is a surgical procedure where either a small piece or the whole tumor is removed. The abnormal tissue is put in a preservative solution and shipped off to a laboratory. A pathologist at the laboratory then examines thin sections of the specimen under the microscope after staining the cells with dyes or chemicals. The microscopic examination will determine the type of abnormal cells that are present, and will report whether there are margins of normal cells surrounding the abnormal ones. Based on the biopsy results, the veterinarian can then predict the probability of whether the abnormal cells will come back or spread elsewhere. Biopsies are expensive but worthwhile—often a biopsy is the key piece of the puzzle in selecting the optimum therapy to protect the horse’s health.

If you notice an area on your horse’s skin that seems abnormal, call your veterinarian. He or she has been trained to decide which “lumps and bumps” are worrisome and which are not. Some of the worrisome lumps or growths are very hazardous to your horse’s health. Fortunately, modern veterinary medicine has several good treatments for many problems if they are caught early.