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SKIN TUMORS IN HORSES
Overview
Skin tumors are relatively common in horses of all breeds and ages. Tumors in horses tend to be locally invasive and slow to metastasize. There are 3 common types of skin tumors: sarcoids, melanomas and squamous cell carcinomas. Many other different types of tumors can also occur but are not as common. Early recognition, accurate diagnosis, and treatment yield relatively good success rates.
Causes
There are different causes depending on which type of tumor is involved. Sarcoids appear to be caused by interaction between genetics and a bovine papilloma virus (a virus that causes warts in cattle). The cause of melanomas is unknown but the various causes leading to its predominance amongst older grey horses are under investigation. There are some known factors that may lead to the development of squamous cell carcinoma. These include ultraviolet radiation, smegma accumulation in the sheath, and prior exposure to tumor-causing substances (carcinogens). Causes of other tumors are less well known.
Signs and Signalment
Sarcoids can occur in horses of any age and are most common in adult Quarterhorses and other closely related breeds. Standardbred horses appear to be very uncommonly affected. There are four different forms of sarcoids, ranging from small nodules under the skin to exuberant tissue. They can occur on any part of the body and sometimes in areas of old or repeated trauma. Melanomas occur in older grey horses almost without exception. They most commonly occur under the tail, in the parotid gland region, and around urogenital structures. Melanomas can be black or be underneath normal skin.
Figure 1. Parotid gland melanoma in an older grey horse
If melanomas occur in colored horses, they are much more likely to be malignant. Squamous cell carcinomas occur most commonly in adult to geriatric horses of any breed. They can be ulcerative or proliferative. They are common in unpigmented (white or pink) areas, such as around the eye (Figure 2), and also around urogenital structures (Figure 3).
Figure 2. A squamous cell carcinoma on the third eyelid of a horse
Figure 3. Squamous cell carcinoma on the penis of a horse. These can ulcerate or proliferate above the normal surface and commonly have a foul smell and bleeding associated with them.
When to Seek Veterinary Assistance
The chance of successful resolution is greatly improved by early diagnosis and treatment. If regular grooming and cleaning reveal a small lump under the skin, a foul odor coming from the penis, bleeding from the penis, difficulty urinating or defecating, or any change in your horse's normal behavior, your regular veterinarian should examine the horse. He may choose to perform a biopsy at the farm and then seek the aid of a board certified surgeon or refer the horse primarily.
Treatment
Skin tumors in the horse can be treated by a variety of different methods depending on the type of tumor, the location, which structures are involved, and economic concerns. Most treatments consist of surgical excision with some form of adjunctive therapy (additional therapy designed to prevent re-occurrence and kill any residual tumor). Other treatments include cryotherapy, lasers, photodynamic therapy, radiation therapy, brachytherapy and drugs ranging from topical creams to oral medications (chemotherapy). ACVS Veterinary Surgeons have expertise in complete surgical removal of skin tumors and obtaining the best cosmetic results possible. They also have access to the latest and most advanced treatments.
You, your referring veterinarian, and your ACVS Veterinary Surgeon form an important team of people dedicated to trying to beat skin cancer in your horse.
Figure 4. A squamous cell carcinoma beside the vulva, before treatment
Figure 5. The same horse in Figure 4 after surgical removal. The tumor had spread deep into the underlying tissues.
Figure 6. The same horse in Figure 5 several weeks later after healing. She had also been treated with oral piroxicam and topical 5-fluorouracil cream
Figure 7. A squamous cell carcinoma that developed on the back of a horse several years after a severe burn. Due to the extensive skin loss from the burn, this horse was not treated surgically and was only treated with oral chemotherapy. Close monitoring and regular veterinary visits were crucial to the eventual successful resolution of the cancer.
Prognosis
The prognosis for most skin tumors is generally good if the tumor is treated early before it is too large. The best outcome is often obtained by a combination of surgery and some form of additional treatment. The best treatment for your horse needs to be determined on an individual basis with you, your regular veterinarian and your ACVS Veterinary Surgeon. While skin tumors are very slow to spread to other areas of the body (metastasize), the risk of metastasis increases if they grow very large or are present for long periods of time.
Figure 8. A large melanoma that had spread from the sheath of a horse to the penis. Both structures were surgically removed (a penile amputation was performed) but the melanoma had already spread to internal organs. Note the very black appearance of the tumor, which is characteristic of melanomas.
Prevention
As with all forms of cancer the key for successful treatment is early diagnosis. Regular grooming and care of the horse will reveal any small lumps or bumps; these should be treated seriously and examined as soon as possible by a veterinarian. Reducing smegma accumulation in the sheath (by regular sheath cleaning), reducing exposure to excessive sunlight in light colored horses, and general cleanliness are important for prevention of tumors, especially squamous cell carcinoma. It is important to seek early veterinary assistance if you notice any skin abnormalities.
—Yvonne A Elce, DVM
Diplomate ACVS
Posted 8/23/2006
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