Equine Occult Sarcoid

Does your horse have a dime to quarter-sized round patch of missing hair that doesn’t ever go away? Although it could be a scar or a callous, don’t forget about the possibility of an equine sarcoid.

Sarcoids are a form of skin cancer caused by the bovine papilloma virus. It is a weird sort of etiology- the virus that causes warts in cattle can cause cancer in horses. While warts are generally self-limiting in cattle, the cancer is a potentially serious life-long issue in horses. Flies can carry the bovine papilloma virus on their mouth piece and inoculate it into the horse’s skin when they bite. Of course, not every horse that gets bitten by a fly carrying papilloma virus gets sarcoids, so much is still unknown about the disease process. It has been postulated that some horses are genetically susceptible to the cancer, or also possibilities that immune status plays a roll. Despite all that we have yet to learn about why they occur, we do know for sure that sarcoids are the most commonly diagnosed tumor of horses, mules and donkeys- representing 36% of diagnosed skin tumors. Studies suggest there is no significant gender or age predisposition, but they are highly prevalent in Quarter horses and Arabians and less common in Standardbreds.

There are six described types of sarcoids:

Occult sarcoids are flat, hairless, crusty lesions that are typically round. They often have a smooth, dark hairless area around them.

Verrucose sarcoids are raised, knobby, dark areas that often spread into poorly defined margins. They can sometimes have ulcerated portions.

Nodular sarcoids are firm and nodular skin lumps which may have normal skin over them.

Fibroblastic sarcoids are often swollen, nodular bloody scabbing lesions that grow off the body much like proud flesh.

Mixed sarcoids are commonly a mixture of two or more of the forms described above.

Malevolent sarcoids are aggressive and invasive lesions that appear on the outside like fibroblastic sarcoids but also invade deep tissues.

Sarcoids can develop anywhere on the body but are most common in the paragenital region (around the sheath and on the inside of the upper hind legs,) the ventral thorax and abdomen (midline), and the head. They frequently are seen at sites of previous injury and scarring. Why? Because these are the places where flies most commonly bite!

Several treatments are available, but none are guaranteed cures. Most commonly, small sarcoids are surgically excised and removed. Often times a laser is used in the surgical process since it can cut the sarcoid out and cauterize the location simutaneously. Cases in which sarcoids are too large to be excised or are in inoperable locations, chemotherapy, radiation and even topical caustic agents can be used to reduce the size of the lesions and kill neoplastic cells. There are even some experimental sarcoid vaccinations in the works- stay tuned! No two sarcoids are treated exactly the same way, so you must consult with your veterinarian to determine which treatment plan is the best option should your horse have a sarcoid.

If your horse has a sarcoid, avoid picking at it and keep the flies away since any irritation can cause it to “wake up” and grow.

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