What the Horse Owner Should Know About Equine Sarcoids

by Dr. Hayley M. Lang DVM

Fibroblastic sarcoid on ear

Grooming your horse one day you notice a small raised growth on your horse’s face. You wonder what it might be? Should you be worried? It looks similar to a growth your friend’s horse had that was diagnosed as a sarcoid! What is a sarcoid anyway and what should you do about it?

Equine sarcoids are an often frustrating condition faced by both horse owners and veterinarians.

Sarcoids are a neoplastic growth, i.e. cancer or a tumor, specific to the horse. It is the most common skin neoplasm of the horse and makes up one-third of all reported tumors. Don’t be too frightened by the “C” word, as this is not a malignant tumor, in other words, it does not metastasize (spread to other organs). Therefore, it is considered a benign tumor. Thank goodness, but wait… sarcoids are a little different from the typical benign tumor – they can become locally invasive and new growths can arise in other locations on the skin, which can be devastating and difficult to treat.

How did my horse develop a sarcoid?

This is a good question and recently the answer to this question has become clearer. The DNA of Bovine Papilloma Virus (BPV) has been present in nearly all equine sarcoids examined. This is a virus that causes warts in cattle. Typically papilloma viruses are species specific: bovine papilloma virus causing warts in cattle, canine papilloma virus causing warts in dogs and other canines, and human papilloma virus (HPV) causing warts in people. Horses even have their own papilloma virus, equine papilloma virus, which is the contagious cause of self-limiting warts often seen in young horses.

Intralesional Cisplatin in oil

Now you must be wondering, “Why does my horse have a cow virus?” As you may know HPV cannot only cause warts in people it can also cause genital cancers. This is because the virus can be oncogenic, causing normal cells to become cancer cells. BPV has this same property in the horse.

There is more to the story!

Although a sarcoid like lesion can be experimentally induced in equids exposed to BPV they often regress spontaneously. You may have noticed that you have never seen a sarcoid on a Standardbred. Quarter horses and other stock breeds are two times more likely to develop sarcoids compared to Thoroughbreds and Warmbloods; Standardbreds are considered to be relatively resistant. This is all due to a genetic predisposition; susceptible horses have a genetic defect that makes their tissues more likely to be mutated by BPV.

Electroscapel removal

Sarcoid means, “flesh like” and is therefore descriptive of a majority of the lesions. Sarcoids most commonly occur on the head (including the muzzle, ears and around the eyes), the lower limbs, and ventral abdomen including the sheath. These are often areas of fly bites or previous trauma, including surgical (castration) sites, where the skin barrier is broken and BPV can be either introduced or triggered if already present.

There are 6 sarcoid subtypes that vary in their appearance, behavior and common locations. Occult sarcoids are flat and occur near the mouth, eye, neck and the inside of the upper limb. Verrucous sarcoids are warty and occur on the face, body and groin. Fibroblastic sarcoids are proliferative and ulcerative resembling granulation tissue or proud flesh and occur on the groin, lower limbs, and eyelids. Mixed sarcoids have characteristics of different types and happen in areas of long-standing or traumatized lesions. Finally, malevolent have multiple nodules with a fibroblastic appearance, and are the most aggressive subtype, infiltrating local tissues.

Verrucous sarcoid on neck

Now that you know more about sarcoids how can you confirm that your horse truly has one? It would be best to have your veterinarian take a look. As this is a fairly common condition, most equine veterinarians can make a presumptive diagnosis based on appearance of the lesion alone. However, there are differential diagnoses such as other skin neoplasias (including squamous cell carcinoma) and even proud flesh. Therefore a skin biopsy, performed by your veterinarian, is the only way to diagnose an equine sarcoid definitively. Not so fast… small flat and verrucous forms can become more aggressive after biopsy. Therefore it is very reasonable to monitor small lesions for changes in appearance and size using serial photographs and measurements. If it changes then biopsy and/ or treatment should be considered.

To Treat or Not To Treat?

To treat or not to treat (and with what), that is the question? There are many treatments with varying success and recurrence rates depending the type of sarcoid and how long the cases were followed out. This means, unfortunately, that there is no one perfect treatment. Common treatments include surgical or laser excision or debulking, cryotherapy (freezing with liquid nitrogen), radiotherapy, immunotherapy (stimulate the immune system to attack the sarcoid), and local chemotherapy. Linear radiation therapy has shown to be very successful in small number of cases, however this is only available at a small number of institutions, is expensive and requires general anesthesia.

Malevolent sarcoid on lower limb

Veterinarians often use a chemotherapeutic drug called Cisplatin, which is either injected with an oil directly into the sarcoid or implanted as a bead, both allowing the drug to stay locally for a longer period of time. Surgical excision or debulking has better results when combined with the use of Cisplatin in one of these forms. Topical chemotherapeutics such as Imiquimod cream (AldaraTM, 3M Pharmaceuticals) and 5-fluorouracil (5-FU) also have had reasonably good success rates but can cause local irritation and must be applied frequently over an extended period of time (usually until resolution of the lesion). Please be careful buying any over-the-counter “Miracle” sarcoid treatments, as any treatment that does not resolve the sarcoid can cause it to become irritated and more aggressive.

The reality is that not all sarcoids are “curable” and recurrences after treatment do happen quite frequently, something that sarcoids are well-known for. Prognosis depends on the sarcoid type and location. Small occult or verrucous lesions may be best treated with benign neglect and monitoring, where aggressive and large malevolent sarcoids can only be kept in check (palliated) at best. Sarcoids near the eye and on the lower limb often carry a poorer prognosis partially due to the fact that more aggressive treatments cannot always be used for fear of damaging important local structures (such as the eye, bone, or joints or tendons). Ultimately, the best treatment option for your horse’s particular sarcoid should be discussed with your veterinarian or referral hospital.

Dr. Hayley Lang DVM

Author biography:
Dr. Hayley Lang is currently a large animal surgery resident at the Cornell University Equine and Farm Animal Hospital. She is a graduate of the University of Saskatchewan, Western College of Veterinary Medicine, in Canada.

Before starting her residency, Dr. Lang, completed an equine veterinary internship at Littleton Equine Medical Center in Colorado.

She has special interest in equine sports medicine, lameness and surgery.

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