Stephen Graham, DVM

April 2006

What is this?  Is it cancer?  Is that a wart gone wrong?  Does it hurt him?  Doc, can you just cut that off?  These are a few questions that we're going to address regarding a few of the more common types of skin tumors.  We commonly find skin tumors on the horse on or around the face (including the eye), the insides of the upper legs, the lower limbs, places where there has previously been or currently is an injury, and on or around the genitalia.  Things you might notice are a loss of hair; bumps in, on or under the skin; oozing; or your horse appears bothered by a particular spot on its body.  To determine if a lump is cancerous and what type of cancer it is, we need a biopsy of the lump or area of concern. 

Sarcoids.  We will start by talking about the most common type of skin tumor, the sarcoid.  This is a skin tumor that can look bad, but it is not one that will kill your horse.  They can be locally aggressive and invade the surrounding skin, but won't spread to other parts of the body.  There are six different types of sarcoids, which all look different on the skin.  They can even show up as a combination of different types.

Sarcoids are most commonly seen in younger horses – greater than 70% of sarcoids develop in horses less than four years old.  They can appear in freshly healing wounds, in previously normal horses, or they can recur at the same location following apparent complete surgical removal up to ten years after surgery.  Studies have shown that the virus responsible for causing warts in cows may be involved.  It has also been suggested that flies may play a role in the development of sarcoids.  Lastly, some studies have shown that horses may be genetically predisposed to getting sarcoids. 

            Occult sarcoids.  This sarcoid is most commonly found in the skin around the mouth, eyes, neck and other relatively hairless areas of the body.  They are often circular and usually contain one or more small nodules with a thickened skin appearance.  The hair over this spot may change colors.  They may be difficult to identify on horses that still have winter coats.  These are characteristically slow growing and may progress to other types of sarcoid if injured.  They may also develop rapidly into a warty or fibroblastic sarcoid without any apparent insult.

            Verrucous (warty) sarcoid.  These have a predilection for the face, body and groin or sheath areas.  They have a characteristic wart-like growth in and on the skin.  This type can affect extensive areas and are often surrounded by an area of slightly thickened skin (possibly reflecting an area of early occult sarcoid with altered, thin hair growth pattern).  Individual warts may be flat-based or with a narrow neck.  Most often they are slow-growing and not very aggressive until traumatized.  Like the occult sarcoid, these too may develop into the fibroblastic type. 

            Nodular sarcoid.  These have a predilection for the groin, sheath or eyelid area.   These are easily recognizable as firm, well defined, spherical nodules under apparently normal skin.  The number of nodules can vary dramatically – they can be found as single, few, several or even hundreds. 

            Fibroblastic sarcoid.  This type has a predilection for the groin, eyelid, limbs, sites of skin wounds at any location, and sites of any other type of sarcoid subjected to trauma or insult.  These are one of the more locally aggressive types and have a fleshy, ulcerated appearance.  They commonly ulcerate and have a serous discharge.  This type can be easily confused with proud flesh (exuberant granulation tissue), as they look very similar.  Surgical wounds are also susceptible to this type of sarcoid.  Although they have an aggressive appearance and can spread locally, they do not metastasize to other parts of the body. 

            Mixed sarcoid.  The skin of the face, eyelid, groin and medial thigh is predisposed to this type of sarcoid.  They are probably the transition phase between verrucous/occult types and the fibroblastic/nodular types.  The different combinations that can be found are great.  It can contain any or all of the above types and is common in long-standing wounds or those that are subjected to repeated minor trauma (such as rubbing).  As this type transitions from its original form to the fibroblastic type, it becomes more aggressive. 

            Malevolent sarcoid.  This is a more recently discovered variation with predilection sites on the jaw, face, elbow and medial thigh.  This type is also seen with a history of repeated trauma to other types of sarcoid.  However, some cases have no such history, with spontaneous development of typical, multiple locally invasive sarcoids of nodular and fibroblastic character. 

Sarcoid Treatment.

As already mentioned, treatment can be difficult as there is often recurrence despite the treatment used.  Treatment can include medical, surgical and radiation treatments.  The recommended treatment is based on the type of sarcoid, its size and its location. 

One of the most common treatments is benign neglect or no treatment at all.  Sarcoids are generally not painful, and as long as they are not causing problems for the horse, they can be left alone.  The other benefit to this is that if they resolve on their own, it seems to be curative and the horse is less likely to develop new sarcoids in the future at the same spot or at a new spot. 

Surgical treatment options include regular excision which has a high rate of recurrence, freezing or cryosurgery (works well with small, well defined lesions), hyperthermia (heating and killing it with radio waves), electrocautery (cutting off with electricity), and surgical excision with a laser. 

Medical treatments include applying heavy-metal compounds, chemotherapy drugs and injecting immune-stimulating materials into the sarcoid so the body attacks it.  A couple of the promising topical compounds that are currently being used are Aldara™ and Xxterra™.  How these two products work is not fully understood, we have had good results and hear other veterinarians using these products are having good results as well. 

Other Skin Tumors.

            Squamous cell carcinoma.  This tumor type is commonly seen around the eye or on the penis.  This type can spread to other parts of the body and is generally a more aggressive type of tumor.  Treatment is very important and can include surgical removal with wide margins or chemotherapy drugs.  The treatment choice depends on its location and rate of growth. 

            Melanoma.  These are non-invasive tumors that originate from melanocytes, which are pigment producing cells.  These are most commonly seen in gray horses.  There is possible genetic predisposition, as some lines of grey horses have a higher incidence than others.  These tumors are commonly found around the anus, vulva, tail and prepuce.  They are commonly diagnosed by their location and appearance.  Treatment can be a method of surgical removal and/or chemotherapy drugs. 

            Papillomatosis (warts).  Warts are sometimes found on our horses, especially the younger and older ones.  They are caused by a virus.  This virus has the ability to cross the placenta in pregnant mares, so it is not uncommon to find warts on the skin of newborn foals.  It is also a contagious virus, and is passed by contact between horses grazing on the same ground.  Warts are most commonly found on the muzzle, face, lips, distal limbs and genitals.  Usually they are not treated.  In three to four months warts will often resolve on their own in young horses.  Treatments such as freezing or surgical removal can be used if they are a problem. 

            Aural plaques.   Aural plaques are also known as pinnal papillomas or papillary acanthosis.  These are very similar to warts and are found on the inside of the ear.  Some of these plaques can develop extensive involvement of the inner skin of the ear.  They usually do not cause problems for the horse.  Though they are similar to warts, they do not usually spontaneously resolve.  The treatment for these is often worse than the plaques themselves. 

Take Skin Tumors Seriously.

Unfortunately, these tumors are just the tip of the iceberg.  There are many others out there that are less commonly seen but should still be considered.  For this reason, it is best to take the "better safe than sorry" approach and have your veterinarian out to examine any masses and decide the best course of action.  The best thing you can do to keep your horses as safe as possible is to be very familiar with and all of their bumps.  Inspect regularly and note any changes.