Equine Sarcoids

Sarcoids are an old, frustrating problem. They are rarely, if ever, life threatening but they are unsightly and disfiguring and they have a habit of occurring in places that make it necessary but difficult to treat them. The lack of any one good therapy to get rid of them has given them a well-earned reputation as a pesky problem to treat.

Appearance and Cause

Sarcoids are the most common type of neoplasia (cancer) in the horse. They do not tend to spread to distant sites (metastasize) but they can be quite locally aggressive, spreading like roots of a tree. Their appearance ranges from a slightly raised hairless region that can resemble ringworm to a large, raised, ulcerated and oozing mass. The appearance of the sarcoid is the basis for how they are classified—occult, verrucous, nodular, fibroblastic, malevolent and mixed. Classification is somewhat arbitrary and can change as the tumors behavior changes. The treatments available don’t really differ between the different classifications of sarcoids and it is unknown why some horses get a less aggressive classification and others get more aggressive forms.

​The cause of sarcoids continues to be elusive but it seems we are getting closer to having some answers. As with many problems, there appear to be multiple factors involved. Sarcoids are frequently encountered in areas were a wound previously existed or areas where flies frequent. Bovine Papilloma Virus (BPV) has been known to be associated with equine sarcoids for some time and could reasonably be expected to be carried by a fly from its natural bovine host to a horse. The theory is that some horses are genetically predisposed to incorporation of an oncogene into their skin cells that they acquire from the virus. This gene causes the rapid and unorganized proliferation of cells that results in the visible tumor. However, there are some sarcoids from which the DNA of BPV cannot be detected and there certainly must be many horses that are exposed to BPV that do not get sarcoids so there is very likely more to this association than simple cause and effect.

​Diagnosis

Despite their uncertain origins, sarcoids are usually not challenging to diagnose. Many can be diagnosed by visual examination alone. Biopsy and histopathology are the gold standard for diagnosis. Additionally, a PCR test is available for detection of BPV DNA in the tumor, which may be an aid in determining the treatment protocol.

​Treatment

There are numerous treatments for these tumors and, in some cases, some very strong opinions about what works the best. The medical literature at this point shows that just about all of the treatments work sometimes, some of the treatments (the better ones) work most of the time but none of the treatments work all of the time. Further confusing the matter of treatment is that reported success rates of the various treatments tend to vary widely from study to study (for example, reported resolution rates for intralesional cisplatin range from 33% to 96%). Regardless of the type of treatment, reported success rates never seem to get much above about 85%. This means that some sarcoids (the worst ones) are not so much cured as managed. The most aggressive types of sarcoids are typically more challenging to deal with and the longer a sarcoid has been present, in general, the more challenging it will be to completely resolve. Below are some of my preferred treatments for these lesions, which is by no means exhaustive of all the options available. Also, keep in mind that each case should be evaluated (and re-evaluated) individually.

  • Surgery + Cryotherapy—Surgical removal and freezing can be a reasonable undertaking if the location of the tumor allows for this option. Sometimes this is the initial treatment that is then followed by one of the other listed below. Some recent research indicates that surgical removal may be more successful when an electrosurgical unit is employed.
  • Intralesional Chemotherapy—Injecting chemotherapeutics directly into the tumor is one of the most effective methods of treatment provided the tumor has enough volume for an effective amount of drug to be injected. Cisplatin is a platinum based chemotherapeutic that is usually combined with oil in an emulsion when used for sarcoids. This is a very viscous formulation that can be difficult to inject through small gauge needles. Cisplatin impregnated beads are one solution to this problem. They can be placed once monthly with similar success rates to injectable cisplatin. Mitomycin C is another chemotherapeutic that has been injected into sarcoids with generally very good results.
  • Topicals—There are many topical options for sarcoids. My personal favorite at this time is acyclovir cream. Acyclovir is an anti-viral that causes no discernible inflammatory response so it is simple to apply and the horses do not resent treatment. It gets very little penetration through the skin so it takes anywhere from 2-8 months to see full resolution and is probably best suited for smaller and flatter lesions. Studies have shown it to be effective about 70% of the time. I think this is a reasonable expectation in the right kind of sarcoid (ie: small, flat and not rapidly growing). The main downside to this treatment in my mind is that it must be applied daily for the duration of treatment. This is a quick and simple matter but may require months of patient application by a dedicated owner. Aldara (imiquimod) and Xxterra (bloodroot extract) are two other topicals that have been used successfully. The main downside to either of these is the inflammatory response that is produced which, depending on the location of the tumor, may make treatment impossible or unsafe.
  • Autologous Implantation—This is an interesting option for some horses. Generally, several small sections of tumor are removed from the horse and placed in liquid nitrogen to freeze them rapidly and completely. The tissue is then thawed and implanted under the mane along the crest of the neck. When it works, the tumors regress and no further treatment is required. Outcomes have been variable and complete resolution has generally been found to be a little lower than some of the other methods above in most studies. With that in mind, I think this remains a good option for horses whose husbandry, behavior or owner’s circumstance precludes them from receiving daily therapy.

Since there is no clear answer on what exactly causes these tumors it is hard to make a recommendation on how to prevent them. My best suggestion would be to have a thorough fly control program (including fly spray, premise treatment and manure management), good wound care practices and early intervention for suspicious skin lesions. If your horse has a sarcoid and you’d like us to take a look or if you are concerned about some other area of your horses health, we would love to help! Give us a call to schedule an appointment and find out what we can do for you!

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