A sarcoid is the most common skin tumor found in horses. It’s a locally invasive growth caused by bovine papillomavirus (BPV) that affects the skin and the tissue just beneath it. Sarcoids don’t spread to internal organs, but they can be persistent, difficult to treat, and prone to coming back after removal.
What Causes Sarcoids
Sarcoids develop when bovine papillomavirus, specifically types 1 and 2, infects a horse’s skin cells. The virus doesn’t cause warts the way it does in cattle. Instead, it triggers abnormal cell growth that forms a tumor. The virus alters the skin cells’ behavior without producing new infectious virus particles, which is part of why the immune system has such a hard time clearing them.
Horses appear to pick up the virus through breaks in the skin. Stable flies (Stomoxys calcitrans) are a likely vector, carrying viral material from infected cattle or other horses and depositing it when they bite. This explains why sarcoids tend to show up in areas where the skin is thinner and more vulnerable to fly bites, like the groin, armpits, and face. Old wounds and scars are also common sites, since damaged skin gives the virus easier entry. Early experiments confirmed that simply introducing bovine papilloma extract into scratched horse skin could trigger sarcoid formation.
There’s also a genetic component. Certain horses seem more susceptible than others, and some breeds appear predisposed. A horse that develops one sarcoid is likely to develop more over time.
Where Sarcoids Typically Appear
Sarcoids can develop anywhere on the body, but they cluster in predictable areas: the groin and sheath region, the lower chest and belly, and the head (especially around the eyes and ears). These locations share two things in common. The skin tends to be thinner there, and flies feed in those areas frequently. Sarcoids also appear disproportionately at sites of previous wounds, surgical scars, or skin trauma, which is one reason vets urge caution about biopsying suspected sarcoids without a clear treatment plan. Cutting into a sarcoid can sometimes make it more aggressive.
What Sarcoids Look Like
Sarcoids vary dramatically in appearance, which is part of what makes them tricky to identify. They’re generally grouped into six clinical types, though a single horse can have multiple types at once, and one type can transform into another.
- Occult sarcoids are flat, roughly circular patches of hairless, slightly thickened skin. They’re easy to mistake for a ringworm infection or a rub mark. The skin surface often looks gray and scaly.
- Verrucous sarcoids resemble warts. They’re raised, rough, and crusty, with an irregular surface that can look like cauliflower. They grow slowly and are sometimes confused with skin plaques.
- Nodular sarcoids are firm, round lumps sitting under the skin. The skin over them may look completely normal, which can make them hard to distinguish from other types of lumps.
- Fibroblastic sarcoids are the most aggressive-looking type. They’re fleshy, often ulcerated masses that can grow rapidly. They bleed easily and attract flies, which can worsen the problem. These frequently develop at wound sites.
- Mixed sarcoids combine features of two or more types in a single growth.
- Malevolent sarcoids are rare but serious. They spread through the skin in cord-like strands, often following lymphatic channels. This type is the hardest to treat.
How Sarcoids Are Diagnosed
A vet can often recognize a sarcoid based on its appearance, location, and behavior, especially if the horse has multiple growths or a known history. However, sarcoids can mimic other skin conditions including squamous cell carcinoma, proud flesh (exuberant granulation tissue), fungal infections, and other tumors. When the diagnosis is uncertain, a biopsy confirms it, though vets weigh that decision carefully. Disturbing a sarcoid with a partial biopsy can provoke faster, more aggressive regrowth. In many cases, a vet will only biopsy when they’re prepared to treat or remove the entire growth at the same time.
Treatment Options
No single treatment works for every sarcoid. The best approach depends on the type, size, location, and number of sarcoids, plus how aggressively they’re behaving. Many horses need a combination of methods, and recurrence is always a concern regardless of the approach chosen.
Surgical Removal
Cutting out a sarcoid is straightforward in theory, but recurrence rates after surgery alone are high because tumor cells often extend beyond the visible margins of the growth. Surgery works best for small, well-defined nodular sarcoids in locations where the vet can remove a wide margin of healthy tissue around the tumor. Laser surgery can improve precision and reduce bleeding compared to a scalpel.
Topical Chemotherapy
A well-known option is the AW4-LUDES cream, commonly called “Liverpool cream” after its creator at Liverpool University. The active ingredient is 5-fluorouracil, a chemotherapy agent, combined with heavy metals, other cytotoxic chemicals, and natural plant oils. A typical course involves four applications over about a week: the first two treatments 24 hours apart, followed by a third 48 hours later, and a fourth another 48 hours after that. The cream causes the tumor tissue to die and slough off over several weeks. It works well for flat and warty sarcoids but requires careful handling because the chemicals are toxic to humans and need to be kept away from the horse’s eyes.
Injected Chemotherapy
For larger or deeper sarcoids, vets can inject chemotherapy drugs directly into the tumor. Cisplatin is the most studied option. The drug is mixed into an oil-based emulsion and injected in a cross-hatched pattern throughout the tumor to ensure even distribution, with sessions repeated every two weeks for a minimum of three to five treatments. In the largest controlled study of equine sarcoid treatments to date, cisplatin injections achieved a 96.3% resolution rate across 573 cases, with only a 3.7% recurrence rate over four years.
Another injectable option works similarly well, particularly for sarcoids near the eye where surgical options are limited. In one study of 59 sarcoid lesions, all nine periocular (near-eye) sarcoids resolved completely with an average of fewer than two injection sessions, and none recurred over the follow-up period of six to thirty months. Side effects were generally mild: some hair turning white at the injection site and occasional swelling as the dead tumor tissue broke down.
Immunotherapy
Because sarcoids involve a virus that evades the immune system, treatments that stimulate a stronger immune response can help. One approach uses BCG (the tuberculosis vaccine) injected directly into the tumor. It works by triggering the immune system to recognize and attack the tumor cells. This method shows particular promise for nodular and fibroblastic sarcoids, with one study reporting a 69% regression rate for those types. Periocular sarcoids, which are notoriously difficult to treat because of their delicate location, also respond well to BCG. Flat and warty sarcoids respond poorly to this approach.
Why Sarcoids Often Come Back
Recurrence is the central frustration of sarcoid management. The bovine papillomavirus persists in the surrounding skin even after the visible tumor is gone, and genetically susceptible horses remain susceptible for life. Incomplete treatment or an overly conservative surgical margin is the most common reason for regrowth, but even aggressive treatment can’t guarantee a permanent cure. Sarcoids that recur after treatment often come back more aggressively than the original, which is why vets typically recommend treating them decisively the first time rather than taking a wait-and-see approach.
Horses with sarcoids also tend to develop new ones over time, independent of recurrence at treated sites. Fly control, prompt wound care, and avoiding unnecessary skin trauma in susceptible horses can reduce the opportunity for new viral exposure, though none of these measures eliminate the risk entirely.