Learn about diagnosing and treating this complex condition

Romeo loves nothing more than huddling with his donkey buddies and getting a good rump scratch from his humans. His serene life was threatened, though, when veterinarians found a fast-growing ulcerated mass on his chest in 2022.

Romeo’s sarcoid wouldn’t kill him—but it could certainly make life miserable for him and his friends, says Vicky Grove, BVSc, lead veterinary surgeon in Surgery at The Donkey Sanctuary in the U.K. As we’ll describe, sarcoids can grow exponentially, eventually interfering with movement, and can spread to other body parts and possibly other equids.

Grove promptly performed laser surgery on the 10-year-old gelding. Two years later he and his donkey pals are all sarcoid-free.

Romeo’s success story is the result of the growing body of scientific knowledge about sarcoids, which we describe in this update.

The Tenacious Topical Tumor

Sarcoids range from smooth bumps and rough, wartlike lesions to fleshy, red ulcerated protrusions, our sources say. Sometimes they move freely under the skin, sometimes not. Researchers have identified five types of equine sarcoids: occult (spots of unusual coat quality), verrucose (wartlike), nodular (under-the-skin nodes), fibroblastic (fast-growing, bleeding, ulcerating, or “cauliflowerlike”), and malevolent (malignant growths spreading to skin elsewhere). Most sarcoids involve a mix of types and even subtypes.

Each type has its own facets, affecting treatment options and recurrence risk. Italian researchers, for example, have determined fibroblastic and mixed sarcoids were more likely to recur after treatment, and verrucous ones tend to be associated with the appearance of other sarcoids elsewhere.

Sarcoids occur on equids of any age, but they’re more common in younger animals—perhaps due to immune responses to the causative virus, says Katie Offer, BVMS, MRCVS, a veterinary surgeon at the University of Glasgow, in Scotland. An estimated 2-10% of horses have sarcoids in their lifetime, and the growths represent up to 67% of tumors diagnosed in horses, including 46% of neoplastic (relating to new, often uncontrolled growth of abnormal tissue) equine cutaneous biopsy samples.

Tumors develop around the ears, eyes, and lips, on the penis or mammary glands, and sometimes on the lower legs or chest. John Munday, BVSc, DSc, PhD, Dipl. ACVP, of Massey University, in Palmerston North, New Zealand, recently discovered sarcoids also occur on horses’ gums.

Sarcoids can warrant euthanasia when tumors ulcerate and secondary infections develop, or when they interfere with vision, eating, breathing, urination, or normal movement, says Derek Knottenbelt, BVM&S, DVM&S, MRCVS, an equine oncologist at Equine Medical Solutions, in Stirling, Scotland. Sarcoids along the saddle or bridle areas can make horses unrideable and sometimes unsaleable.

Sarcoids and Bovine Papillomavirus

In 1935 South African veterinarian Cecil Jackson, BSc, BVSc, coined the term “equine sarcoids” to describe unsightly, sometimes bulbous skin growths on horses and donkeys that often led to a tumor erupting from a deeper layer of skin. The tumors grew incessantly but never spread to muscles or other organs.  

Jackson and his early contemporaries assumed sarcoids were caused by a papillomavirus, but it wasn’t until the 1950s that scientists understood it was bovine papillomavirus (BPV), which usually affects cattle.

In the early 2000s scientists found the DNA specifically of BPV types 1 and 2 in equine sarcoids (TheHorse.com/132632) and, in 2013, a Brazilian scientist also identified the DNA of BPV type 13 in South American horses. Five years later Christoph Koch, DrMedVet, Dipl. ACVS, ECVS, a senior clinician in the University of Bern Vetsuisse Faculty Swiss Institute for Equine Medicine, and his colleagues found identical BPV genomes in both cattle and equids—meaning they’re exchanging the same virus.

Most of the world’s hundreds of papillomaviruses—including the equine papillomavirus (EPV)—create benign warts on a single species. But there are five known delta papillomaviruses, which infect species outside their normal hosts. We now know that BPV-1, -2, and -13 are deltas. The other two are BPV-14 and a sheep papillomavirus, which cause masses in cats and pigs, respectively, Munday recently found.

Researchers as of late determined BPV type varies geographically. While there are differences by country, in general BPV-1 is most common in European sarcoids, while BPV-2 appears more often in North America.

Types might affect body regions differently, says Munday. BPV-2 causes lesions in cows’ muzzles, and that’s the type associated with the first known sarcoid in a horse’s mouth, which he discovered last year.

Flies, Friends, and Injuries—How Sarcoids ‘Spread’

Scientists have long known that BPV infects the skin’s middle layer, the dermis, which it accesses through wounds—even very small ones, our sources say. They also know flies can carry BPV and might transmit it between animals. That makes sense, Grove says, since common sarcoid locations—eyes, ears, nose, and between the upper hind limbs—are fly hotspots.

Even so, there’s no scientific evidence so far that sarcoids spread between equids, says Koch. The presence of multiple sarcoids in a herd or in the same animal could have other explanations—such as exposure to many sources of BPV. Importantly, scientists have yet to find any proof of infectious BPV production or replication in sarcoids.

In 2011 Austrian scientists found BPV can infect the outer skin layer, the epidermis, and even maintain silent infections in apparently healthy equine skin near sarcoids. Today scientists wonder whether that explains how sarcoids can jump at opportune moments—such as in the face of new wounds—to activate, even in the absence of flies.

Recently, Munday proposed equids might transmit BPV not only by touching or licking themselves or each other but also by touching or chewing BPV-laden surfaces. 

Grove seconds the idea. “We’ve found the virus on flies, on the hair, on the grooming kit,” she says. “So these are all ways … it can spread. And spreading is a real issue.” One of her patients, for example, had sarcoids on her udder and now has them on her ear.

Donkeys might be particularly likely to spread the disease to herdmates due to the closely bonded pairs they form, Grove adds. While there’s currently no scientific evidence to support this, she’s noticed repeatedly if one donkey gets sarcoids, the bonded partner usually gets them as well.

Diagnostic Difficulties

If you biopsy a lesion, that creates a wound, which could make sarcoids proliferate. But treating the sampled area without a confirmed diagnosis might invite BPV infection. Plus, you might also subject horses to “aggressive” treatments that simply aren’t necessary, Koch says.

Therein lies the dilemma: Sarcoid-looking lesions could be granulomas, eosinophilic lesions due to insect bites, or small foreign objects in the skin such as an ingrown hair, thorn, or melanomas, he says.

In a multicenter international study involving more than 100 veterinarians, equine practitioners misdiagnosed 18% of lesions suspected to be sarcoids—the percentage even higher among less experienced vets, Koch says. So in 2018 he created a free downloadable checklist indicating the likelihood of lesions being sarcoids on a scale of 0 to 15, and the importance—or not—of risking a biopsy. Overall, vets using the chart significantly improve their diagnoses, he says.

Surgery, Radiotherapy, and Creams

So far, scientists have published little on which treatments work, and in which situations, Offer says. Most veterinarians still treat according to their own experience, leading to a wide range of approaches.

With more than 40 therapies in use, sarcoid treatment options could be their own article. Today’s top treatments include surgical removal, chemotherapy injections and creams, and immunomodulating injections and creams. Practitioners say a more recent treatment is a radiation approach known as interstitial brachytherapy.

Koch and Grove usually perform laser surgery on sarcoids, resecting a 1.5-centimeter margin around the growths to catch migrating tumor cells, and often add a chemotherapeutic cream. Immunomodulating injections (such as Bacillus Calmette-Guerin, or BCG, made from a strain of Mycobacterium bovis—also used to create the tuberculosis vaccine) are good approaches for eye tumors, and creams are more convenient for the belly where the skin isn’t as deep. Grove says she has also recently found success giving a mitomycin B polymer gel injection into a lip tumor. Regardless of the treatment, “we monitor that site forever,” she says.

Grove says early treatment tends to be more successful. Each successive recurrence becomes more difficult to manage. “We don’t treat beyond three episodes, as the recurrence rate increases each time,” she says. “It’s not fair to put the animal through that.”

Ideally, future research will provide clearer evidence of treatment efficacy for different sarcoid types and locations, Offer says.

How Often Do Sarcoids Return?

Recurrence rates range from 10% to 80%, depending on the scientific report, Munday says. Trying to rectify such a discrepancy, his team recently followed 49 sarcoid cases after both complete and incomplete surgical removal, sometimes accompanied by other treatments. They found an overall recurrence rate of 24%—although the fibroblastic type recurrence rate was 75%.

Surprisingly, incomplete removal didn’t affect recurrence, Munday says. But mitotic count (MC)—the number of cells dividing and growing at a single moment evident on biopsy—did. He explains that MCs of at least 20 (per 2.37 mm^₂) led to an 80% recurrent rate, compared to an 18% recurrent rate for MCs under 20.

Koch’s team determined that when dealing with mild disease in adolescent horses, “it’s nearly impossible to predict which ones will progress and get possibly much worse, which ones will stay stable for very long periods, and which tumors will regress without any treatment” (see below).

In general, however, experts agree sarcoids that haven’t recurred within six months are unlikely to do so, Grove says.

Do Sarcoids Ever Just Go Away?

Despite popular belief, many sarcoids can, in fact, resolve on their own. Koch’s team recently discovered that 48% of untreated sarcoids in young horses disappeared within five years.

Critically, however, Koch’s cases were observed by experts in the field—not by owners. In other words, many sarcoids might develop and resolve without anyone really paying much attention to them, he says.

In horses that have multiple and aggressive lesions, however, “it is very unlikely that these regress spontaneously,” he adds.

Cases that spontaneously resolve might represent a “different syndrome altogether,” Knottenbelt says. But meanwhile, it’s risky to wait for sarcoids to resolve, given their tendency to spread.

Grove agrees. “You think it’s an innocuous lump,” she says. “But there are a lot of welfare implications to the principle of monitoring sarcoids.”

For Koch, the point isn’t to delay treatment but, rather, understand the immune system plays an important role. “Horse owners need to be well-informed about the situation and all possible scenarios to make their decision and then consistently pursue a management approach with full conviction,” he says.

“Treatments can be expensive, fairly aggressive—leaving big wounds that take months to heal—and require (regular) monitoring and follow-up,” he adds. This isn’t something all owners and veterinarians are prepared to handle, and continuously villainizing all sarcoids as being “cancer and terrible … like a mantra” is neither professional nor constructive, he explains.

“This will make owners and veterinarians who have had other experiences lose faith in what ‘experts’ claim,” Koch says. “As a result, we lose our greatest asset—namely the trust of the owners, who must pursue the same goal together with us and do their best for their animals.

“We need to realize that it can go either way, and that owners need to be vigilant if they decide to wait and see,” he explains. “I’m against benign neglect, but I’m also against hysteria when it comes to this disease because that does just as much harm.”

Take-Home Message

Sarcoids in horses are an unusual type of skin tumor appearing in various forms that arise from a cattle virus. Scientists are still learning about the disease processes at play. Although some tumors resolve spontaneously, their spread to other body regions and potentially other animals merits rapid treatment. Ongoing research should help clarify disease mechanisms and identify the best treatments—including chemotherapeutic agents, radiotherapy, and immune therapies—to improve equine health and welfare. Horse owners need to follow their veterinarians’ instructions and be proactive about managing sarcoid cases.