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