Equine Sarcoids

Sarcoids can be difficult to diagnose and treat in horses, and probably are transmitted from horse-to-horse.
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Sarcoids seem to be contagious  from horse to horse, but researchers don't understand how they are transmitted. And not all horses in a herd will get sarcoids even if several horses have them.

These growths are sometimes mistaken for warts and vice versa. “Sarcoid can look like many different things. In the beginning, some can look like ringworm or skin nodules or warts," said Stephen D. White, DVM, DACVD, Professor and Chief of Service, Dermatology, School of Veterinary Medicine University, University of Californi, Davis. "Sometimes they look terrible—large and bloody. The important thing to understand is that sarcoid growths are a cancer. It is uncommon for them to metastasize and spread to other locations in the body, but horses may develop sarcoid tumors in various areas at the same time.

“These growths are somewhat contagious from horse to horse, but we don’t understand how they are transmitted," continued White. "It’s not unusual to see several horses in a herd develop sarcoids, yet some other horses do not. Sometimes you might see just one horse that has it out of a large group. We don’t know how they are transferred from one horse to another—whether it is by close contact or spread by insects, or some other means. There are many theories, but we really don’t know."

White said that for a horse owner to determine if a growth is actually a sarcoid, veterinarians can take a biopsy. "But probably about a quarter to a third of the cases that are biopsied in early stages become more aggressive," said White. "It doesn’t happen all the time with a biopsy, and with the growths that are already very aggressive it doesn’t matter, but owners need to realize that taking a biopsy may stimulate the sarcoid to grow even more. Yet in order to definitively diagnose a growth, we need a biopsy. 

"There are other things that look like sarcoid that turn out to be ringworm or bacterial infections, and vice versa,” added White.

“Once a diagnosis of sarcoid is confirmed, then the owner and veterinarian must decide on a treatment," White said. "If it’s an old horse in the beginning stages of sarcoid, it may progress slowly and treatment may or may not be advised. In most cases, however, some kind of treatment is tried.” 

White said the only things that have been clinically shown to work and be statistically successful is wide surgical excision with the horse under anesthesia so the surgeon isn’t rushed. White said it is important to get rid of the entire growth.

He also said cisplatin is used, which is a chemotherapeutic drug. "It should only be administered by a veterinarian who has experience with it," White said.

“There is some evidence to show that topical ointments such as imiquimod (anti-tumor medication) may help in some cases," added White. "This is a local immune stimulator.” 

Another product is the so-called Indian bloodroot (Sanguinaria canadensis). “This has also been shown to help, in some cases," said White. "Studies are lacking, however, to show that every mass that was helped by treatment of these two products had been confirmed on biopsy to be sarcoid. While topical treatments are attractive options, we need to remember that these tumors are in the dermis—the deepest layers of the skin—and not as easily reached by products applied to the epidermis.”

White said there is no best way to treat sarcoids, although there is a lot of information in the literature. "The two veterinarians that I feel know the most about sarcoid treatment are Alain Theon, DVM, PhD, DACVR, who is at the University of California, Davis, and Derek Knottenbelt, MRCVS, OBE, BVM&S, DECEIM, DACVIM, who is in the United Kingdom," said White. "For anyone who wants to know more about sarcoid, these would be the two people to ask."

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