By Garrett Metcalf, DVM
Sarcoids are the most common cutaneous skin tumor of horses that come in various forms with varying degrees of virulence. Sarcoids are small gray to cherry red tissue that appears similar to granulation tissue or proud flesh that does not seem painful and do not spontaneously regress. These masses range from a very flat hairless area to very large raised aggressive forms. There are actually six forms or classifications of sarcoids in horses based on the appearance and behavior the mass exhibits. Sarcoids can appear around the face, eyes, ears, lower limbs, neck, groin and areas of previous injury or wounding.
How Do Sarcoids Form?
Sarcoids are commonly found more often in younger horses and Quarter Horses or stock breeds. It was been found that Quarter Horses are twice as likely to develop sarcoids compared to thoroughbreds. Research has linked genetic causes for the development of sarcoids in certain breeds and the lack of a particular gene makes other breeds less susceptible to sarcoidosis. It has also been discovered that close housing of a group of donkeys resulted in sarcoids being transmitted to donkeys without sarcoids. A fly vector of transmission was believed to lead to the spread of the sarcoids.
Viral etiologies have been known for some time to cause warts and plagues on horses. A common disease among young horses is called papillomatosis, or grass warts, that form around the muzzle and face caused by Equine Papillomavirus (EPV). The warts are commonly self-limiting with the maturation of the horse and their own immune system to eliminate the disease. This information led to further research that discovered that almost all sarcoid masses contained the DNA of a virus called Bovine Papillomavirus (BPV). Horses do not develop diseases from bovine types of papillomaviruses, but rather the viral infection is nonproductive leading to the virus to remain latent in the skin of horses without causing disease. It is hypothesized that trauma to the skin or a wound can cause a trigger event leading to the formation of sarcoid masses with the help of this latent BPV. Papillomaviruses are known to cause oncogenic (tumor formation) transformation of normal tissue into cancer cells in many species including humans, so it is logical to assume or to investigate if this also occurs in horses.
To make a definitive diagnosis of sarcoidosis in a horse is to take a biopsy of the mass and perform histopathology at a lab. Many times the mass can be correctly identified based on the appearance, location, response to treatments and age of the horse. Depending on the location of the mass, it may be possible to fully remove the mass and submit it to the lab for evaluation. This is important because excision alone has a high recurrence rate in several journal reports. Masses located around the ears or eyes are sites where quick and early diagnoses are needed to begin treatment as early as possible. There are some other masses or growths that can mimic similar lesions as sarcoid, further making it important to perform biopsies.
There is a plethora of treatment plans, protocols and methods to treat sarcoids in horses with a wide variation of success, but no universal treatment plan has overwhelming successful. Many times multiple methods need to be implemented to treat the same sarcoid to reach a successful outcome. Unfortunately, these treatments need to be performed multiple times, leading to frustration by owners and low compliance to stick to the treatment plan. Although most of these techniques or treatment protocols have been validated through scientific research, there are some methods that are not, so it is hard to evaluate the success of all of the available methods. The options include surgical excision, laser ablation, cryotherapy, hyperthermia, radiotherapy, immunotherapy, intralesional chemotherapy, topical chemotherapy and brachytherapy. Other therapies that are used are herbal topical products, other chemical infusions such as formaldehyde, autologous vaccines or implantations. Veterinarians often must choose a treatment method that is 1) tailored to the type of sarcoid 2) the locations of the sarcoid 3) the availability of equipment or products and 4) their comfort level to use these various methods. Some of the techniques or equipment are not readily available and come with varying cost differences per treatment.
Read more in the September 2019 issue of Oklahoma Farm & Ranch.