Equine Sarcoid

Sarcoid is a tumor of the skin of horses, donkeys and mules, presumably caused by bovine papilloma virus infection, characterized by proliferation of neoplastic fibroblasts and thickening and/or ulceration of the skin.  It is the most frequently diagnosed tumor in horses. Surveys have estimated the prevalence of sarcoid at 20% of all equine neoplasms and 36% of all skin tumors.

Sarcoids occur at any age but are more common in young adult horses. These tumors may be single or multiple and, although they occur most frequently on the head, limbs and abdomen they can occur anywhere on the body including sites of trauma and healed wounds. Even though all breeds are affected, a genetic predisposition to sarcoid development may exist.  Quarter horses, Appaloosas and Arabians may be at greater risk and Standardbreds at lower risk to develop sarcoids.  There is no gender, coat color, seasonal or geographic predilection for the occurrence of sarcoids.

The appearance of sarcoids varies from small, sessile or pedunculated growths that have a warty appearance to firm, freely moveable, nodular dermal masses. Sarcoid diagnosis is based on the characteristic microscopic arrangement of fibroblasts and collagen fibers and their orientation to the overlying epidermis. There are many approved methods for treating sarcoids, some of which include surgical excision, cryotherapy, immunotherapy, radiotherapy, laser therapy, hyperthermia, topical chemotherapy and intratumoral chemotherapy. Treatments may have to be repeated. Sarcoids are locally invasive and frequently recur after surgical excision, but do not metastasize.

During the period between January 1993 and December 1998, 465 cases of sarcoids were diagnosed at the University of Kentucky Livestock Disease Diagnostic Center. Breed information was supplied in 93% of the cases. Twenty different horse breeds, ponies, mules, donkeys and crossbred horses were represented.  Twenty-four percent of the tumors occurred in Thoroughbreds.  Some of the other breeds affected, in decreasing order of frequency, were Quarter horses (22%), Tennessee Walking Horses (8%), American Saddlebred Horses (7%), Arabians (6%) and mixed-bred horses (6%). It appears that Quarter horses and Arabians which represented 5% and 2% of the accessions during the reporting period, respectively, are at greater risk to develop sarcoids than are Thoroughbreds which accounted for 66% of equine accessions.

Horses in the Central Kentucky area with sarcoid ranged in age from 122 days to 26 years. Forty-four percent of the sarcoids occurred in animals between three and six years of age and 85% in animals ten years of age or less (Figure 3).  Four cases occurred in animals less than one year of age. There was no gender or seasonal predilection for the occurrence of sarcoid.


The anatomical distribution of sarcoids in this survey was similar to that reported in other surveys. Lesions were most frequent on the head and ears (51%), followed by the limbs and shoulders (25%), and the neck and trunk including the male and female genitalia (24%) (Figure 4). Of sarcoids on the head, the eyelids and ears were the most frequently affected sites.

Dr. David C. Bolin, (606) 253-0571,
Livestock Disease Diagnostic Center