Sarcoids, the most common tumor of the horse, are fibroblastic, wart-like s
kin lesions that show variable manifestations. They are often invasive and
recurrent, although they do not fulfill all criteria of malignancy. Due to
their anatomic location, these tumors can sometimes cause loss of use of th
e horse. There is very strong evidence that sarcoids are caused by viruses
closely related or identical to bovine papilloma viruses, and genetic studi
es have shown associations between genes in or near the equine major histoc
ompatibility complex (MHC) and susceptibility to sarcoid. Several types of
treatments have been successful in treating sarcoids, although the response
to therapy is not consistent. Current treatment of sarcoids primarily invo
lves antitumor therapy, but the development of preventative measures in the
future may be directed against the causitive papilloma virus. Sarcoid cont
inues to be an important clinical entity for the equine practitioner.