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  • The most common cutaneous tumor, and probably the most common tumor overall of the horse.
  • 'Locally aggressive, fibroblastic tumor of equine skin with a variable epithelial component and a high tendency to recurrence' (Jackson, 1936). 
  • Significant cause of loss of commercial value.
  • Probably the most common cutaneous reason for euthanasia, although the condition is seldom fatal in its own right. 
  • Cause: unknown although there is strong evidence for the involvement of bovine papilloma virus (BPV-1/BPV-2). There is certainly a genetic component to the disease, with some horses having a known genetic predisposition and a known heritability index for some breeds. 
  • Signs: clinical signs vary from faint circular areas of alopecia and hair coat color and density changes to small or larger nodules, to large ulcerated fibromatous and invasive tumors. The rate of progression is very variable - some remain static for years others exacerbate rapidly. Inappropriate interference is a common cause of progression towards the more aggressive forms. 
  • Diagnosis: because biopsy can → exacerbation, diagnosis on clinical grounds is probably reasonable; confirmation by biopsy may be justifiable in difficult locations and where there is a doubt about the diagnosis; if biopsy is performed a definite plan for prompt therapy must be available. Where a horse has several different lesions (a diagnosis that can be classified as typical sarcoid), a tentative diagnosis can be made.
  • Treatment: therapeutic options are severely limited. If treatment is clinically difficult or may have significant welfare implications and where treatment carries a poor prognosis, some cases may be best left alone. A small proportion of sarcoids (<1%) will resolve spontaneously. Different treatments have different success rates for the various forms of the disease and many veterinarians have their own preferred treatments. 
  • Prognosis: most individual lesions become larger, more extensive and more aggressive if left alone (and particularly if traumatized at all) and most cases will develop increasing numbers of lesions with time. A few lesions remain static and a few horses remain with low numbers. In all cases the prognosis should be very guarded.
Print off the Owner factsheets on Cancer in horses, Lumps and bumpsSarcoids - what you need to know, and Common skin problems in the horse to give to your clients. Clinical tip
Question: What treatment options should be considered for sarcoids?
Answer: Radiation therapy, teletherapy, topical treatments, surgery, laser surgery, cryosurgery, immune-mediated therapy, intralesional therapy.



  • Epidemiology and behavior of sarcoids suggest involvement of an infectious agent.
  • Papilloma virus (BPV-1/BPV-2) Papilloma virus is strongly associated with the development of sarcoids, and BPV DNA has been isolated from up to 100% of sarcoids sampled. In addition, the amount of BPV DNA reflects the severity of the sarcoid, and viral genes associated with neoplastic transformation are consistently found in equine sarcoid lesions. The virus is believed to be introduced into the horse via fly transmission. However, the exact role of BPV in the etiology and pathogenesis of sarcoids remains unclear. There is certainly a genetic predisposition towards the disease and it is likely that both host and viral factors combine to cause the development of sarcoid lesions.
  • Lesions resembling sarcoid have been produced experimentally by scarification of equine skin with homogenates containing bovine papillomavirus. However, these lesions tended to resolve spontaneously with a host mounted immune response - neither of which appears to occur in natural sarcoid.
  • It is possible that sarcoid represents a transmissible cell that can be transmitted against the protection of the MHC of the recipient animal.
  • Recent research suggests that macrophages may be involved in transfer of viral particles to remote sites via the bloodstream.

Predisposing factors


  • Sarcoids occur more commonly at sites where the skin is thin, where sweating occurs, where there is no/little hair cover and where flies feed without hindrance.
  • It has been suggested that sarcoid lesions tend to occur on areas of skin prone to traumatic insult.
  • Wounds are a common site for sarcoid transformation - wound healing is delayed/inhibited and complexes with granulation tissue and sarcoid admixtures are common. Sarcoid should be considered whenever there is incipient and unexplained wound healing failure - there is little clinical difference between granulation tissue (proud flesh) and sarcoid.


  • Locally invasive, non-metastasizing, fibroblastic tumors of the skin with a spectrum of clinical presentations.
  • Capacity for infiltrative expansion in the dermis and subcutis.
  • Metastatic spread does not occur, but some reports of multiple small lesions occurring after incomplete surgical removal of one sarcoid or after autogenous vaccine usage.
  • Malevolent form shares some characteristics with aggressive, locally invasive neoplasms.
  • Fibroblastic types have long sinuous pegs of tumor tissue extending beneath the intact epidermis.
  • High tendency for sarcoids to recur.
  • Six distinguishable types (see under clinical signs).


  • Lesions tend to persist; some static, some worsen over time.
  • Spontaneous resolution is very rare (<1%).
  • May recur years after apparently successful removal.
  • Occasional tumors exacerbate rapidly, especially after traumatic damage, including biopsy. This may be because incomplete excision activated latent BPV which stimulates sarcoid growth.


  • Flies have been suggested to be involved in the transmission of an etiologic agent.
  • This corresponds with the concept that sarcoids multiply on an individual horse over the summer and grow over the winter to become visible in the following spring and summer.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Hollis A R & Berlato D (2017) Initial Experience with High Dose Rate Brachytherapy of periorbital sarcoids in the horse. Equine Vet Educ WileyOnline.
  • Compston P C, Turner T, Wylie C E & Payne R J (2016) Laser surgery as a treatment for histologically confirmed sarcoids in the horse. Equine Vet J 48 (4), 451-456 PubMed.
  • Taylor S & Haldorson G (2013) A review of equine sarcoid. Equine Vet Educ 25 (4), 210-216 WileyOnline. 
  • Stadler S et al (2011) Successful treatment of equine sarcoids by topical aciclovir application. Vet Rec 168 (7), 187 PubMed.
  • Barrelet A, Foote A & Littlewood J D (2010) Common equine skin tumours. UK Vet 15 (6), 9-17.
  • Haralambus R et al (2010) Intralesional bovine papillomavirus DNA loads reflect severity of equine sarcoid disease. Equine Vet J 42 (4), 327-331 PubMed.
  • Brandt S, Haralambus R, Schoster A, Kirnbauer R & Stanek C (2008) Peripheral blood mononuclear cells represent a reservoir of bovine papillomavirus DNA in sarcoid-affected equines. J Gen Virol 89 (6), 1390-1395 PubMed.
  • Brandt S et al (2008) A subset of equine sarcoids harbours BPV-1 DNA in a complex with L1 major capsid protein. Virology 375 (2), 433-441 PubMed
  • Pilsworth R C & Knottenbelt D C (2007) Equine sarcoid. Equine Vet Educ 19 (5), 260-262.
  • Gobeil P, Gaoult E A, Campo M S et al (2007) Equine sarcoids are not induced by an infectious cell line. Equine Vet J 39 (2), 189-191 PubMed.
  • Hewes C A & Sullins K (2006) Use of cisplatin-containing biodegradable beads for treatment of cutaneous neoplasia in equidae: 59 cases (2000-2004). JAVMA 229 (10), 1617-1622 PubMed.
  • Knottenbelt D C (2006) Clinical forum - the etiology, diagnosis and treatment of the equine sarcoid. UK Vet 11 (3), 8-17.
  • Hikita M, Ishikawa Y, Shibahara T & Kadota K (2004) Mesenteric myofibroblastoma in a horse. Vet Rec 154 (25), 795-796 PubMed.
  • Knottenbelt D C & Kelly D F (2000) The diagnosis and treatment of periorbital sarcoid in the horse: 445 cases from 1974-1999. Equine Ophthal 3 (2-3), 169-191 PubMed.
  • Knottenbelt D C, Edwards S & Daniel E A (1995) The diagnosis and treatment of the equine sarcoid. In Pract Suppl Vet Rec 123-129 (an excellent account of equine sarcoid, its diagnosis and treatment).
  • Knottenbelt D C & Walker J A (1994) Topical treatment of the equine sarcoid. Equine Vet Educ 6 (2), 72-75.
  • Theon A & Pascoe J R (1994) Iridium 192 interstitial brachytherapy for equine periocular tumours: treatment results and prognostic factors in 115 horses. Equine Vet J 27, 117-119 PubMed.
  • Torrontegui B O & Reid S W J (1994) Clinical and pathologic epidemiology of the equine sarcoid in a referral population. Equine Vet Educ 6 (2), 85-88.
  • Hardy J (1993) Factors associated with the risk of developing sarcoid tumors in horses. Equine Vet J 25 (2), 169 PubMed
  • Mohammed H O, Rebhun W C & Antczak D F (1992) Factors associated with the risk of developing sarcoid tumors in horses. Equine Vet J 24 (3), 165-168 PubMed.
  • Pascoe R R (1991) Equine nodular and erosive skin conditions - the common and the not so common. Equine Vet Educ 3 (3), 153-159.
  • Trenfield K Spradbrow P B & Vanselow B (1985) Sequences of papillomavirus DNA in equine sarcoids. Equine Vet J 17 (6) ,449-452 PubMed.
  • Lavach J D, Wheeler C & Lueker D C (1985) BCG treatment of periocular sarcoid. Equine Vet J 17 (6), 445-448 PubMed.
  • Gorman N T (1985) Equine sarcoid - time for optimism. Equine Vet J 17 (6), 412-414 PubMed.
  • Turrel J M & Stover S M (1985) Iridium 192 interstitial brachytherapy of equine sarcoid. Vet Radiol 26, 20-24.
  • Wyn-Jones G (1983) Treatment of equine cutaneous neoplasia by radiotherapy using iridium 192 linear sources. Equine Vet J 15, 361-366 PubMed.
  • Wyn-Jones G (1979) Treatment of periocular tumours of horses using radioactive gold 198 grains. Equine Vet J 11, 3-10.
  • Lane J G (1977) The treatment of equine sarcoids by cryosurgery. Equine Vet J 9 (3), 127-133 PubMed.

Other sources of information

  • Knottenbelt D C & Pascoe (2009) Pascoe's Principles and Practice of Equine Dermatology. W B Saunders, UK.
  • Tamzali Y, Teissie J & Rols M P (2003) First Horse Sarcoid Treatment by Electrochemotherapy: Preliminary Experimental Results. In: Proc 49th AAEP Convention. pp 381-384.
  • Pascoe R R & Knottenbelt D C (1999) Manual of Equine Dermatology. W B Saunders, UK.
  • Theon A P (1997) Cisplatin Treatment for Cutaneous Tumours. In: Current Therapy in Equine Medicine. 4th edn. Ed: Robinson N E. W B Saunders, UK. pp 372-377.
  • Knottenbelt D, Edwards S & Daniel E A (1995) The Diagnosis and Treatment of the Equine Sarcoid. In Pract. Suppl Vet Rec. pp 123-129 (An excellent account of equine sarcoid, its diagnosis and treatment).

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