Sarcoid in Horses (Equis) | Vetlexicon


  • 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.

Presenting signs

  • The earliest forms appear simply as roughly circular alopecic skin lesions with a slightly scaly surface texture. Even in these however, small granules and/or thickened skin can usually be palpated.
  • Cutaneous verrucose, nodular or fibroblastic forms are the most frequent presentations recognized by both owners and practitioners.
  • Numbers of lesions vary from few to many hundreds or even thousands; single lesions can occur in a few cases and there is no typical presentation/type for these single lesions.
  • Lesions can be found on any area of the body but the distal limbs and upper body trunk and dorsum are probably less often affected. Lesions are commonly found on the head (especially periorbital, lips and muzzle), ventral body (especially paragenital region and axillae).
  • Six forms of sarcoid exist - occult, verrucose, fibroblastic, nodular, mixed and malignant.
  • Individual horses may have several different types of sarcoid lesion and even single anatomical locations may have several forms concurrently.

Geographic incidence

  • Worldwide.
  • Significant geographical variations occur in both prevalence and types of lesion.
  • Multiple lesions are more common in the UK than in Australia, USA and continental Europe.
  • Occult and verrucose lesions are uncommon in Africa, Australia and North America, but especially common in the UK.
  • There is an increasing prevalence of the condition worldwide.

Age predisposition

  • Although most cases are identified first around 2-10 years of age, de novo cases can develop at any age, at any time, in any horse that is genetically liable to the condition.

Breed/Species predisposition

  • Incidence related to specific genetic susceptibility associated with the major histocompatibility complex (MHC) antigen ELA-B1, also ELA-W3, W5 and W13. Genetic susceptibility relates to the presence of 'susceptibility' genes.
  • Appaloosa Appaloosa, Arabian Arab and Quarterhorses Quarterhorse have been shown in studies to be at a higher risk of developing sarcoid than Thoroughbreds Thoroughbred.
  • Standardbred Standardbred horses have a lower incidence of sarcoids than Thoroughbreds Thoroughbred.
  • It is reasonable to assume that all breeds can be affected.
The disease also affects zebra, donkeys, mules and onegars.

Cost considerations

  • Equine sarcoid is a cause of huge losses to the equine industry.
  • Treatment if implemented tends to be prolonged, may need to be repeated, and may be unsuccessful → major expense to the owner.
  • Lesions may interfere with ability of horse to work.
  • Performance animals may perform less well than expected.
  • The sale of a sarcoid-affected horse is highly problematic and failure to detect the lesions at prepurchase examination is a common cause of litigation and professional indemnity claims - these are usually successful.
  • Horses with sarcoids are undoubtedly worth much less than an equivalent horse without them.
  • Breeding from affected horses is ethically questionable and so even this side has a cost implication.
  • Furthermore, chemotherapy or surgical treatment during pregnancy inevitably carries hazards.

Special risks

  • High rate of recurrence and possibly even exacerbation of the lesions after any insult, eg surgical excision, biopsy Dermatology: biopsy or trauma.
Biopsy should not be performed if this is not likely to alter the treatment options selected, unless the diagnosis is really uncertain.In any event, when biopsy is performed the general assumption that it MAY be a sarcoid should support the availability of a prompt treatment if the diagnosis is confirmed.



  • 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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