The sarcoid affects all members of the equid family worldwide; this includes horses, mules, donkeys, zebra, Przewalski Horse and exotic asses. It has several implications, and is a form of cancer. Affecting between 2-8% of horses, it is a common disease. Sarcoids can occur almost anywhere on the skin, although there are some regions that are more liable to their development.

Sarcoids are likely to multiply on a horse, and may enlarge quickly or slowly; especially when the sarcoid is traumatised or otherwise interfered with! Some sarcoids can remain static/quiescent for many months/years. Trauma can trigger the development of the most aggressive forms of aggressive sarcoid, from even the earliest, mildest and smallest lesions. The tumours may ‘spread’ across the horse, meaning that more will develop over time. On any one horse there may be different types and ages of sarcoid due to the fact that sarcoid continues to spread over the life of the equine at an unpredictable rate.

Further research has been devoted to the investigation of the sarcoid, both microscopically and genetically. There are still large ‘unknown black holes’ in our knowledge.

Ways that sarcoids affect horses

  • They can become ulcerated and/or infected.
    • This may be debilitating and metabolically exhausting; and can become a serious welfare issue.
    • Some horses are so badly affected that they develop systemic effects; including low blood proteins, and anaemia. Such cases are sometimes destroyed on humane grounds.
  • They can cause severe functional problems when they develop at highly dangerous body sites.
    • These sites include the eyelids, over joints, and coronary bands.
    • Treatment at these sites is usually restricted, and more problematic.
    • Eyelid sarcoids can be very aggressive and difficult, with extensive roots penetrating the muscles that control eyelid movement.
  • Cosmetic difficulties.
  • Location of the sarcoid can make it difficult/impossible to wear tack/rugs.
  • Flies are attracted to sarcoids.
    • Particularly in the summer months. This can be highly distressing for the horse.
  • Some horses perform less well with sarcoids than without them.
    • Sarcoids may limit the working life of horses.
  • The price of a horse with sarcoids on the open market is usually less than the same horse without sarcoids
    • They may also be unsellable.
    • Sarcoid is a common cause of loss of commercial value, even in high performance horses.


Common areas

Sarcoids occur at all body sites, but there are some areas where the disease is more common:

  • At sites where flies attack and can feed without difficulty.
  • Where the skin is thin.
  • Where there is little/no hair.
  • Where there is a propensity for sweating.
  • At wound sites where the skin has been broken (the wound may not have been recent)
    • We know that sarcoid formation is a common cause for incipient wound healing failure. If a wound fails to heal it should be tested for the presence of sarcoid.
    • Wounding is a very important factor in the development of sarcoid at abnormal locations such as the neck, upper body, and distal parts of the legs.

Sarcoids never develop in internal organs and do not spread to these from cutaneous sites even when they are extremely aggressive or extensive; this means that affected horses are not usually ‘sick’. They are rare on the upper trunk, back, and neck – when they do occur it tends to be where there has been skin damage (injection sites, rub marks or injury sites).

Fly's hovering around an infected eye

Sarcoid should be viewed as a form of cancer

There are several features of sarcoid that are consistent with the clinical principles of a genuine neoplastic (cancerous) disease:

  • The behaviour of the tumours.
  • The difficulties encountered in treatment.
  • The tendency to recurrence.
    • There are very few cases which spontaneously resolve.
  • The spread across the body from site-to-site.
    • Some sarcoid lesions are small and have a benign clinical behaviour. They may remain static for many months or even years.

The sarcoid is a tumour of fibroblasts – the cells that provide support and structure to tissues. The skin and the tissues directly underneath seems to be the only place where this tumour can exist. It does not spread to the internal organs. Sarcoid can infiltrate between muscles and can sometimes be very extensive and aggressive locally. Pathologists recognise it immediately, even in very small samples!

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