Spontaneous regression of equine sarcoids is an exceptional event

Sarcoids are benign, yet locally aggressive skin tumours that commonly affect horses and other equid species. The lesions are induced by bovine papillomavirus types 1, 2, and probably 13 in conjunction with other factors including trauma and a genetic predisposition. Although sarcoids have a substantial impact on the health and welfare of affected equids, information on the immune response to bovine papillomavirus infection and resulting sarcoids is limited. However, there is evidence that sarcoid disease is associated with an impaired immune response to bovine papillomavirus infection. This observation agrees with the common perception of sarcoids as persistent lesions. Contrary to this general view, there are reports on the spontaneous regression of skin tumours diagnosed as sarcoids, giving rise to the question whether sarcoids can self-resolve, and if so, under which circumstances. This review addresses this unresolved issue.