Sarcoids.

Sarcoids are the most common skin tumors seen in horses worldwide. The pathogenesis of sarcoids is multifactorial, including an association with bovine papillomavirus types 1 and 2 and a genetic susceptibility to tumor development. Clinical manifestations vary and include occult, verrucous, nodular, fibroblastic, mixed, and malignant (malevolent) types. The tumor is nonmetastasizing but can become very aggressive locally. Multiple tumors are common. All clinical types can be present in the same horse. No treatment protocol is universally effective. The tumor has a high risk of recurrence. Recurrent and large tumors are associated with poorer prognoses.