A Suggested Clinical Classification for the Equine Sarcoid

The equine sarcoid, a fibroblastic tumor, is the most prevalent equine skin tumor and can affect all equidae. The sarcoid should be regarded as a tumor of fibroblasts. Involvement of the epidermis is a feature of most types but is not an obligatory feature. There are six visually recognizable types that correlate closely with the histology. The occult (superficial) type features alopecia, scaling, and skin thickening. The verrucose type has a prominent warty or verrucous appearance. There are two subtypes of the nodular form. Type A nodules are usually isolated individual or lobulated congregations of spherical subcutaneous masses. Type B nodules have cutaneous involvement that precludes independent movement of the overlying skin. Fibroblastic sarcoids have a fleshy fibrovascular appearance, often closely resembling granulation tissue. Type 1 fibroblastic sarcoids are pedunculated, whereas type 2 have a broad locally invasive base. The malignant sarcoid is an aggressive locally invasive form that extends widely into adjacent skin and subcutis. Mixed forms are common, and it is probably true that the large majority of sarcoid lesions have mixed components of two or more types.