Fig. 1. The equine sarcoid is unique in respect to its etiology and its epidemiology. It also occurs in several distinct forms and individual animalsmayhave several ormanylesions and thesemaybe varied in appearance. A,Anulcerated TypeB1sarcoid. There is skin involvement but limited/no root extension beyond a pathologically recognizable fibrocellular capsule. B,Amixed sarcoid with an early broadly based (Type 2) fibroblastic sarcoid within a wide area of occult sarcoid that is developing verrucous features. C, The whole medial thigh is infiltrated with occult sarcoid. There are fibroblastic lesions distally and Type A and Type B nodules proximally overlying the saphenous vein.
linked has yet to be established and sarcoids also occur de novo where no cattle are present. The recent discovery of the BPV in flies around affected horses has been suggested to explain the develop-
ment of de novo sarcoids but it is unlikely that it is as simple as this! The equine sarcoid is a fibroblastic tumor and the clinical behavior of the transformed cells is typical of
Fig. 2. A and B, Flies appear to feed preferentially on sarcoid lesions if they can. All types of sarcoid seem attractive feeding sites. The role of surface feeding and biting flies is probably complex and currently not understood. Transmission is probably much more likely via surface feeding flies than those that feed on blood. Biting flies do not appear on sarcoid sites. Where multispecies of flies attack a site on the horse, sarcoid could be present or could develop.
