The genetic material of the bovine papillomavirus (BPV) can be detected in virtually all equine sarcoids.Eight different types have been described, all inducing benign proliferation of epithelium in cattle. BPV-1 and -2are less strictly species-specific and can induce equine sarcoids in horses. Historically, association betweenBPV and equine sarcoids has been demonstrated using inoculation studies and detection of BPV DNA and BPVgene expression. The BPV genome is composed of 6 early and 2 late genes, with E5 and E6 being the mostimportant transforming genes. Specific BPV-1 variants associated with equine sarcoids have been reported,suggesting circulation of the virus between horses. In horses, a non-productive BPV infection occurs, withonly transcription of early genes, responsible for genome maintenance, regulation of cell growth and celltransformation. There is no formation of new infectious virus particles as is the case in the natural host.