Bluetongue virus core proteins, molecular model. Bluetongue virus (BTV) is an Orbivirus that causes a potentially fatal disease in sheep, cattle and other ruminants. It does not affect humans. The disease is transmitted between infected animals by biting midges. The core is the central protein case that contains the genetic material. The secondary structure of the protein is shown. Alpha helices are shown as spirals, and beta sheets are shown as parallel arrows. This structure was determined by X-ray diffraction studies.