Orf Virus (ORFV or OV) belongs to the Poxviridae family, genus Parapoxvirus. Orf Virus injection causes the orf disease, a zoonotic disease that occurs primarily in sheep and goats, and sometimes also in humans and other animals such as dog, cat, musk ox, reindeer, the red squirrel, chamois, etc. The infection of orf virus usually occurs when the broken skins get in touch with virus from the infected animals or the surface of equipments contaminated with orf virus, the injection is often confined to epidermis of the skin and commonly without systemic symptoms. Infection of orf virus causes a purulent-appearing papule locally in humans, which can be located in fingers, hands, arms and faces. The papule may persist for 7 to 10 weeks and then spontaneously resolves, which may be difficult to diagnose. All the human infections are animal to human injections, and no human to human infection cases are reported.
Fig. 1 An orf virus particle under negative-stained transmission electron micrograph.
The orf virus is a DNA virus with a genome size of 134-139kbp. Like other poxvirus, the orf virus genome consists of a large central coding region bonded by two identical inverted terminal repeat (ITR) regions. But the orf virus genome contains 15-16 open reading fragments (ORFs), which is quite different from other poxvirus or cellular proteins. Orf virus genome contains about 130 putative genes. 88 of these genes are conserved among all the characterized chordopoxviruses. Orf virion consists of the viral genome and a protein capsid, it is a large particle with a size of >200 nm. The structural proteins of orf virion were organized into a tubular thread structure. The tubular thread structure formed a protein wall to construct the capsid in a manner of left-hand spiral coil of a single thread. Detailed structure of orf virion still remains unclear, but it was believed to be similar with vaccinia virus (VACV). Genome sequencing indicates that the structural genes of orf virus have homologues of almost all of structural genes in vaccinia virus.
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