Orthoreovirus is a genus of viruses, in the family Reoviridae, in the subfamily Spinareovirinae. Vertebrates serve as natural hosts. There are ten species in this genus. Diseases associated with this genus include mild upper respiratory tract disease, gastroenteritis, and biliary atresia. Mammalian orthoreovirus 3 (strain dearing-T3D) induces cell death preferentially in transformed cells and therefore displays inherent oncolytic properties. History The name "orthoreovirus" comes from the Greek word ortho, meaning "straight" and the reovirus, which comes from taking the letters: R, E, and O from "respiratory enteric orphan virus". The Orthoreovirus was named an orphan virus because it was not known to be associated with any known disease. It was discovered in the early 1950s when it was isolated from the respiratory as well as gastrointestinal tracts of both sick and healthy individuals Classification Orthoreovirus is part of the family Reoviridae. Its genome is composed of segmented double-stranded RNA (dsRNA), thus it is classified as a group III virus according to the Baltimore classification system of viruses. This family of viruses is taxonomically classified into 15 distinct genera. These genera are sorted out taking into account the number of dsRNA genomes. The Orthoreovirus genus has 10 segments that have been isolated from a large range of hosts including mammals, birds, and reptiles. These genera are further divided into two phenotypic groups: fusogenic and non-fusogenic. The way that they are determined to belong to a specific group is if the virus is able to cause multinucleated cells known as syncytial cells. According to this classification, mammalian orthoreoviruses (MRV) are known to be non-fusogenic, meaning it does not produce syncytia, while other members of this genus are fusogenic, such as avian orthoreoviruses (ARV), baboon orthoreoviruses (BRV), reptilian orhtoreoviruses (RRV). Taxonomy The following ten species are assigned to the genus: Avian orthoreovirus Baboon orthoreovirus Broome orthoreovirus Mahlapitsi orthoreovirus Mammalian orthoreovirus Nelson Bay orthoreovirus Neoavian orthoreovirus Piscine orthoreovirus Reptilian orthoreovirus Testudine orthoreovirus Structure Mammalian orthoreovirus virions are non-enveloped with icosahedral symmetry created by a double-layered capsid reaching about 80 nm wide. Each capsid contains 10 segments of double stranded RNA (dsRNA) genome. The inner capsid or core particle (T=2) contains five different proteins: σ2, λ1, λ2, λ3, and μ2 and is approximately 70 nm in diameter. One hundred and twenty copies of the λ1 protein arranged in 12 decameric units make up the shell of the inner capsid structure. This shell is stabilized by