Avian orthoreovirus, also known as avian reovirus, is an orthoreovirus from the Reoviridae family. Infection causes arthritis and tenosynovitis in poultry. It can also cause respiratory disease. Avian orthoreovirus infection is more common in young birds, because resistance begins to develop from as young as two weeks of age. It is also reportedly more common in broilers. Distribution of avian orthoreovirus is worldwide and it is present in most poultry flocks. It can be transmitted horizontally via the faeces or rarely, vertically. It is not a zoonosis. The most common symptom is lameness. There may also be swelling or bleeding around the joints. Gastrointestinal, respiratory and neurological signs have also been reported. Presumptive diagnoses may be made based on the observation of clinical signs. They can be confirmed using virus isolation, complement fixation, ELISA, immunodiffusion or histopathology (following postmortem exam). Description Avian reovirus belongs to the genus Orthoreovirus, and Reoviridae family. They are non-enveloped viruses that undergo replication in the cytoplasm of infected cells. It has icosahedral symmetry and contains a double-shelled arrangement of surface protein. Virus particles can range between 70 and 80 nm. Morphologically, the virus is a double stranded RNA virus that is composed of ten segments. The genome and proteins that are encoded by the genome can be separated into three different sizes ranging from small, medium, or large. Of the eleven proteins that are encoded for by the genome, two are nonstructural, while the remaining nine are structural. Avian reoviruses can withstand a pH range of 3.0–9.0. Ambient temperatures are suitable for the survival of these viruses, which become inactive at 56 °C in less than an hour. Common areas where this virus can survive include galvanized metal, glass, rubber, feathers, and wood shavings. Avian reovirus can survive for up to ten days on these common areas in addition to up to ten weeks in water. Cultivation and observation of the effects of avian reovirus is most often performed in chicken embryos. If infected into the yolk sac, the embryo will succumb to death accompanied by hemorrhaging of the embryos and cause the foci on the liver to appear yellowish-green. There are several primary chicken cell cultures/areas that are susceptible to avian reoviruses, which include the lungs, liver, kidney, and fibroblasts of the chick embryo. Of the following susceptible areas, liver cells from the chick embryo have been found to be the most sensitive for