Orbivirus

African horse sickness virus Bluetongue virus Epizootic hemorrhagic disease of deer virus Equine encephalosis virus Ibaraki virus Palyam virus

The genus Orbivirus is a member of the Reoviridae family. This genus contains 19 species and at least 130 different serotypes. Orbiviruses can infect and replicate within a wide range of arthropod and vertebrate hosts.

Virus Particles
Orbiviruses are named after their characteristic doughnut shaped capsomers (In Latin Orbi means ring). They have non-enveloped particles that are between 70-80nm in diameter. The virus particles are spherical in appearance and have icosahedral symmetry. An outer and an inner capsid layer surround the genome.

Genome
These viruses have double stranded RNA genomes and therefore are classified as Class III viruses. Their genome is linear and is segmented into 10 segments of various lengths. One copy of each gene segment is packaged per virion. In most cases each gene segment encodes a single open reading frame (ORF).

Replication
Many Orbiviruses preferentially infect vascular endothelial cells. Orbiviruses enter the host cell by endocytosis and the outer capsid is subsequently removed. The whole cycle of viral replication takes place within the cytoplasm of the host cell. Transcription of the viral genome into mRNA occurs within the core particle and mRNA is translated into proteins using the host cell ribosomes. Viral proteins are synthesized 2-14 days after initial infection. New virons self-assemble within the cytoplasm and are then released from the host cell by budding. During the budding process they transiently acquire a lipid envelope which can be detected for a short period of time following their release but this is subsequently lost.

Pathogenesis
Orbiviruses primarily cause diseases in animals. The different Orbivirus species have different host specificities. Orbiviruses are vector-borne pathogens transmitted between vertebrate hosts by vectors such as mosquitoes, midges, gnats, sandflies and ticks. Bluetongue virus (BTV) is an Orbivirus that causes bluetongue disease in sheep, cattle, goats and wild ungulates. BTV has been in the forefront of molecular studies for last three decades and now represents one of the best understood viruses at the molecular and structural levels. Other species of Orbiviruses are responsible for other diseases of animals such as African Horse sickness and Colorado Tick Fever.