Orbiviruses

Orbiviruses

Orbiviruses are primarily animal pathogens that cause disease mainly in animals. They are so named for their ring-shaped (Latin wordorbi: ring) structure. These viruses are differenti-ated from the orthoreoviruses by their protein structure and their transmission by arthropod vectors. The genus Orbivirus consists of 19 species and at least 130 subspecies. The orbi-viruses are nonenveloped viruses with two-layered capsids. 

The genome consists of 10 segments of double-stranded RNA. The virion measures 70–80 nm in diameter.

Unlike other reoviruses, the orbiviruses are sensitive to low pH. The orbiviruses mainly cause disease in animals, such as sheep, cattle, goats, and wild ungulates. They are associated with African horse sickness in horses, donkeys, and dogs; blue tongue disease in sheep; and epizootic hemorrhagic feverin deer.

Only few of orbiviruses are linked to disease in humans. Kemerovo virus has been implicated in neurological infections in Russia and Central Europe. Lebombo virus, a virus isolated in Africa, has been implicated with clinical disease in humans. Orbiviruses cause a severe flu-like disease, encephalitis, and polyradiculitis in humans. Fever, headache, and myalgia are the common manifestations. Rarely, the cases are complicated by other neurological manifestations. Till date, fewer than 50 cases of orbivirus infection in humans have been reported from Africa, South and Central America, Russia, and Eastern Europe. No case is reported from India. Humans acquire infec-tion by the bite of arthropod vectors including mosquitoes, ticks, gnats, and midges. Oklahoma tick fever or Kemerovo or Lipovnik virus infections are transmitted by bite of ticks, specifically Ixodes ticks. However, bite of mosquitoes transmits orbivirus infections in Africa and South America. Orbivirus infections are mostly asymptomatic.

Changuinola virus has been implicated in a single documented case of an acute self-limited febrile illness in Panama. The disease is transmitted by Phlebotomus flies. Orungovirus found in parts of Sub-Saharan Africa causes an acute illness characterized by head-ache, fever, and myalgia. The virus is transmitted primarily by Aedes mosquitoes. Oklahoma tick fever, reported in Oklahomaand Texas in the United States, is transmitted by ticks. The condi-tion manifests as fever, nausea, and abdominal pain.

The diagnostic facilities for orbivirus infections are available only in a few reference laboratories.

      No specific antiviral agents are available. Use of insect repel-lants, protective clothing, and avoidance of tick bite prevent transmission of orbivirus infections.