O’nyong-nyong virus is an arthropod-borne virus in the genus Alphavirus of the family Togaviridae. It is transmitted by Anopheles mosquitoes and causes an acute febrile illness with rash and polyarthralgia in humans.
Epidemiology and Virology
O’nyong-nyong virus (ONNV) is closely related to the chikungunya and Ross River viruses within the Semliki Forest antigenic complex. The virus has a single‑stranded positive‑sense RNA genome enclosed in a spherical icosahedral capsid and is surrounded by a lipid envelope. Unlike most alphaviruses that rely on Aedes or Culex species, ONNV is transmitted mainly by Anopheles funestus and Anopheles gambiae mosquitoes. These vectors thrive in humid, rural environments, and transmission occurs when infected mosquitoes feed on humans. ONNV was first isolated during a large epidemic in Uganda in 1959 and has since been documented in sporadic outbreaks across East Africa. Humans appear to be the principal vertebrate hosts; no animal reservoir has been identified. After an incubation period of about 3–7 days, infection presents as fever, headache, rash, and marked joint pain. Lymphadenitis, especially of cervical nodes, is a common feature that helps distinguish ONNV infection from chikungunya. Viremia is short‑lived, and most patients recover fully within weeks. There is no vaccine or specific antiviral therapy.
Historical Epidemics and Clinical Features
Large ONNV outbreaks have occurred twice. The first epidemic between 1959 and 1962 affected more than two million people in Uganda, Kenya, Tanzania, Zaire and Malawi. The name “o’nyong‑nyong” comes from the Acholi language, meaning “severe joint pain and weakness,” which reflects the debilitating arthralgia experienced by infected individuals. A second major epidemic occurred in 1996–1997 in Uganda, followed by smaller outbreaks in Ethiopia and Sudan. Patients typically develop high fever, maculopapular rash, and symmetric joint pain affecting the wrists, knees, and ankles. Cervical and axillary lymph node enlargement is frequent, and fatigue may last for several weeks. Unlike some related alphaviruses, ONNV has not been associated with neurologic disease or mortality.
After recovery, patients develop lasting immunity. No licensed vaccine exists, so control efforts focus on reducing mosquito populations and limiting human exposure through bed nets and insect repellents. Laboratory diagnosis is achieved by reverse‑transcription PCR or serology. Because symptoms overlap with chikungunya and dengue, clinical diagnosis requires laboratory confirmation.
In summary, O’nyong‑nyong virus is a mosquito‑borne alphavirus endemic to sub‑Saharan Africa. It is unique among alphaviruses in its reliance on Anopheles mosquitoes for transmission and its association with large epidemics characterized by fever, rash and incapacitating polyarthralgia. Ongoing surveillance and vector control are key to preventing future outbreaks.
Related Terms: Ross River virus, Chikungunya virus, Semliki Forest virus, Mayaro virus, Alphavirus