O'nyong'nyong fever is caused by O'nyong'nyong virus and is causing epidemics in Africa time after time.
O'nyong'nyong virus (ONNV) belongs to family Togaviridae, genus Alphavirus and was first isolated from human blood and anopheline mosquitoes in Gulu, Uganda, in 1959. There were three major epidemics of O'nyong'nyong fever reported in Africa – in 1959 in East Africa, with more than 2 million cases but no known fatalities; in 1996 in southern Uganda and in 2002 in Central Uganda, near Lake Wamala. Some other outbreaks in Kenya, Tanzania, Malawi and Mozambique were also reported.
Clinically the features of ONNV infections include a fever, symmetrical polyarthralgia, lymphadenopathy, generalized papular or maculopapular exanthema, and joint pain. Other symptoms are lymphadenitis, eye pain and reddening with no discharge, chestpain, and general malaise. All age groups and both sexes are equally affected. No deaths have been reported, but several miscarriages have been associated with infection.
O’nyong-nyong virus is unique among mosquito-borne alphaviruses in being primarily transmitted by mosquitoes of the genus Anopheles – e.g. Anopheles gambiae or An. funestus while alphaviruses and flaviviruses are typically transmitted by culicine mosquitoes (Aedes, Culex or others). Humans are thought to be the only natural host of ONNV, as other vertebrate reservoirs have not been identified.
O’nyong-nyong virus is closely related to chikungunya virus (CHIKV). Another virus, Igbo Ora, was also thought to be a separate virus but closely related to ONNV. However, sequence analysis of this virus indicates that it is a strain of ONNV.