O'nyong-nyong Fever in South-Central Uganda, 1996–1997: Description of the Epidemic and Results of a Household-Based Seroprevalence Survey

O'nyong-nyong (ONN) fever, an acute, nonfatal illness characterized by polyarthralgia, is caused by infection with a mosquito-borne central African alphavirus. During 1996–1997, south-central Uganda experienced the second ONN fever epidemic ever recognized. During January and early February 1997, active case-finding and a household cluster serosurvey were conducted in two affected and two comparison areas. A confirmed case was defined as an acute febrile illness with polyarthralgia occurring within the previous 9 months plus serologic confirmation or isolation of ONN virus from blood. In affected (n = 129) and comparison (n = 115) areas, the estimated infection rates were 45% and 3%, respectively, and the estimated attack rates were 29% and 0%, respectively, for an apparent: inapparent infection ratio of nearly 2 in affected areas. In villages sampled near Lake Kijanebalola, Rakai District, the estimated infection and attack rates were 68% and 41%, respectively, and 55% of sampled households had ⩾1 case of ONN fever. In conclusion, this epidemic was focused near lakes and swamps, where it was associated with high infection and attack rates.