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.

O'nyong-nyong fever in south-central Uganda, 1996-1997: clinical features and validation of a clinical case definition for surveillance purposes.

O'nyong-nyong (ONN) fever, caused by infection with a mosquito-borne central African alphavirus, is an acute, nonfatal illness characterized by polyarthralgia. During 1996-1997, south-central Uganda experienced the second ONN fever epidemic ever recognized. Among 391 persons interviewed and sampled, 40 cases of confirmed and 21 of presumptive, well-characterized acute, recent, or previous ONN fever were identified through active case-finding efforts or during a household serosurvey and by the application of clinical and laboratory criteria. Among confirmed cases, the knees and ankles were the joints most commonly affected. The median duration of arthralgia was 6 days (range, 2-21 days) and of immobilization was 4 days (range, 1-14 days). In the majority, generalized skin rash was reported, and nearly half had lymphadenopathy, mainly of the cervical region. Viremia was documented in 16 cases, primarily during the first 3 days of illness, and in some of these, body temperature was normal. During this epidemic, the combination of fever, arthralgia, and lymphadenopathy had a specificity of 83% and a sensitivity of 61% in the identification of cases of ONN fever and thus could be useful for surveillance purposes.