Emergence of epidemic O'nyong-nyong fever in Uganda after a 35-year absence: Genetic characterization of the virus

O'nyong-nyong (ONN) virus is an alphavirus (family Togaviridae, genus Alpha virus) classified in the Semliki Forest virus (SFV) antigenic complex. ONN was initially isolated in northern Uganda in 1959 during the early stages o f an explosive arbovirus epidemic in which >2 million cases were reported. No additional epidemics or human isolations of ONN were reported until 1996 , when it was isolated from an epidemic in southern Uganda. We report the c omplete nucleotide and deduced amino acid sequence of one of these 1996-199 7 ONN isolates (SG650) and that of the related alphavirus Igbo Ora virus. T he data indicate that the recent ONN virus isolate is closely related to th e previously published ONN strain isolated in 1959. In addition, phylogenet ic analysis of the sequence data reveals that Igbo Ora virus, previously th ought to be a separate virus closely related to ONN and Chikungunya (CHIK), clearly is a strain of ON N. The sequence data also reveal that unlike the published ONN (1959) sequence, all ONN strains from the 1996-1997 epidemic possess a stop codon at the nsp3-nsp4 junction. (C) 1998 Academic Press.