Abstract

In late 1997 and early 1998, a large outbreak of hemorrhagic fever occurred in East Africa. Clinical samples were collected in Kenya and southern Somalia, and 27 of 115 (23%) hemorrhagic fever patients tested showed evidence of acute infection with Rift Valley fever (RVF) virus as determined by IgM detection, virus isolation, detection of virus RNA by reverse transcription-polymerase chain reaction (RT-PCR), or immunohistochemistry. However, two patients (one from Kenya and the other from Somalia) whose illness met the hemorrhagic fever case definition yielded virus isolates that were not RVF. Electron microscopy suggested these two virus isolates were members of the family Bunyaviridae. RT-PCR primers were designed to detect bunyavirus RNA in these samples. Regions of the S and L segments of the two isolates were successfully amplified, and their nucleotide sequences exhibited nearly complete identity with Bunyamwera virus, a mosquito-borne virus not previously associated with severe human disease. Unexpectedly, the virus M segment appeared to be reassorted, as the sequences detected exhibited 32-33% nucleotide and 28% amino acid differences relative to the corresponding M segment sequence of Bunyamwera virus. The association of this reassortant bunyavirus, proposed name Garissa virus, with severe disease is supported by the detection of the virus RNA in acute-phase sera taken from 12 additional hemorrhagic fever cases in the region.