Highly sensitive and broadly reactive quantitative reverse transcription-PCR assay for high-throughput detection of Rift Valley fever virus.

Rift Valley fever (RVF) virus is a mosquito-borne virus associated with large-scale epizootics/epidemics throughout Africa and the Arabian peninsula. Virus infection can result in economically disastrous "abortion storms" and high newborn mortality in livestock. Human infections result in a flu-like illness, with 1 to 2% of patients developing severe complications, including encephalitis or hemorrhagic fever with high fatality rates. There is a critical need for a highly sensitive and specific molecular diagnostic assay capable of detecting the natural genetic spectrum of RVF viruses. We report here the establishment of a pan-RVF virus quantitative real-time reverse transcription-PCR assay with high analytical sensitivity (approximately 5 RNA copies of in vitro-transcribed RNA/reaction or approximately 0.1 PFU of infectious virus/reaction) and efficiency (standard curve slope = -3.66). Based on the alignments of the complete genome sequences of 40 ecologically and biologically diverse virus isolates collected over 56 years (1944 to 2000), the primer and probe annealing sites targeted in this assay are known to be located in highly conserved genomic regions. The performance of this assay relative to serologic assays is illustrated by testing of known RVF case materials obtained during the Saudi Arabia outbreak in 2000. Furthermore, analysis of acute-phase blood samples collected from human patients (25 nonfatal, 8 fatal) during that outbreak revealed that patient viremia at time of presentation at hospital may be a useful prognostic tool in determining patient outcome.

Marburgvirus genomics and association with a large hemorrhagic fever outbreak in Angola.

In March 2005, the Centers for Disease Control and Prevention (CDC) investigated a large hemorrhagic fever (HF) outbreak in Uige Province in northern Angola, West Africa. In total, 15 initial specimens were sent to CDC, Atlanta, Ga., for testing for viruses associated with viral HFs known to be present in West Africa, including ebolavirus. Marburgvirus was also included despite the fact that the origins of all earlier outbreaks were linked directly to East Africa. Surprisingly, marburgvirus was confirmed (12 of 15 specimens) as the cause of the outbreak. The outbreak likely began in October 2004 and ended in July 2005, and it included 252 cases and 227 (90%) fatalities (report from the Ministry of Health, Republic of Angola, 2005), making it the largest Marburg HF outbreak on record. A real-time quantitative reverse transcription-PCR assay utilized and adapted during the outbreak proved to be highly sensitive and sufficiently robust for field use. Partial marburgvirus RNA sequence analysis revealed up to 21% nucleotide divergence among the previously characterized East African strains, with the most distinct being Ravn from Kenya (1987). The Angolan strain was less different ( approximately 7%) from the main group of East African marburgviruses than one might expect given the large geographic separation. To more precisely analyze the virus genetic differences between outbreaks and among viruses within the Angola outbreak itself, a total of 16 complete virus genomes were determined, including those of the virus isolates Ravn (Kenya, 1987) and 05DRC, 07DRC, and 09DRC (Democratic Republic of Congo, 1998) and the reference Angolan virus isolate (Ang1379v). In addition, complete genome sequences were obtained from RNAs extracted from 10 clinical specimens reflecting various stages of the disease and locations within the Angolan outbreak. While the marburgviruses exhibit high overall genetic diversity (up to 22%), only 6.8% nucleotide difference was found between the West African Angolan viruses and the majority of East African viruses, suggesting that the virus reservoir species in these regions are not substantially distinct. Remarkably few nucleotide differences were found among the Angolan clinical specimens (0 to 0.07%), consistent with an outbreak scenario in which a single (or rare) introduction of virus from the reservoir species into the human population was followed by person-to-person transmission with little accumulation of mutations. This is in contrast to the 1998 to 2000 marburgvirus outbreak, where evidence of several virus genetic lineages (with up to 21% divergence) and multiple virus introductions into the human population was found.