Ebola virus, Marburg virus, Lassa fever, Rift Valley fever, yellow fever, and dengue are well known viruses from four different viral families that can cause hemorrhagic fever. But an international team of researchers report in PLoS Pathogens today that the rhabdovirus family, which typically causes brain swelling or flulike disease, can join the club of hemorrhagic fever agents, which are among the most virulent pathogens known to humans.
Using sophisticated RNA sequencing technology, the researchers discovered evidence that links a rhabdovirus to acute hemorrhagic fever in three people from the village of Mangala between 25 May and 14 June 2009. The first two people afflicted with the frightening disease died. But a third, a male nurse who treated both patients at a health center, survived.
The researchers did not actually isolate a rhabdovirus, but instead plucked out RNA sequences from the surviving man's blood sample and reconstructed what they contend is the genome of the pathogen that caused the disease. "It looks fairly solid," says Thomas Ksiazek, an epidemiologist and virologist who specializes in hemorrhagic disease at the University of Texas Medical Branch in Galveston. "Clearly, they have identified a virus in one of the three patients they describe. But trying to make more out of this is speculative. Is the disease due to the agent? You can speculate all you want, but until you have a virus in hand it's hard to answer that."
Delwart's group randomly amplified genetic material with the polymerase chain reaction, sequenced the products, and then checked what they created against databases of known genomes. "We got one very intriguing read," Delwart says. "It was clearly a rhabdovirus." But his team couldn't amplify any more meaningful sequence, so they passed the sample on to Charles Chiu, head of a viral discovery center at the University of California, San Francisco. Chiu sequenced 140 million fragments of genetic material and then merged related fragments to assemble the genome of the Bas-Congo virus.
Virologist Joseph Fair, who works with Metabiota, says the researchers could not isolate the actual virus because it took so long for the nurse's sample to arrive at a lab that could probe for agents. "We do the best we can with what we have here," Fair says, speaking by phone from Isiro, Congo, where he's working on a new Ebola outbreak. "We're trying to build capacity to build cold chains in cases like this."
Fair points to several lines of evidence that the Bas-Congo virus is real. He notes that the research teams exhaustively searched for other pathogens. "We found nothing except this rhabdovirus," he emphasizes. Based on genomic copies of Bas-Congo virus, the nurse also had high levels of the bug. The researchers then made a lab tool known as a pseudovirus, which contained the surface protein from Bas-Congo inside of a vesicular stomatitis virus, to run a confirmatory antibody test.
Specifically, they showed that antibodies taken from the nurse in January 2012, as well as from an asymptomatic nurse who cared for him, prevented the pseudovirus from infecting cells in a culture dish. Fair and his co-authors also note that another hemorrhagic fever virus, Lujo, was first found in 2009 using similar genetic techniques to those they relied on.
But Fair acknowledges the importance of isolating the virus itself. "Since we weren't able to get an isolate, that limits what we can know about how it kills people and the effect it has on the immune system," Fair says. "Along with our Congolese colleagues, our next step will be to mount an expedition to find this virus."