Ebola is evolving, but not becoming deadlier.
Researchers analyzed the genetic blueprints of Ebola viruses in hundreds of blood samples from infected patients, comparing these data with each other and previously published Ebola genomes. This work revealed how the virus traveled and changed biologically in the first long-term Ebola epidemic on record. The findings may help inform future vaccine development and public health responses.
As the epidemic progressed, Ebola split into at least four genetic lineages. Mapping these lineages over time, the researchers confirmed that Ebola initially spread from Guinea into Sierra Leone, Liberia and Mali (SN: 9/20/14, p. 7). Once in Sierra Leone, the virus spread mostly within the country’s borders, rather than through international introductions, the study in Cell finds. Understanding this transmission pattern might help contain future outbreaks, says virologist Thomas Hoenen of the National Institutes of Health’s Rocky Mountain Laboratories in Hamilton, Mont.Studies from early in the outbreak had indicated an unusually high viral mutation rate. But viral evolution looks different over short and long time periods, says geneticist Daniel Park of the Broad Institute of Harvard and MIT, coauthor of the study in Cell . Longer observations allow for more accurate estimates of evolution rate, says evolutionary biologist Andrew Rambaut of the University of Edinburgh, a coauthor of both studies.
Ebola is actually evolving at the expected rate for viruses of its type, Rambaut says. This rate is over 50 percent higher than the mutation rate seen since the last outbreak in 1976. Despite developing many new mutations during the epidemic, Ebola did not appear to become more infectious or deadly, says virologist Miles Carroll of Public Health England in Salisbury, England. “The death rate did not change as the virus evolved,” says Carroll, coauthor of the paper in Nature. Patient survival rates were between 16 and 19 percent for samples examined in that study, regardless of viral lineage.
Mutations that weaken Ebola are weeded out of its genome over time, the studies suggest. Less successful variants of the virus, and their genes, are removed from the population in the long run. This process seems to drive Ebola’s evolution, says disease ecologist Sébastien Calvignac-Spencer of the Robert Koch Institute in Berlin.
Both studies observed changes in genetic regions that encode a protein that interacts with the human immune system. “This is obviously something that should be examined further,” says Calvignac-Spencer. Such genetic shifts could reflect the emergence of new ways for the virus to evade the immune system, or the effects of the immune system’s attempt to scramble and destroy viral genes, the study in Cell suggests. Still, Park and Rambaut say that genetic variability in these regions is common. Future research would be needed to determine what these genetic changes mean, Park says.
In the future, it will be important to have the capacity to rapidly analyze viral genetic data from within affected countries, Hoenen says. Both studies used samples from Sierra Leone, and the study in Nature also took samples from Guinea and Liberia. All samples were sent to American and European laboratories for deeper analysis, and delays in processing damaged some samples for the study in Cell. Carroll says that collecting, diagnosing and recovering samples for his team’s study required dynamic international effort and collaboration. “We should have a small celebration here,” he says.
The Ebola outbreak continues in Guinea and Sierra Leone. The World Health Organization reports 24 confirmed cases in these countries since June 7, representing a slight decrease from the previous week but an overall increase since May.