The Ebola virus raging through West Africa is mutating rapidly as it tears a deadly path through cities, towns and villages, but the genetic changes are for now not giving it the ability to spread more easily.
Concern that the virus could gain capability to transmit through the air - creating a nightmare scenario of the disease being able to spread like a flu pandemic, killing millions - was fuelled by a top infectious disease expert in the United States.
Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said in an opinion article he believed the risk of airborne Ebola is real, and warned: "Until we consider it, the world will not be prepared to do what is necessary to end the epidemic."
Yet many other virus and infectious disease specialists say that while the prospect of an airborne Ebola virus is not impossible, it is extremely remote.
"This is way down on the list of possible futures for Ebola and in all probability will never happen," said Ian Jones, a virologist at Britain's University of Reading.
Ebola is contagious, but spreads via direct contact with the bodily fluids of an infected person, such as their blood, faeces or vomit. The virus has infected 5,357 people in West Africa this year, killing 2,630 of them, in the worst Ebola epidemic the world has seen.
Ben Neuman, a Reading virologist who has been monitoring the Ebola epidemic since it began in Guinea, noted that under carefully controlled laboratory conditions, scientists have shown it is feasible to make Ebola transmit through air, but added: "So far there is no solid evidence that it actually happens out there in the real world."
"One clue is how slow the virus is spreading," he told Reuters. "Compared to this Ebola outbreak, the H1N1 swine flu had already spread to an estimated 10,000 times as many people in its first 10 months."
That's not to say the Ebola virus isn't mutating. It is, rapidly, all the time.
In a study published in the journal Science late last month, a team of researchers sequenced 99 Ebola virus genomes isolated from blood samples of 78 patients in Sierra Leone - one of the four countries at the heart of the epidemic.
They found what they described as "a rapid accumulation of interhost and intrahost genetic variation" - in other words, a large number of frequent changes in the virus - even in the initial few weeks of the outbreak.
"SLOPPY" RNA VIRUS
Unlike some other nasty viruses such us smallpox and hepatitis B, Ebola, like HIV and flu, is an RNA virus - one whose genetic material is contained in ribonucleic acid (RNA) rather than deoxyribonucleic acid (DNA).
RNA viruses are renowned for their rapidly changing nature and are often described by virologists as "sloppy" viruses because when they replicate, they make copies of themselves that are full of errors.
But most of these mistakes, or changes, are just "irrelevant mutations", explained Anthony Fauci of the United States National Institute of Allergy and Infectious Diseases.
Fauci told a U.S. senate hearing this week that the changes so far observed in Ebola in this outbreak, while prolific, were generally "not associated with a biological change or a biological function" of the virus, meaning they were highly unlikely to give it an entirely new skill, such as the ability to transmit in droplets in the air.
"It is an unusual situation where a mutation would completely change the way a virus is transmitted," Fauci said. "It's not impossible, but it would be unlikely."
Jones added that the so-called tropism of the Ebola virus - the tissue it prefers to infect - is the vasculature, not the airway surfaces.
"As a result it is not in the right place to make the leap to a new transmission route," he told Reuters. "In fact very few viruses do this. Most stay in the niche they have established over evolutionary time."
Experts stress, however, that keeping close tabs on the mutations in the Ebola virus is vitally important, particularly for those working on developing potential drugs to treat the infection, or vaccines to prevent it.
The researchers who sequenced the Ebola genomes and published their findings in Science said many of the mutations they found had altered protein sequences or other biologically meaningful targets within the virus.
"They should be monitored for impact on diagnostics, vaccines, and therapies critical to outbreak response," they wrote.
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