The diagnosis of Ebola in a second health worker in Texas raises questions about how well researchers understand how the virus spreads and whether the virus is changing in a way that makes it easier to transmit.
“I suspect it may have become more transmissible,” said Ira Longini, a biostatistician at the University of Florida, who cited recent cases among health workers who thought they were taking careful precautions. The Dallas health worker case announced today “does raise a red flag, but you can’t prove anything from it,” he said.
Longini is studying the probability that the secondary infection rate -- or how many fellow household members get the disease from an infected person -- is higher in the current outbreak than it has been in the past. A greater number of secondary infections per case could indicate that the virus has become more transmissible, he said.
The Ebola virus circulating in West Africa is already different from previous strains. While scientists don’t fully understand what the changes mean, some are concerned that alterations in the virus that occur as that pathogen continues to evolve could pose new dangers.
Researchers have identified more than 300 new viral mutations in the latest strain of Ebola, according to research published in the journal Science last month. They are rushing to investigate if this strain of the disease produces higher virus levels -- which could increase its infectiousness.
So far, there is no scientific data to indicate that. The risk, though, is that the longer the epidemic continues, the greater the chance that the virus could change in a way that makes it more transmissible between humans, making it harder to stop, said Charles Chiu, an infectious disease physician who studies Ebola at the University of California at San Francisco.
“If the outbreak continues for a prolonged period of time or it becomes endemic, it may mutate into a form that is more virulent,” Chiu said. “It is really hard to predict.”
Viruses such as Ebola, whose genomes are made from ribonucleic acid, are constantly mutating. Some mutations are good for the virus and some are bad for the virus, said Ian Mackay, a virologist at the University of Queensland. It’s the ones that are good for the virus that tend to stick around.
“Viruses don’t think. They make mutations that are good for them,” he said. “If it helps the virus spread or replicate faster it will be around more.”
“It is a numbers game, the more cases you have the more likely there are going to be mutations that could change the virus” in a significant way, said David Sanders, a professor of biological sciences at Purdue University who studies Ebola. “The more it persists, the more likely we are going to be thrown a curve.”
So far, the patterns of transmission are consistent with the previous Ebola epidemic, Chiu and several other researchers said.
Ebola virus spreads through direct contact with bodily fluids, such as vomit or diarrhea. Patients are most infectious at the late stages of the disease when large quantities of the virus are present inside the body. This explains why health-care workers and family members tending to gravely ill patients are particularly at risk of getting infected.
The current outbreak is centered in the west African countries of Liberia, Sierra Leone and Guinea. Almost 9,000 people have been infected with the virus in the three countries, with about 4,500 deaths, according to the World Health Organization.
There’s no evidence yet that the virus is spreading faster than in earlier outbreaks, said Cliff Lane, deputy director for clinical research and special projects at the U.S. National Institute of Allergy and Infectious Diseases in Bethesda, Maryland.
One dire scientific scenario is that the virus could somehow mutate in a way that allowed it to be transmitted through tiny particles that linger the air, in the same way that measles is thought to spread.
That is highly unlikely to happen for a variety of reasons, five Ebola virus researchers interviewed by Bloomberg News said.
Ebola would have to “do something pretty drastic” to change its route into the body, said Robert Garry, a virologist at Tulane University. It is not designed for getting into respiratory cells, he said.
“We know which parts of the virus bind to human cells,” Garry said. “Those parts are not changing.”
A sixth researcher, Sanders of Purdue University, said the possibility of Ebola becoming airborne “cannot be ruled out.”
“There are a lot of unknowns” about Ebola, Sanders said. The previous outbreaks have been small and they generally have been in remote areas, making detailed scientific study difficult.
Other changes could occur short of the virus becoming airborne that could make it more transmissible. For example, the virus could change to become hardier so it survives longer outside of the body or on surfaces, Garry said.
The question of whether the current strain can produce higher amounts of virus inside the body is difficult to answer because researchers haven’t developed precise measurements for Ebola viral blood levels, NIAID’s Lane said.
While infectiousness can also be studied in animals, the common lab model for Ebola infection uses monkeys, which must be infected intravenously -- not the usual way humans get the disease, he said.
“What’s critically important is that we get some good research so we can ask these questions, and ask them clearly and definitively,” Lane said in a telephone interview.
“We’re looking all the way back from 1976 to the present and see if we can detect any changes, which might be a good indicator or how transmissible it is,” Longini said. One problem is the current outbreak has been so intense that researchers don’t have reliable estimates on secondary infection rates.
Ronald Corley, chairman of microbiology at Boston University is skeptical of speculation that Ebola virus has mutated in any medically significant way. It is already highly deadly, so it is hard to imagine it becoming more virulent, he said. Nor is there any evidence suggesting that it could become airborne, like the measles, he said.
“I’m seeing all these reports about changing virulence and mode of transmission,” he said. “I just don’t see how that would work from a fundamental scientific perspective.”