While the largest Ebola outbreak in history has slowed down, shifting global attention away from the West African countries devastated by the deadly hemorrhagic fever, the debate and confusion over whether airborne transmission of the Ebola virus is possible has returned following a new report by American scientists.
Led by public health scientist Michael T. Osterholm at the University of Minnesota, the report, published Thursday in the journal mBio, is titled "Transmission of Ebola Viruses: What We Know and What We Do Not Know." The paper states that based on existing evidence, the deadly virus is primarily transmitted through human to human contact, or contact with infectious bodily fluids. The authors, however, assert that more research is needed on aerosol transmission, environmental contamination, and other key areas.
"In this review, we address what we know and what we do not know about Ebola virus transmission," they write in the paper's abstract. "We also hypothesize that Ebola viruses have the potential to be respiratory pathogens with primary respiratory spread."
Throughout the paper, the authors make reference to the likelihood of aerosol transmission, yet their paper only poses hypotheses — it does not put forth new clinical data or independent research. The article is labeled as "opinion/hypothesis" and the authors support their ideas with information from a collection of previous studies analyzing Ebola in human outbreaks and experiments with animals.
"It is very likely that at least some degree of Ebola virus transmission currently occurs via infectious aerosols," the researchers state, noting that this hypothesis has been challenging to either definitively demonstrate or rule out entirely.
'It's a cool thing for scientists to think about but not something people should worry about.'
Using this statement, many media reports have taken the paper to mean that it is likely Ebola could go airborne. The researchers, however, make no such statement. The confusion between aerosol and airborne is both common and understandable as there is a fine line between the two.
Ian Mackay, an Australian virologist at the University of Queensland, explains the term aerosol in his blog Virology Down Under. As Mackay and other collaborators wrote in a post during the height of the outbreak, aerosol signifies "any collection of particles suspended in air, and particle sizes vary enormously." Aerosols contain droplets of various sizes — those larger than 5 to 10 millionths of a meter hit the ground quickly without evaporating. Spray paint, for example, has larger size droplets designed to coat a wall, not to remain in air for a period of time.
The mBio paper itself explains that, airborne transmission of any virus or bacteria "occurs by dissemination of either airborne droplet nuclei or small particles in the respirable size range containing infectious agents that remain infective over time and distance." Droplet nuclei are leftover particles that can contain bacteria and virus. These particles are the remnants of much smaller aerosol droplets that linger in the air and then evaporate.
To date, research shows that droplet nuclei from the Zaire strain of Ebola — currently responsible for the West African outbreak — is not infectious. As Mackay notes, droplet nuclei containing Ebola have never been shown to transmit that strain of the virus, even when tracked through a home where the disease was present. Some airborne diseases include tuberculosis, chickenpox, and the flu.
For a recent and tangible example, University of Reading virologist Ben Neuman points to the case of Thomas Eric Duncan, who in September became the first person during the current outbreak to be diagnosed with Ebola in the US. Duncan contracted the disease while in his native country of Liberia, but traveled via plane to the US and spent days in Dallas before developing symptoms.
Shortly after exhibiting symptoms — the point at which current science shows an Ebola-infected individual is first contagious — Duncan went to the hospital on two separate occasions, spent time in an emergency room with other patients, and was treated by individuals who were not wearing protective suits. Despite the open contact with many individuals, the only two people to contract the disease from Duncan were nurses who cared directly for the 42-year-old man and likely came in contact with his bodily fluids. According to Neuman, had the virus been airborne, we would likely have seen more people in the emergency room or other locations contract Ebola.
Osterholm's paper itself asserts the difference between airborne and aerosol, while also highlighting that airborne transmission is unlikely. Aerosol transmission, however, remains plausible.
"Leading public health agencies have stated that airborne transmission… of Ebola viruses is unlikely to occur in the future because this would require specific genotypic changes in the virus," the paper explains. The researchers agree that this is an improbable scenario, but they note that aerosol transmission with various size droplets in close proximity is more likely.
Mackay has often pointed out that the inability of scientists to relay to the public the difference between aerosol and airborne has become problematic. This sentiment was shared again in a series of tweets from the virologist on Thursday after the paper was published.
According to Neuman, looking at what we know about the Ebola virus, there's no logical explanation as to why the hemorrhagic fever couldn't become airborne, but there is simply no evidence to indicate this is happening.
"Things would be a lot different if it had been… It'll spread a lot more quickly and it'll spread widely," he explained. "It's a cool thing for scientists to think about but not something people should worry about."
While interesting from a scientific perspective, Neuman said talking about the possibility that Ebola might travel through the air in Africa right now is irresponsible speculation.
"Studying rare exceptions can give us a deeper understanding of a virus like Ebola, but public health policy needs to be guided by the practical concern of how to stop the bulk of virus transmission," he said. "We all let our minds wander at times, and when there is a deadly virus around, inattention has consequences."
Follow Kayla Ruble on Twitter: @RubleKB
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