The paper's lead author, Michael T. Osterholm, an epidemiologist at the Center for Infectious Disease Research and Policy at the University of Minnesota, touched off a small furor and was condemned by some experts last Sept. 11 when he raised the same possibility in an op-ed piece in the New York Times as concern over the spread of the deadly disease was increasing rapidly.
Less than a month later, Thomas Eric Duncan, a Liberian infected with Ebola in his home country, died in a Dallas hospital, but not before two nurses who treated him became infected, sparking fears about how prepared U.S. hospitals were to handle the disease. Public health authorities reassured Americans they were in no danger of contracting the hemorrhagic disease from casual contact with others. Ebola is transmitted by contact with infected body fluids -- mainly blood, feces and vomit -- experts around the world have said. This is why health care workers and people who had contact with victims were most likely to become infected in the current epidemic, they said.
"There was almost a rush to ensure the public that we knew a lot more than we did," Osterholm said in an interview Wednesday night, repeating a theme he has raised many times before. "But we're saying you can’t rule out respiratory transmission."
Osterholm's September opinion piece focused on the possibility that the virus could mutate and eventually become airborne, a theory that other experts widely dismissed as extremely unlikely. In contrast, Thursday's review examines the idea that minuscule droplets of body fluid containing the virus could hang in the air and be inhaled by others, providing an unrecognized, if minor, pathway for the virus.
This time Osterholm was joined by Gary P. Kobinger of Canada's Public Health Agency, Pierre Formenty of the World Health Organization's pandemic response unit and Clarence J. Peters, of the Galveston National Laboratory at the University of Texas Medical Branch, among many others.
The paper, titled 'Transmission of Ebola Viruses: What We Know and What We Do Not Know," takes pains to note that respiratory transmission of Ebola is unproven and that contact with infected body fluids is by far the most common way that the virus is passed from one person to another. Indeed, as health experts and aid workers have persuaded West Africans to adopt safe burial practices and isolate people infected by the virus, the disease has virtually disappeared in Liberia, though it is still more prevalent in Sierra Leone and Guinea.
As evidence, the research notes that Ebola virus has been found on the outside of face masks worn by health workers caring for victims of the disease. It also points out that the virus has been passed between animals via respiration. And the authors say that Ebola can infect certain cells of the respiratory tract, including epithelial cells, which line body cavities, and macrophages, a type of white blood cell that consumes pathogens.
The paper notes that breathing, sneezing, coughing and talking can release droplets of fluid from the respiratory tract that travel short distances and most likely cause infection by settling on a mucous membrane. Those actions also release smaller airborne particles capable of suspension in mid-air that can be inhaled by others. Technically, both qualify as aerosols, the paper says.
The debate has centered on whether Ebola can be transmitted via those smaller particles. William Schaffner, an infectious disease specialist at Vanderbilt Medical Center who did not take part in the study, praised it for raising the issues "in a thoughtful fashion" and predicted it would be "very, very widely read."
He said he could imagine the possibility of respiratory transmission of Ebola from close-in contact, perhaps a distance of three or four feet. Even so, Schaffner said, it would be rare; as the study points out, it has never been demonstrated in humans.
The common mode of transmission--contact with body fluids--"those are the highways of transmission," Schaffner said. "Could respiratory transmission occur? Yes. But it's probably a byway, a little trail in the forest."
Asked why many more people who were near Ebola victims had not become infected, Osterholm said the Ebola virus may be much less contagious than other diseases spread by respiration, such as measles. He likened it to tuberculosis, which is more difficult to contract this way.
In an e-mail, Kobinger said that "we hope that this review will stimulate interest and motivate more support and more scientists to join in and help address gaps in our knowledge on transmission of Ebola (and other filoviruses). Important policies and biosafety regulations must be evidence-based, not [by] using opinions and beliefs as guiding principles."
The review itself points out that "to date, investigators have not identified respiratory spread (either via large droplets or small-particle aerosols) of Ebola viruses among humans. This could be because such transmission does not occur or because such transmission has not been recognized, since the number of studies that have carefully examined transmission patterns is small."
But it concludes by adding: "The West Africa Ebola epidemic surprised even the most astute infectious disease experts in the global public health community; we should not assume that Ebola viruses are not capable of surprising us again at some point in the future."