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Author Topic: Limited airborne transmission of Ebola is ‘very likely,’ new study says  (Read 797 times)

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A team of prominent researchers suggested Thursday that limited airborne transmission of the Ebola virus is "very likely," a hypothesis that could reignite the debate that started last fall after one of the scientists offered the same opinion.

"It is very likely that at least some degree of Ebola virus transmission currently occurs via infectious aerosols generated from the gastrointestinal tract, the respiratory tract, or medical procedures, although this has been difficult to definitively demonstrate or rule out, since those exposed to infectious aerosols also are most likely to be in close proximity to, and in direct contact with, an infected case," the scientists wrote. Their peer-reviewed study was published in mBio, a journal of  the American Society of Microbiology.

The study'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 study 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 in the paper 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 study, 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 of Wednesday, Ebola had sickened 23,253 people, killing 9,380 of them, all but a handful in the three West African countries, according to the World Health Organization.

As evidence, the study 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.

Droplet transmission involves respiratory droplets carrying infectious pathogens that travel directly from the respiratory tract of the infectious individual to susceptible mucosal surfaces of the recipient, generally over short distances. Airborne transmission 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. In further understanding the complexities of disease transmission, two other terms should be considered. The first is aerosol transmission, which involves inhalation of infectious aerosols suspended in the air either near a person or at a distance and can involve aerosol particles of various sizes that either land on mucosal surfaces, such as the nose and mouth, or are inspired deeper into the respiratory tract (69). Both traditional models of droplet transmission and airborne transmission can fit under the broader category of aerosol transmission.

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 study 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."
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