WHO, CDC dissect Ebola transmission risk


In the wake of suggestions from some experts that the Ebola virus could evolve into an airborne pathogen, the World Health Organization (WHO) took pains to explain today that the virus is known to spread only through contact with bodily fluids—mainly blood, feces, and vomit.

The agency also said that the virus has been detected in breast milk, urine, and semen, and that even saliva and tears may pose some risk.

Meanwhile, the US Centers for Disease Control and Prevention (CDC), in updated guidance for infection control in hospitals, said the virus can persist on environmental surfaces for a few days. But with daily cleaning and disinfection in a US hospital environment, it would be unlikely to survive longer than a day, the agency said.

Questions about how, and how easily, the virus spreads have been raised widely since the epidemic in West Africa exploded into a global public health emergency. Some experts have observed that the widespread epidemic is giving the virus unprecedented opportunities to evolve, raising the alarming possibility of its morphing into an airborne-transmissible virus.

The WHO perspective on what's known

The WHO tackled the topic today with a statement titled "What we know about transmission of the Ebola virus among humans."

The virus spreads only through direct contact with infected bodily fluids, "the most infectious being blood, faeces and vomit," the agency said.

"The Ebola virus has also been detected in breast milk, urine, and semen," the statement adds. "In a convalescent male, the virus can persist in semen for at least 70 days; one study suggests persistence for more than 90 days.

"Saliva and tears may also carry some risk. However, the studies implicating these additional bodily fluids were extremely limited in sample size and the science is inconclusive. In studies of saliva, the virus was found most frequently in patients at a severe stage of illness. The whole live virus has never been isolated from sweat."

The WHO also said contact with contaminated objects can lead to transmission, but the risk is low and can be reduced further by cleaning and disinfection.

The agency insisted that Ebola is not an airborne pathogen, meaning one you can catch by inhaling an infectious dose from "a suspended cloud of small dried droplets." In contrast with viruses like measles and chickenpox, this type of transmission has not been seen in Ebola studies over several decades.

"Theoretically, wet and bigger droplets from a heavily infected individual, who has respiratory symptoms caused by other conditions or who vomits violently, could transmit the virus—over a short distance—to another nearby person," the WHO said.

But it said it has not seen any studies that demonstrate this type of transmission—whereas solid studies from previous outbreaks show that all cases resulted from "direct close contact with symptomatic patients."

Mutation called just speculation

The WHO went on to assert that in the light of experience with other viruses, the suggestion that Ebola may evolve into an airborne pathogen amounts to baseless speculation.

"Scientists are unaware of any virus that has dramatically changed its mode of transmission," the agency said. For example, it noted that the H5N1 avian influenza virus has infected billions of birds for at least 20 years, and yet its transmission pattern remains essentially unchanged.

"Speculation that Ebola virus disease might mutate into a form that could easily spread among humans through the air is just that: speculation, unsubstantiated by any evidence," the WHO asserted.

Today's WHO comments followed a much shorter statement with a similar message, issued Oct 3 by the United Nations Mission for Ebola Emergency Response (UNMEER).

CDC peers at contaminated surfaces

Meanwhile, the CDC says the role of contaminated surfaces in spreading Ebola virus "has not been established," but evidence for that mode of transmission is lacking so far. The agency discusses the question in its guidance for hospital infection control in relation to Ebola, updated on Oct 3.

"There is no epidemiologic evidence of Ebolavirus transmission via either the environment or fomites that could become contaminated during patient care (eg, bed rails, door knobs)," the guidance says. But given the low infectious dose, disease severity, and other factors, strict precautions are warranted.

Only one laboratory study has scrutinized Ebola virus survival time, and it found that, under ideal conditions, the virus could persist for up to 6 days, the CDC reported. A follow-up study, however, showed the virus was "quite sensitive" to inactivation by ultraviolet light and drying.

Further, just one study has examined Ebola virus survival in a "real world" patient-care environment, the CDC said. In that one, researchers did not find either viral material (through DNA amplification) or viable virus (through culturing) in any of 33 samples collected from sites that were not visibly bloody.

"Based upon these data and what is known regarding the environmental infection control of other enveloped RNA viruses, the expectation is with consistent daily cleaning and disinfection practices in U.S. hospitals that the persistence of Ebola virus in the patient care environment would be short—with 24 hours considered a cautious upper limit," the CDC said.

See also:

Oct 6 WHO statement on Ebola transmission

Oct 3 UNMEER statement

CDC guidance for environmental infection control for Ebola

Related Sep 12 CIDRAP News story

Related Sep 17 commentary published by CIDRAP

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