Ebola: Has the Virus Gone Airborne?

Since my last blog, the World Health Organization (WHO) has reported 68 new cases and 29 new deaths from Ebola in West Africa, bringing the new total to 1,779 infected and 961 dead. These numbers include suspected, probable, and confirmed cases alike. Given the magnitude of this crisis, the recently formed WHO Emergency Committee met for two days via teleconference this past week and unanimously decided that the present outbreak constitutes a Public Health Emergency of International Concern. Importantly, no registered medicines or vaccines currently exist to combat Ebola. This Monday, however, a WHO-organized panel of medical ethics experts will meet to discuss the prospect of using experimental treatments to quell the outbreak.   

As for the title of this blog, while the ongoing Ebola outbreak continues to produce record-breaking numbers of infections, much talk has begun circulating as to the reasons for such a severe outbreak. Among the different hypotheses I’ve heard is the idea that this particular Ebola virus has mutated into an airborne form. Were this hypothesis true, it would of course spell catastrophe for the globe. For this reason, and as promised in my last blog, I’ve decided to dedicate the following paragraphs to addressing this concern.

In general, the assertion of airborne Ebola appears to be mostly conspiracy, although there is some merit to the argument which deserves mention. First, the Ebola virus responsible for the present outbreak is indeed of a strain we have not previously seen, though it may have existed in the region for quite some time. This new strain, however, shares 98% homology with Zaire ebolavirus, which we have seen numerous times over the last 38 years, and which would require no mutations for transmission to be viable through the air. Rather, experiments have already shown Zaire ebolavirus to be stable and infectious in small-particle aerosols. This was observed both intentionally and accidentally in laboratory monkeys at the United States Army Medical Research Institute in 1995, for instance.

When considering such evidence of airborne transmission in monkeys, it is important to keep in mind that a virus’s ability to infect through the air is largely dependent on the viral burden of an animal’s lungs and that animal’s efficiency at aerosolizing particles through exhalation (there are other factors as well). It may well be, in the case of monkeys, that their physiology is such that airborne transmission is an important pathway of exposure. In short, observing such transmission in monkeys does not necessarily mean this route is driving transmission in humans, although admittedly this should raise some eyebrows.

To better answer the question regarding humans, it is more informative to turn to epidemiological studies. Historically, such studies have shown Ebola to spread in close circles, usually to family members or nurses giving care to infected individuals, as well as those involved in the handling or funeral preparations of deceased Ebola victims. In fact, cases of Ebola in people who never had direct contact with other infected people are quite rare, even when infected individuals live in the same household. Where there is no direct contact, there is usually no spread of disease. Even among people who have limited direct contact with patients, transmission is still low, as was learned following the 1979 outbreak in Sudan.

To further consider the “airborne hypothesis,” it is useful to look at the rate of geographic spread of the current epidemic. Were Ebola truly an airborne killer, we would expect the virus to spread much like other airborne killers from the past, such as the SARS epidemic which spread internationally across 26 countries in 2003, or the swine flu which traveled the entire 3,000 mile length of the U.S., infecting all 48 states in a matter of 7 months. By contrast, we are now over five months into the present Ebola outbreak and the virus has remained confined to four nations of western Africa. While four nations is not insignificant, three of these nations happen to share close borders with the epicenter of the outbreak. In brief, the rate of spread of this outbreak, though of grave concern, does not follow the pattern we’d expect of an airborne killer.

In light of low evidence for airborne transmission of Ebola, can we completely rule out the air as a pathway of exposure? No. And this is largely what has aroused suspicion and criticism among the public regarding statements made by the Center for Disease Control and Prevention (CDC). Before hastening to criticism, however, one must note that direct contact (versus airborne) as defined by the CDC includes large airborne droplets, such as those generated from sneezing and coughing. So when they are denying airborne exposure, they are not necessarily denying airborne exposure as perceived by the public. Their acceptance of the virus’s ability to spread through airborne droplets is in fact reflected in their 2005 medical guidelines for managing Ebola which identify the importance of eye and mouth protection. Definitions aside, it would make little sense for the CDC to risk panicking the nation by uttering the word “airborne”, particularly when such transmission accounts for the fewest of Ebola cases and that the media is known for making mountains out of molehills when it suits their interests.

A likely reason for low airborne transmission of Ebola is that the infection does not cause coughing, meaning the generation of infectious airborne particles from the lung is relatively low. This is in stark contrast to SARS which caused incessant coughing and spread rapidly to many countries. So if the virus hasn’t gone airborne, to what can we attribute the explosiveness of the current Ebola outbreak? Are such epidemics likely in the future? In my next blog I will discuss this topic as well as present an informative graph I created that really puts this current epidemic in perspective. See you soon!  

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                                                                                     -Shahir Masri, MS