After posting my last commentary on the ongoing Ebola outbreak in West Africa, I listened to the netcast, This Week in Virology (www.twiv.tv), for September 14, 2014.  TWiV sessions, hosted by Vincent Racaniello, a well-known virologist at Columbia University, are generally highly informative, typically offering thoughtful discussions about recently published studies pertaining to viruses or addressing broad areas of virus-related research.The TWiV team took note of the New York Times op-ed (9/11/14) by Michael Osterholm and voiced substantial skepticism about Osterholm’s discussion of the possibility that Ebola virus might evolve the ability to spread through the respiratory route.  They fairly note that there is no documented case of a virus that infects humans changing its route of spread.  Furthermore, the TWiV discussants suggest that a mutation causing a major change in route of transmission might also lead to decreased virulence.

In a letter to the editor in the New York Times (2014) in response to Osterholm’s article, two microbiologists at Boston University School of Medicine similarly took issue with Osterholm’s concern about the possibility that, in the current outbreak, Ebola virus would mutate in ways that facilitate respiratory spread.  As Connor and Mühlberger note, “existing viral diseases like influenza, polio, hepatitis C and H.I.V. have not evolved to change their established route of transmission.”

Perhaps Osterholm’s key point can be re-framed as follows: the unprecedented size and context of the current outbreak provides expanded opportunities for exploration of Ebola virus genome sequence space.  In other words, with several-fold more infected people than in previous outbreaks of Zaire ebolavirus, there is an increased number of Ebola genomes that are replicating and generating progeny genomes with mutations.  Therefore, more variants are being produced and subjected to selection.

The broader concern then, is that one or another mutation will increase transmission or virulence (although the latter it is already incredibly high) through whatever mechanism.  A related set of concerns relates not to any changes in the virus itself but to the possibility that social, cultural, or other virus-extrinsic factors related to the circumstances of the current outbreak will tend to make the trajectory of the ongoing outbreak worse than might be predicted based on the experience with previous outbreaks.

In this context, a new study published online by Thomas House (2014), of Warwick University, provides an analysis of all completed Ebola outbreaks in terms of new introductions, case fatality ratio, and efficiency of person-to-person spread.  This study was based on an approach based on the time between outbreaks, number of deaths, and the final number of cases for 24 previous Ebola outbreaks.

While I cannot critically assess the mathematical details of the analysis, on the assumption that the approach is reasonable, the conclusions are of interest.  As quoted by Alice Robb in The New Republic (2014), House states that ““What the results seem to indicate is that it isn’t just an extreme event”.”  And, ““It’s becoming more and more likely that there’s something different this time.””

What could be different is either the intrinsic ability of the virus to spread, a quantity known as the basic reproduction ratio, R0, (the average number secondary cases per primary case in the initial phase of an outbreak) or the effective reproductive ratio, Rt, that takes account of both R0 and other virus-extrinsic factors, such as population density, the behaviors of ill individuals and their close associates, and the effectiveness of medical interventions.  It may be of value to determine more fully and quantitatively which if any of these factors is contributing to the size and scale of the current Ebola epidemic.  Nevertheless, the most important priority is probably to assemble, as quickly as possible, sufficient personnel and resources to implement the relatively low-tech measures that were shown to be effective in controlling previous Ebola outbreaks.

References

Racaniello V, Despommier D, Dove A, Condit R, and Spindler K. TWiV 302: The sky is falling. September 14, 2014. www.twiv.tv (last accessed on 9/21/14)

Osterholm MT. What we’re afraid to say about ebola. New York Times, September 11, 2014. http://www.nytimes.com/2014/09/12/opinion/what-were-afraid-to-say-about-ebola.html?ref=opinion (last accessed on 9/12/14)

Connor JH, Mühlberger E. New York Times, September 27, 2014. http://www.nytimes.com/2014/09/27/opinion/transmission-of-ebola-two-microbiologists-weigh-in.html?ref=opinion

House T. Epidemiological dynamics of Ebola outbreaks. eLIFE 2014;10.7554/eLife.03908 doi: http://dx.doi.org/10.7554/eLife.03908

Robb A. Ebola is spreading way faster than anyone predicted.  The New Republic, September 19, 2014. http://www.newrepublic.com/article/119513/ebola-spreading-unprecedented-speed (last accessed on 9/21/14)