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I don't know if this question has an answer at this time, but I would like to know if there is some biological reason why the current Ebola outbreak is so much more complicated and widespread than in the past. Traditionally it has been initially very virulent and has then has rapidly waned. I hope what I am saying is accurate, and if so why dont we see the same pattern now ?

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Can you provide some references/proof for your statements (e.g. fatality rates over time of the current outbreak), and also explain more accurately what you mean (e.g. complicated in what way)? –  fileunderwater Sep 4 '14 at 7:57

3 Answers 3

This is too long for a comment, so I put this in here: The main reasons are sociological. From the data I have read so far, this outbreak (actually these are two independent outbreaks, one in West-Africa and another one -not connected- in the Democratic Republic of Congo) is not exceptionally deadly in terms of Ebola. The death rate is about 60% which is extremely high compared to other diseases, but Ebola had reached up to 90%...

I think the main differences are that this outbreak struck in a much more populated area which has a very poor health care system (if you can call it like this at all). Then it happened in a border region, I recall reports about a traditional funeral of a Ebola victim which infected visitors, some of them crossed the border afterwards.

Then it was completely underestimated in the beginning, because a lot of cases stayed unknown. This led to a wider spreading as people who got sick infected relatives and medical personnel subsequently. And although the reaction has been powered up a lot since then, it is still not adequate to contain the outbreak.

Former outbreaks happened in rural areas which where easier to contain with a smaller number of personal, which is not possible this time. If you want to read further about this, I recommend the blog of Tara C. Smith which has been doing research about Ebola for some time and also worked in crisis response teams. Especially interesting are these two:

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You are correct that this seems to be two separate outbreaks, see who.int/mediacentre/news/ebola/2-september-2014/en –  fileunderwater Sep 4 '14 at 7:47

For some background, it is essential to know that Ebola is actually a group (genus) of ebolaviruses, each with different fatality rates. There are five known species of Ebola, and four are known to cause disease humans (WHO: Ebola virus disease; wikipedia).

The known species of Ebola includes:

  • Zaire ebolavirus (or just ebolavirus)
  • Sudan ebolavirus
  • Bundibugyo ebolavirus
  • Tai ebolavirus (only one known human case)
  • Reston ebolavirus (can infect but does not seem to cause disease in humans)

Zaire ebolavirus is the deadliest strain (with Sudan second), and also the one that is causing the current outbreak. A table of all known ebola outbreaks can be found in the WHO report (WHO: Ebola virus disease), which shows that the historical fatality of the Zaire strain has been between 44% and above 90%.

From my limited understanding, when dealing with ebola it is essential for infected individuals to be identified early so that they can get intensive care (most importantly intravenous fluids because of dehydration), but also to prevent further spread. This is probably what has failed during this outbreak, but another problem is also that the size of the outbreak has put tremendous strain on an already weak healthcare system. These outbreaks have also occurred in well populated areas in countries with very weak governments, that also lacked experience in dealing with ebola (both in the administration and among the population).

In many former outbreaks, the initial death rate is very high since early victims are found at late stages of the disease, but when the outbreak is more controlled the death rate drops due to supporting care of infected individuals. I don't know to what extent this has been the case during the current west african outbreak. However, the overall fatality rate of the current outbreak is 52% (tabulated Aug. 28 2014), with 44% in Sierra Leone and 66% in Guinnea (WHO: Ebola virus disease update - west Africa), and this lies well within the normal fatality rate of the Zaire ebolavirus.

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I know that it started out with a huge population base around 1k infected individuals and it mutated significantly in those subpopulations into around 100 different adaptations with about 300 total mutations. 300 is a huge number of new SNPs considering there are only 20 thousand nucleotides total. These have spread further and faster than normal because it reached cities or more dense regions of populace.

The 2014 Zaire Ebola virus outbreak is unprecedented both in its size and in its emergence in populated areas. Previous outbreaks had been localized mostly to sparsely populated regions of Middle Africa, with the largest outbreak reporting 318 cases in 1976. The present outbreak has manifested in more densely populated West Africa. Since it was first reported in Guinea in March, 2,127 cases have been reported, with 1,145 deaths, as of the end of last month.


“Although we don’t know whether these differences are related to the severity of the current outbreak, by sharing these data with the research community, we hope to speed up our understanding of this epidemic and support global efforts to contain it.”

-Harvard gazette

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You seem to be implying that the current outbreak is more severe due to evolutionary changes in the virus. Do you have anything to back this up (preferably not from the Harvard Gazette)? To show genetic changes is one thing (they will most often be there), to show a functional change is another thing. Otherwise this is pure speculation. –  fileunderwater Sep 4 '14 at 8:12
@fileunderwater you seem to be implying that the current outbreak is not more severe because of evolutionary changes. Do you have anything to support evolution not advancing the effectiveness of viruses? –  caseyr547 Sep 4 '14 at 8:52
@caseyr547 As far as I am aware, there is no proof that the mutations affect the efficiency of the virus in spreading and infecting people. It is possible that this happened (or will happen) but not yet proven. Or do you have a reference for this that I missed? –  Chris Sep 4 '14 at 8:57
@caseyr547 Sorry, I don't understand this comment? –  Chris Sep 4 '14 at 9:03
Survival of the fittest has no influence on the mutations per se. It has an effect on the survival - this may sound like nitpicking, but it makes a difference. For the selection process it is important, that a mutation has no negative effect, it can also be neutral (for the moment). As RNA viruses as Ebola generally have a high mutation rate, a number of mutations occuring with higher case numbers is something you would expect. The is the same for HIV-this is a zoonotic virus which was transfered humans a long time ago. It is not a dangerous as Ebola, so it got more time to adapt to its host. –  Chris Sep 4 '14 at 9:37

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