How lethal is ebolavirus?

Ebola seropositivity GabonAfter we discussed newly discovered entry factors for ebolavirus and hepatitis C virus on TWiV 166, the New York Times covered part of the story in Key protein may give Ebola virus its opening. Given my recent interest in the case fatality ratio of avian influenza H5N1, I was intrigued by the following introductory statement:

Of the pathogens that keep worried scientists awake at night, few rival Ebola for ruthless efficiency. The virus contains just seven genes, yet it manages to kill up to 90 percent of the people it infects.

Is it true that the fatality rate of ebolavirus is ‘up to 90 percent’? According to the WHO page on Ebola haemorrhagic fever,

Zaïre, Sudan and Bundibugyo species have been associated with large Ebola haemorrhagic fever (EHF) outbreaks in Africa with high case fatality ratio (25–90%) while Côte d’Ivoire and Reston have not. Reston species can infect humans but no serious illness or death in humans have been reported to date.

There have been roughly 1850 recorded cases with over 1200 deaths since ebolavirus was discovered, an average fatality rate of 65%. But have there been only 1850 human infections?

The answer is clearly no. The results of several serological surveys have shown that many individuals have antibodies against Zaire ebolavirus – purportedly the most lethal. The results of one study revealed antibodies in 10% of individuals in non epidemic regions of Africa. A similar seroprevalence rate (9.5%) was reported in villages near Kikwit, DRC where an outbreak occurred in 1995. In addition, a 13.2% seroprevalence was detected in the Aka Pygmy population of Central African Republic. No Ebola hemorrhagic fever cases were reported in these areas.

A more recent study examined sera from 4,349 individuals in 220 villages in Gabon. Antibodies against Zaire ebolavirus were detected in 15.3% of those tested, with the highest levels in forested regions (see map). The authors believe that the seropositive individuals had mild or asymptomatic ebolavirus infection:

The high frequency of ‘immune’ individuals with no disease or outbreak history raises questions as to the real pathogenicity of ZEBOV for humans in ‘natural’ conditions.

These findings indicate that the fatality rates of Zaire ebolavirus that are quoted widely are likely to be vast overestimates. Why the infection is more lethal during outbreak conditions is not known. One possibility is related to the size of the viral inoculum received. During outbreaks the virus is spread by contact with the blood, secretions, organs or other body fluids of infected individuals, which contain very large quantities of virus. In contrast, infections in nature – by contact with contaminated fruit, for example – may involve far less virus.

Whether we are discussing avian H5N1 influenza, ebolavirus, or even the fictitious MEV-1, do not assume that widely quoted fatality rates are correct – check the scientific literature!


Should we fear avian H5N1 influenza?

Becquart P, Wauquier N, Mahlakõiv T, Nkoghe D, Padilla C, Souris M, Ollomo B, Gonzalez JP, De Lamballerie X, Kazanji M, & Leroy EM (2010). High prevalence of both humoral and cellular immunity to Zaire ebolavirus among rural populations in Gabon. PloS one, 5 (2) PMID: 20161740

Comments on this entry are closed.

  • Alan Dove

    Besides the inoculum size, the entry route might matter. Outbreak cases are often family members and healthcare workers who’ve had direct contact with the blood and other body fluids of patients with severe disease. Perhaps the “natural” route of infection involves oral or respiratory entry of animal body fluids instead.

    In any case, the literature on this virus, H5N1 flu, and some other “nightmare” viruses suggests that we need more seroprevalence studies on all of these pathogens. If it turns out that the really severe outcomes are freak incidents that don’t represent the normal course of infection, then we should try to determine the factors that push the disease one way or another, and aim treatment and prevention efforts there.

  • Rustling

    SOPA and PIPA – Learn more
    From Wikipedia, the free encyclopedia< Wikipedia:SOPA initiativeWhat effect is the blackout having, so far?The purpose of the blackout is twofold: to raise awareness of SOPA and PIPA among the general public, and to encourage people to share their views with their representatives. It's too early to tell what the ultimate impact will be, but here are some early indicators, as of 1PM PT January 18:More than 10,000 people have commented on the Wikimedia Foundation's blog post announcing the blackout. We have not done a content analysis, but at-a-glance it looks like the overwhelming majority support the blackout;Almost immediately after the blackout launched, it became a trending topic on Twitter, globally, with hashtags including #factswithoutwikipedia, #SOPAstrike and#wikipediablackout. Trendistic says at one point, #wikipediablackout constituted 1% of all tweets, and Hotspots shows that SOPA has accounted for a quarter-million tweets hourly since the blackout began;Google News contains 7,200 articles on the blackout;More than 90 million people have seen the Wikipedia blackout pageMore than five million people have looked up their elected representatives' contact information via the Wikipedia tool.Why is Wikipedia blacked-out?Wikipedia is protesting against SOPA and PIPA by blacking out the English Wikipedia for 24 hours, beginning at midnight January 18, Eastern Time. Readers who come to English Wikipedia during the blackout will not be able to read the encyclopedia. Instead, you will see messages intended to raise awareness about SOPA and PIPA, encouraging you to share your views with your representatives, and with each other on social media.What are SOPA and PIPA?SOPA and PIPA represent two bills in the United States House of Representatives and the United States Senate respectively. SOPA is short for the "Stop Online Piracy Act," and PIPA is an acronym for the "Protect IP Act." ("IP" stands for "intellectual property.") In short, these bills are efforts to stop copyright infringement committed by foreign web sites, but, in our opinion, they do so in a way that actually infringes free expression while harming the Internet. Detailed information about these bills can be found in the Stop Online Piracy Act andPROTECT IP Act articles on Wikipedia, which are available during the blackout. GovTrack lets you follow both bills through the legislative process: SOPA on this page, and PIPA on this one. The Electronic Frontier Foundation, a non-profit organization dedicated to advocating for the public interest in the digital realm, has summarized why these bills are simply unacceptable in a world that values an open, secure, and free Internet.Why is the blackout happening?Wikipedians have chosen to black out the English Wikipedia for the first time ever, because we are concerned that SOPA and PIPA will severely inhibit people's access to online information. This is not a problem that will solely affect people in the United States: it will affect everyone around the world.Why? SOPA and PIPA are badly drafted legislation that won't be effective at their stated goal (to stop copyright infringement), and will cause serious damage to the free and open Internet. They put the burden on website owners to police user-contributed material and call for the unnecessary blocking of entire sites. Small sites won't have sufficient resources to defend themselves. Big media companies may seek to cut off funding sources for their foreign competitors, even if copyright isn't being infringed. Foreign sites will be blacklisted, which means they won't show up in major search engines. And, SOPA and PIPA build a framework for future restrictions and suppression.Does this mean that Wikipedia itself is violating copyright laws, or hosting pirated content?No, not at all. Some supporters of SOPA and PIPA characterize everyone who opposes them as cavalier about copyright, but that is not accurate. Wikipedians are knowledgeable about copyright and vigilant in protecting against violations: Wikipedians spend thousands of hours every week reviewing and removing infringing content. We are careful about it because our mission is to share knowledge freely. To that end, all Wikipedians release their contributions under a free license, and all the material we offer is freely licensed. Free licenses are incompatible with copyright infringement, and so infringement is not tolerated.Isn't SOPA dead? Wasn't the bill shelved, and didn't the White House declare that it won't sign anything that resembles the current bill?No, neither SOPA nor PIPA is dead. On January 17th, SOPA's sponsor said the bill will be discussed in early February. There are signs PIPA may be debated on the Senate floor next week. Moreover, SOPA and PIPA are just indicators of a much broader problem. In many jurisdictions around the world, we're seeing the development of legislation that prioritizes overly-broad copyright enforcement laws, laws promoted by power players, over the preservation of individual civil liberties.How could SOPA and PIPA hurt Wikipedia?SOPA and PIPA are a threat to Wikipedia in many ways. For example, in its current form, SOPA would require Wikipedia to actively monitor every site we link to, to ensure it doesn't host infringing content. Any link to an infringing site could put us in jeopardy of being forced offline.I live in the United States. What's the best way for me to help?The most effective action you can take is to call your representatives and tell them you oppose SOPA and PIPA, and any similar legislation. Type your zipcode in the locator box to find your representatives' contact information. Text-based communication is okay, but phone calls have the most impact.I don't live in the United States. How can I help?Contact your local State Department, Ministry of Foreign Affairs, or similar branch of government. Tell them you oppose SOPA and PIPA, and any similar legislation. SOPA and PIPA will affect sites outside of the United States, and actions to sites inside the United States (like Wikipedia) will also affect non-American readers — like you. Calling your own government will also let them know you don't want them to create their own bad anti-Internet legislation.Is it still possible to access Wikipedia in any way?Yes. During the blackout, Wikipedia is accessible on mobile devices and smart phones. You can also view Wikipedia normally by disabling JavaScript in your browser, as explained on this Technical FAQ page. Our purpose here isn't to make it completely impossible for people to read Wikipedia, and it's okay for you to circumvent the blackout. We just want to make sure you see our message.I keep hearing that this is a fight between Hollywood and Silicon Valley. Is that true?No. Some people are characterizing it that way, probably in an effort to imply all the participants are motivated by commercial self-interest. But it's obviously not that simple. The proof of that is Wikipedia's involvement. Wikipedia has no financial self-interest at play here: we do not benefit from copyright infringement, nor are we trying to monetize traffic or sell ads. We are protesting to raise awareness about SOPA and PIPA solely because we think they will hurt the Internet, and your ability to access information online. We are doing this for you, because we're on your side.In carrying out this protest, is Wikipedia abandoning neutrality?We hope you continue to trust Wikipedia to be a neutral information source. We are staging this blackout because (as Wikimedia Foundation Trustee Kat Walsh said recently), although Wikipedia’s articles are neutral, its existence is not. For over a decade, Wikipedians have spent millions of hours building the largest encyclopedia in human history. Wikipedia is a tremendously useful resource, and its existence depends upon a free, open and uncensored Internet. SOPA and PIPA (and other similar laws under discussion inside and outside the United States) will hurt you, because they will make it impossible for sites you enjoy, and benefit from, to continue to exist. That's why we're doing this.I have a question that isn't answered here, or, I would like to send feedback to Wikipedia.You can reach Wikipedia editors at info-en(at)wikimedia(dot)org. If you need a response, please be patient: we may have trouble keeping up with the mail.What can I read to get more information?Try these links:Wikipedia's articles on SOPA and PIPAStatement from Wikipedia editors announcing decision to black outWikimedia Foundation press releaseBlog post from Wikimedia Foundation Executive Director Sue GardnerElectronic Frontier Foundation blog post on the problems with SOPA/PIPAAs of 6AM PT, January 18, Google has more than 4,600 articles about the blackout. Here are a few:Why is Wikipedia staging a blackout and what is SOPA?, from the National PostWikipedia joins blackout protest at US anti-piracy moves, from the British Broadcasting CorporationWikipedia blackout over US anti-piracy bills and FEATURE: Websites blackout over 'SOPA censorship', from Al JazeeraWikipedia, Craigslist, other sites go black in SOPA protest, from the Los Angeles TimesGoogle Rallies Opposition to Murdoch-Backed Anti-Piracy Bill, from BusinessWeekSOPA protest: The Net strikes back, from PoliticoWikipedia blackout a 'gimmick', MPAA boss claims, from the GuardianWikipedia 24-hour blackout: a reader and Why we're taking Wikipedia down for a day, from the New StatesmanInternet-wide protests against SOPA/PIPA are kicking up a storm, by the Hindustan TimesSOPA, PIPA: What you need to know, from CBS NewsProtest on Web Uses Shutdown to Take On Two Piracy Bills, from the New York TimesProtesting SOPA: how to make your voice heard, from Ars TechnicaWhy We've Censored, from Wired

  • Brian Hanley

    Hmm. Remember a case report in South Africa of a patient who contracted Ebola, apparently from a room in a hospital where a previous patient had died of Ebola. The room had been decontaminated with bleach and left vacant for, I believe, 3 months prior. That inoculum could not have been large.

    Perhaps the virus has multiple variants that present the major antigens?

  • Guest


  • md

    Vast overestimate? Well, if the “up to 90%” case fatality rate holds true for outbreak conditions then it isn’t a vast overestimate. If you talking about non-outbreak conditions, then obviously that’s different. Something works to change the virus’ virulence in outbreak conditions.

  • There is no evidence that viral virulence is different in outbreak vs non-outbreak conditions. You are assuming this incorrectly.

  • Pingback: Diseases That Threaten Humanity | End Time Bible Prophecy()

  • Emilia Hazel

    Agreed… To me that, and the ‘transmission route’ making a difference does sound very unlikely as it effects the entire body rather than just whatever part it enters through.

  • Emilia Hazel

    Perhaps outbreaks are more likely to occur in populations that do not have a large occurrence of these genes that cause immunity or partial-immunity. It’s certainly strange that 15% of people in areas that appear to have never had an outbreak of Ebola have any traces of it in them at all.

  • Qui

    given that ebola is a RNA virus, could the difference between outbreak and asymptomatic infection be due to a small mutation that is influential enough to interfere with some of the processes, but not enough to interfere with something like the tropism of the virus?

  • Pingback: How lethal is Ebola virus? | Virology and Bioin...()

  • Pingback: Why Ebola’s nothing to worry about: The disease the media can’t live without |()

  • Z

    Considering the possibility that many people who died from Ebola infection in the poor countries were never given a correct diagnosis, the fatality rate may be under estimated.

  • GreyFool

    No doubt someone else will have asked this, but what is the unchecked morbidity rate for ebola? In other words, without any medical or sanitation or PPE or quarantine controls, what % of the population would contract ebola? Is it only folk with natural immunity who would be spared? I can’t find the answer with searches or on the news.

  • William Smith

    There is also the obvious factor of not receiving care needed to survive the worse during an epidemic. With most people sick the few remaining have far more to do and far less resources to do it with. If one person out of twenty is not sick they are responsible for giving the sick all their water, removing the dead, feeding the sick, bandaging all their wounds; while if the reverse is true then there are twenty individuals left to care for the single person ill.

  • William Smith

    Same holds true for risk of contagion as well.

  • Pingback: Is the Ebola virus mutating? | Cosmoso - Explore Science & Technology()

  • Pingback: Ebola may be mutating: don’t panic | netgueko()

  • Pingback: Explainer: is the Ebola virus mutating? | Scroll CMS()

  • Pingback: Is Ebola really mutating? | netgueko()

  • Pingback: Is the Ebola Virus mutating? | Health and medical info in Kigali()

  • Pingback: Explainer: is the Ebola virus mutating?()