In case you missed it, over the past couple of days there have been reports of an outbreak of Ebola hemorrhagic  fever virus in Uganda. As of this writing, the most recent report I’ve seen puts the death toll at 16, with a few other suspected cases. Ebola is terrifying for a number of reasons – it’s readily transmissible, it has a remarkably high and rapid lethality (25-90% case fatality rate within days to weeks), and the way it kills is gruesome - causing massive bleeding from all orifices. There’s no vaccine or cure.

The good news from an epidemiology standpoint though, is that Ebola is a kinda terrible pathogen in humans. It has a short incubation time, and infected people are obvious (what with the bleeding from their eyes). Further, it’s not airborne, so you actually have to come into contact with bodily fluids from infected individuals to become infected yourself. Once an outbreak has been identified, health workers can take fairly simple precautions and dramatically reduce their risk of infection. What this means in a practical sense is that, after the first terrifying days of lots of sick people turning up, people know what to do to avoid infection, and the outbreak burns itself out. Since its discovery in 1976, Ebola has killed less than 2000 people, with the worst outbreak in 2000 killing 224. Contrast that with your garden variety influenza, which kills on average 40,000 people per year in the US alone, and hundreds of thousands more around the globe.

Remember, the goal of a pathogen is not to kill you. Your death is an unintended side-effect of the real goal: replication and transmission. On this front, Ebola sucks. It’s great at evading your immune system and replicating like crazy, but then it makes you terrifying to other potential hosts and incapacitates or kills you before you have a chance to spread it very far. The most successful pathogens strike a balance between replication and transmission. Influenza generally leaves you healthy enough to walk around and shake hands, press the elevator button at work, and use an ATM. Influenza doesn’t want you dead – the only reason it kills so many people is that so many more people become infected. Successful pathogens that make most of their victims really really sick or dead have more efficient ways to be transmitted that don’t require you to be healthy and mobile. Malaria has mosquitoes do the work of getting the infection mobile, cholera gives folks massive diarrhea to spread itself in water sources, Bacillus anthracis (anthrax) can form spores that survive time, heat and desiccation for decades while waiting for a new host.

So, Ebola is not really as scary as it seems. The really scary thing about this outbreak and others is that it reminds us that brand new diseases come out of previously isolated areas, and that globalization and urbanization means that the distance between a rural village in Africa and every major city on the planet is small and shrinking. This most recent outbreak of Ebola had victims in Kampala – the capital of Uganda. It’s fairly easy to imagine an international traveler hoping a plane from Kampala to Paris and then… Well, Ebola probably wouldn’t get much further for the reasons I already mentioned, but some other virus?

New infectious diseases generally pass into humans from animals, but they’re poorly adapted for our immune systems. Generally, this means that they either don’t replicate very well, or are difficult to transmit, or they’re super deadly and wipe out their hosts too quickly to be transmitted. The trouble with cities is that high population density lowers the bar on transmission, allowing more virulent bugs to make it to the big time. And rapid global transit means that any local outbreak has the potential to cause a global pandemic.

Comments

  1. #1 Tara C. Smith
    August 3, 2012

    “…the way it kills is gruesome – causing massive bleeding from all orifices.”

    Though this gets hyped, this manifestation is actually much less common than people think. Usually it kills due to multi-organ failure, not necessarily massive bleeding. Indeed, one reason this current outbreak went unrecognized initially was because of the lack of hemorrhagic manifestations.

    And while not Ebola, there are at least two known cases of people (unknowingly) infected with Marburg who were able to travel to Europe and the US. The US case actually didn’t get diagnosed until quite some time later via serology. So it could potentially get farther than you might think, since the incubation period can be fairly lengthy in some cases.

    • #2 Kevin Bonham
      August 3, 2012

      Fair point, but the fact that a hemorrhagic fever virus made it to the US without causing an outbreak exemplifies the point. The stuff is contagious, but contagion is preventable using fairly standard hygiene standards. My subconscious thoughts about viruses like Ebola are mostly exemplified by the movie “Outbreak,” where the doctors are in BSL4 type pressure suits and they have to napalm the village to keep the infection contained… Clearly, these viruses don’t quite rise to that level of terror.

  2. #3 Mike Olson
    August 3, 2012

    Okay, somewhat off topic, but, congratulations, you’ve encouraged me to check out a film I always went past. I’ve never really cared for Dustin Hoffman and always have trouble taking him seriously…there just always seemed something overly dramatic and strident about him I just found unappealing. Oddly, another actor I think is a real flake and have a hard time taking seriously is Tom Cruise. But, somehow watching Cruise portray a self-centered yahoo and Hoffman as a completely, utterly, otherwise brain damaged savant in, “Rainman,” was pretty cool. Somehow they both seemed to really fit the roles. At any rate, I’ll give, “Outbreak,” a shot. After I’m done with, “The Game of Thrones.” All props to HBO, Between, “The Wire,” “Treme,” and “Generation Kill,” and now, “Game of Thrones,” I’m seeing some exceptionally good drama in a media other than books. And honestly, the hardest good fiction to come by is always science fiction, hard science fiction or speculative fiction. People seem to prefer science as being similar to evil magic, selfish frankensteinian scientists, or irrelevant within the context of a story that is otherwise completely about science. The sailor with the coolest job, as far as I’m concerned and who had the best job as a career in the Navy was a guy named Shannon Putnam who last I knew was working, I believe in Indonesia on infectious diseases. Ebola is certainly a current, rather horrific manifestation of a process we need to really come to grips with. A war between humanity and viruses or microbiology sounds dramatic and cool, but the reality is we should probably concern ourselves with how we can adapt to organisms that adapt to utilize us as part of their life cycle. We can’t kill ‘em all and by doing so we’d lead to our own demise.

    • #4 Kevin Bonham
      August 4, 2012

      Mike, please don’t get me wrong, I never said “Outbreak” was a good movie… To be honest, I rather like Dustin Hoffman, but I don’t remember if that movie is any good (its possible that I watched it because I had an adolescent crush on Rene Russo – I was 11 when that movie came out)

  3. #5 Thomas
    August 4, 2012

    Ebola is doing so much right to make it a global killer, and every outbreak gives it new chances to mutate or acquire genes from other human pathogens to solve the remaining weaknesses.

    • #6 Kevin Bonham
      August 4, 2012

      What do you mean by “doing so much?” You’re absolutely right that outbreaks in humans give the disease a chance to mutate, but those mutations are just as likely to reduce virulence. In fact, Ebola would probably be scarier if it was a bit less deadly.

      I should be clear – I don’t mean to imply that Ebola is nothing to worry about, just that it doesn’t reach “OMG we’re all gonna die!!” territory

  4. #7 harold
    August 4, 2012

    Ebola is something to worry about – if you live in rural Uganda and are at high risk of becoming an early case in an outbreak.

    Ebola is something to worry about – in the sense that ALL viral pathogens could give rise to descendant strains that are more problematic, and all pathogens that cause severe symptomology could be deliberately spread.

    I don’t know how modern life support, therapy for coagualopathy, etc, impact Ebola mortality. I”m guessing the mortality rate is mainly among people with no access to intensive modern care for sequelae. Of course a hypothetical outbreak in a big city could cause intensive care capacity to be exceeded.

    As for Americans who are obsessively worried about Ebola, I hope they take strong precautions against lightening strikes, which are a far more serious risk in the US.

  5. #8 ArchTeryx
    Ann Arbor, MI
    August 5, 2012

    If you really want a good Hollywood illustration of a major emergent pandemic, watch Contagion. Outbreak is fluff; Contagion is as close to the real thing as Hollywood ever will get. (It’s pathogen is a Nipah-like virus with the transmissibility of measles…a virus that causes encephalitis and spreads throughout the world in a matter of days? Now THERE is a scary thought).

    Ebola is a fairly minor player. In epidemiology terms, it’s R-o makes it a terrible pandemic candidate (thank goodness!) after a few iterations. What people worry about is that it will become airborne, or at least take a sudden jump in infectivity, and thus turn itself into a pandemic. That’s always possible, I suppose, but not very likely. Like most enveloped viruses, it needs body fluids to stay viable and doesn’t spread all that well via fomites.

  6. #9 MuleHeadJoe
    Sack of Tomatos
    August 6, 2012

    I always thought that the book “The Hot Zone” by Richard Preston was the coolest story (generally regarded as non-fiction, however it was written very dramatically, like a thriller) … and although I can’t find any evidence to support this notion, I alway was under the impression that the movie “Outbreak” was primarily adapted from, or at least influenced by, THZ. I’ve probably seen Outbreak, but I don’t actually recall it for sure. Anyhow … if you haven’t read The Hot Zone, give it a whirl (decent synopsis on Wikepedia, the book is available on Amazon).

  7. #10 Rafael Science
    Brazil
    August 6, 2012

    Very interesting post, I admitted that the new Ebola spread is a new information for me. Do you have some interesting material and paper to support the phrase: ” The trouble with cities is that high population density lowers the bar on transmission, allowing more virulent bugs to make it to the big time ”

    Thanks