Ebola Virus causes a nasty disease. The current Ebola Virus Disease (EVD) epidemic in Western Africa is running almost unchecked through a population ill-prepared to counter it.
It is unlikely that EVD will cause a global pandemic. The progression of the EVD epidemic is slow, and those who are sick are easily contained. There is also an effective vaccine on the near horizon. The EVD epidemic will be stopped; my notoriously unreliable crystal ball tells me it will be fully contained by next summer. Unfortunately it is out of control now, and it is likely that tens of thousands of people will die of EVD in Western Africa before it is stopped.
It is so likely that tens of thousands will die that it is possible to assign a statistical confidence to that prediction. Simply plotting the reported numbers yields a trace that displays an uncanny adherence to an exponential growth rate. Through a statistical process known as regression analysis one can generate an average trace of the data (shown as the black lines in the following chart).
|Total cases and mortality data from WHO-reports as of 5 October 2014 Click to view larger|
The mechanism for generating the average trace also generates a statistic –called “R-squared”- that tells us how well the average line describes the data. An R-squared value of 0 would mean that the trace described none of the data, and an R-squared value of 1 would suggest that the average line described 100% of the variability of the data. The R-squared value I generate for the average trace of the mortality data from the last several months of the EVD epidemic is 0.9951.
An R-squared value of 0.8-or-so for an exponentially increasing average line would strongly suggest that the epidemic was out of control. An R-squared of 0.9951 suggests that none of the response measures implemented to date have had any measurable effect on the progression of the epidemic. There is no fine detail in this trace, and it may be that unexpected setbacks have perfectly offset any effective countermeasures. However you look at it these data are very “not good” in a bad way.
The trace also provides a strong reason not to worry about this EVD epidemic breaking out and creating a global pandemic. The upward “slope” of the trace is just not steep enough. Almost a hundred years ago today, peaking in October and November of 1918, the Spanish influenza epidemic swept through the world’s population. In just a few months it infected as many as half a billion people; killing as many as 60 million. By the time the Spanish influenza epidemic was over it infected as many as 800 million people and killed as many as 100 million; all between January 1918 and December 1920. EVD is not fast enough to outpace the world’s response in the way that Spanish Influenza did.
Notice, in the above description of the Spanish Influenza epidemic, my use of wishy-washy soft-data language. I use “as many as” four times. This is because we do not really know how many people died of the Spanish Influenza. In 1918 the social structure needed to even count the people dying became too overburdened to operate effectively. The lower number of people killed worldwide is half as large as the maximums I provided. There are mass graves.
The social machinery which counts the number of EVD deaths in Western Africa is showing signs that it is being pushed beyond its breaking point. There is informed speculation that as many as several EVD cases actually exist for every case reported. There have been reports of bodies left to rot in the street, or forgotten to ripen in some back bedroom of some apartment.
The World Health Organization (WHO) was, until September, providing disease outbreak news every couple of days; until some WHO healthworkers caught EVD, and died. Now they are providing Situation reports every two-to-six days; sometimes significantly revising the numbers of previous reports. Without a means of protection at least as effective as a good vaccine some of the first to succumb to an epidemic are the people needed to properly track the epidemic’s progress. With a super-rapid epidemic like Spanish Influenza in the informationally-challenged early 20th century the obstacles to effective data management must have been insurmountably huge.
To understand why EVD is less contagious, and therefore less likely to cause a global pandemic, than a disease like Spanish Influenza it is worthwhile to look at the basic biology of viruses.
Viruses are not really alive. They are essentially a sticky capsule of bad instructions. The instructions essentially say “make more virus”, and the sticky outside of the capsule cause it to attach to cells that can load and run the bad instructions. In some cases, like for HIV, the instructions say something like “make more virus slowly for a long time”; for viruses like influenza and Ebola the instructions say “make more virus as fast as you can until you explode”. The sticky capsule can be specific for certain types of cells. The tiny Polio virus capsules preferentially attach to nerve cells, and so the polio disease is characterized by paralysis. Influenza capsules attach to cells in the throat, and so the virus is shed into the phlegm that is converted into airborne droplets when someone with the Flu coughs or sneezes.
Ebola virus attaches to many different cells in the body. As it circulates through the body in the bloodstream it is presented first to capillary walls. When the thin capillary walls are weakened by the liquefaction of cells that died while furiously making new virus the capillaries themselves begin to leak. At this early stage of infection the body mobilizes defenses which cause the temperature of the body to rise. It is possible for a person who has been properly exposed to Ebola to fight off the disease in this early stage, before it manages to cause much damage; otherwise exploding capillary cells continue to spew more and more virus into the bloodstream, and the infection spirals out of control.
Most of the noticeable symptomology of hemorrhagic diseases like EVD occur in places where there are large concentrations of capillaries. Most EVD patients die in this intermediate stage of failures in capillary-rich internal organs, like kidneys. Given luck, or competent supportive care, an EVD patient can live into the final stages of the disease.
The fact that only about 5% of western African patients are surviving to the spectacular terminal phase of EVD probably suggests a lack of comprehensive supportive care. The first active Ebola case in the US (Thomas Eric Duncan ) received such comprehensive supportive care that relatives declined to view him via remote video (he was in an isolation unit) because it was “too disturbing”.
In the terminal stages the capillaries become completely porous, and begin leaking blood as well as fluid. The sclera of the eye fills with blood; there may be blood tears. Blood leaks through the capillary-rich intestinal linings into the digestive tract, and from there gushes out the rectum; the patient uncontrollably vomits blood. Blood begins to ooze from mucous membranes; the gums bleed, the nose bleeds, the genitals will bleed. The skin itself will begin to boil in blood-filled blisters. Grape-sized blisters will erupt from the skin, and burst to release virus laden bloody fluid.
All of these copious fluids are infectious. They are not easily handled. Sticky virus-laden goo gets in everything. Untrained individuals can get it in their hair, and noses, and in their car, and bring it home, and all without noticing. It is unsurprising that EVD spreads; even without the formation of the respiratory droplets which worked so well for Spanish Influenza.
The voraciousness with which viruses like Influenza and Ebola eat through their target cells actually makes it more likely that effective vaccines can be made against them. Good vaccine candidates have already been created for Ebola. Ramping up production of at least one of those candidates will only take months rather than years. It will likely be the fielding of a vaccine in the first half of 2015 that will end this epidemic.
The fielding of the vaccine will stop the epidemic in its tracks. Over a hundred thousand corpses will likely be left in its wake, but it will be impossible for any future EVD epidemics of the Zaire strain to reach the numbers we see even today.
So, should you be scared of Ebola? Yes. Ebola causes a terrible-horrible-scary disease which will kill tens of thousands of people in the coming weeks. That sort of thing is scary. It scares me.
Will Ebola destroy civilization as we know it? No. Not even close.