“Is Ebola virus going to kill me?”

If you’re asking that question, this is for you.

John Skylar, PhD
Jul 31, 2014 · 12 min read

“Ebola virus EM” by CDC/ Dr. Frederick A. Murphy — This media comes from the Centers for Disease Control and Prevention’s Public Health Image Library (PHIL), with identification number #1833.

Note: I’ve organized this primer on the 2014 Ebola epidemic to work for people who want to read it all, or who just need to scroll through for one answer—whatever works best for you.

The Epidemic: Basic Facts

The largest outbreak of Ebola virus in history is currently spreading across a few countries in West Africa.

This epidemic began in the country of Guinea in March, though there is some conjecture that sub-epidemic transmission began in December 2013 with a toddler who may be the “patient zero” for this epidemic. Since then, the outbreak has spread to Sierra Leone and Liberia, both on the coast. A few cases have been detected in Nigeria, all of which stem from an infected person who had just stepped off a plane arriving from Liberia. Nigeria does not share a border with any of these countries.

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The World Health Organization (WHO) reports that through 19 August 2014, there were 2240 confirmed cases of Ebola Viral Disease (EVD), and of these, 1229 patients died. That gives a case fatality rate of about 55%.

How Does This Compare to Other Ebola Epidemics?

The virus in this epidemic has been identified as a close relative of Zaire ebolavirus (EBOV). EBOV has been responsible for 16 EVD epidemics in the last 38 years. This is the first time that EBOV has caused an epidemic outside of sub-Saharan Africa, and it is also the largest epidemic of EVD of all time.

Other EBOV epidemics have had case fatality rates between 47% and 90%, so the 56% case fatality rate seen here is near the low end. Dr. Stephan Monroe of the CDC had this to say:

The mortality rate in some outbreaks can be as high as 90 percent, but in this outbreak, it is currently around 60 percent, indicating that some of our early treatment efforts may be having an impact.

Keep in mind that case fatality rates in small epidemics are often higher than the real percentage of people who die of a virus. Not all patients who get sick go to a hospital, and usually the ones who do have worse disease. This means that sometimes only the sickest people get into the count, making the official fatality rate look higher than the actual odds that a given patient will die.

As of right now, this epidemic is definitely headline-worthy. This doesn’t mean you need to be afraid of it—we’ll come to that in a moment.

Ebola Virology & Disease

EVD, Ebola virus disease, is caused by some ebolaviruses. One of these, Zaire ebolavirus (EBOV), appears to be the cause of the current epidemic in Western Africa. There are other viruses in the genus ebolavirus which also cause disease, such as Sudan ebolavirus (SUDV). Not all ebolaviruses cause disease in humans.

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EBOV Virology

EBOV is what is called a “filamentous, enveloped, nonsegmented negative-sense RNA virus.” That means it looks like a long filament, is covered in an envelope or membrane taken from the cell membrane of its host, and that it has a long, single molecule of RNA that contains its whole genome.

Our genome is made of DNA, but we use RNA to communicate messages from the genome to the cell factories that build our bodies. For ebolaviruses, both the message and the genome are made of RNA. The genomes of ebolaviruses are what’s called “negative-sense,” which means that the RNA of ebolaviruses has to be copied by the virus into “messenger” RNA in order to force the host cell to start making virus components. If that is confusing, the basics of RNA, DNA, and protein are available in this adorable animated video. Also, this website will take you through the experiments that proved these things to be true.

Scientists are still working to understand exactly how EBOV gets into cells, and how it makes people get sick with EVD. Still, we know that EBOV has a set of host proteins that it needs in order to enter cells, one of which (“Niemann-pick C1,” or NPC1) is part of a normally harmless route that cells use to process cholesterol.

How EBOV Causes Death

EBOV infects the cells that line human blood vessels, as well as some immune cells and liver cells. The infection of circulatory and immune cells is thought to contribute to the disease. By killing immune and circulatory cells, scientists think EBOV makes them release signals that activate a strong and sudden immune response that can cause shock and tissue breakdown. The details are somewhat more complicated, but this is meant to be a brief introduction to the basics of the disease caused by this virus.

The disease itself begins with flu-like symptoms which become more and more severe. Eventually, these symptoms give way to central nervous system problems like confusion, agitation, headaches, seizures, or even coma. As symptoms worsen, rashes and bruising can appear. In patients with the worst prognosis, hemorrhaging starts to occur. In other words, the patients who are most likely to die start bleeding internally, and sometimes externally.

Though this is an alarming and harmful symptom, it is not usually the main cause of death. The shock produced by EBOV infection causes problems with organ systems as cells that line blood and lymph vessels die, and fluid builds up in the body. This multi-system organ failure is generally the direct cause of death.

How do people catch EBOV?

EBOV and other ebolaviruses, are now thought to originate in bats. Bats can pass the virus to other species, and by some unknown route it sometimes enters human populations. The CDC has released a graphic which explains how this works.

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There are a couple of points here that are worth mentioning. While this image calls Zaire ebolavirus “Ebola virus,” this isn’t how the International Committee on the Taxonomy of Viruses actually recommends you refer to the virus, which is why this article does not follow the CDC convention. Also, the “(non-human)” note on Reston ebolavirus is not entirely accurate. Reston ebolavirus does not cause disease or death in humans, but it can still infect humans. Reston ebolavirus, for this reason, could potentially be used to develop ebolavirus vaccines.

That said, this is the general way that ebolaviruses get into human populations. From there, they can spread person to person, which is one of the reasons that ebolavirus epidemics are worrisome—viruses that can spread directly between humans are harder to contain in a world where international travel is common. Remember that in this epidemic, there has already been one case where a man sick with EVD got onto a plane and flew to Nigeria.

Luckily, EBOV does not spread through the air, and that limits the danger of widespread human infection. EBOV spreads through contact with bodily fluids from an infected person. This means the virus can spread among families, healthcare practitioners, and other people who have close contact with the ill. It also means that EBOV can be spread by sharing of needles, which is sometimes still practiced in some areas of Africa where syringes and needles are in short supply. Another route for infection is sexual transmission. In fact, EBOV can be transmitted by an infected survivor to sexual partners for up to two months after infection—even if disease has subsided.

However, the requirement for fluids has one advantage. In general, EBOV-infected people cannot spread the virus until they start showing symptoms, because they are not producing large amounts of virus-laden fluids until they get sick. That’s good news.

UPDATE: But I read something about Ebola virus transmitting in the air?

There is a lot of misinformation going around about this. The virus that is currently causing disease in West Africa is not airborne and is very unlikely to become so.

Some of the misinformation is based on evidence that Reston ebolavirus, which does not cause disease in humans, can be transmitted through the air. Reston ebolavirus, discussed above, is a very different virus from EBOV. If EBOV were to become more like Reston ebolavirus, it would mean mutating to become more like a virus that doesn’t cause disease in humans.

Another element to this rumor comes from a paper published in 2012. In that experiment, pigs were infected with EBOV and then rhesus macaques (monkeys) were exposed to those pigs in a setting where they could not touch each other. All of the monkeys became ill.

However, these monkeys were unable to infect other monkeys, and the authors themselves suggested that there were other possibilities for how they became infected than just transmission through the air:

Under conditions of the current study, transmission of [Z]EBOV could have occurred either by inhalation (of aerosol or larger droplets), and/or droplet inoculation of eyes and mucosal surfaces and/or by fomites due to droplets generated during the cleaning of the room.

This is a fancy way to say that it’s possible that fluids from the pigs got onto surfaces in the room and were stirred up by cleaning, and this is how the monkeys got infected.

Another important point is that pigs produce what the authors call “large droplets” in their breath, something which humans and monkeys do not do. It is likely that if there was in fact air-based transmission of EBOV in this setting, it had something to do with pigs’ status as efficient transmitters of virus in large respiratory droplets. In the past, pigs have transmitted deadly viruses in this way. This hypothesis is supported by the fact that the monkeys, which do not have this large droplet size, were unable to transmit the virus to one another by aerosol.

As a result, pig-rearing farms are considered a potential threat for the transmission of new viruses. The first outbreak of Nipah virus, which I personally study, was caused by transference of the virus from bats to pigs on a farm, and eventually to humans. This study on EBOV transmitting from pigs to monkeys was done to determine whether or not pigs could transmit EBOV in a farm setting.

And that is all we can take home from that study: that pig farms can incubate EBOV and potentially transmit it to other animals. It does not indicate that EBOV can be transmitted through the air between humans, and this has never happened. EBOV only transmits between people through contact with fluids from infected people, including blood, mucus, and sexual fluids.

UPDATE: What about the Ebola patients who have been brought back to the US?

There are two US citizens who have become infected with Ebola virus while working in the outbreak area. They have been returned to the United States, to a hospital in Atlanta operated by Emory University and are being treated in collaboration with the CDC.

A lot of people are understandably worried about bringing Ebola patients to the US. For this reason, the CDC and Emory have gone to great lengths to make sure that no one will be in danger. The hospital in Atlanta has a special isolation unit that is intended for dealing with these types of dangerous diseases, and medical containment technology these days is sophisticated and effective. No one is being put at risk by bringing these sick workers home.

There have been some people loudly complaining about this putting US citizens in danger and saying the pair should have been left in Africa, where it’s quite possible they would die due to a lower standard of care. Donald Trump, in particular, has been calling for this. Particularly as there is no danger of the virus spreading in the US, Mr. Trump’s perspective would do little but put the two patients’ lives at risk.

The pair are both medical professionals who were working abroad in a clinic run by missionaries. They are people who put themselves at risk in order to help others, and they contracted Ebola virus as a result of that. One of the things that the US generally guarantees our citizens who work in this kind of international aid is that if something happens, we will do everything we can to get them home. They deserve the highest standard of care, and they will be getting it in a setting where their infection poses no risk to the general population.

So, should I be worried about the epidemic spreading to the US or other countries outside Africa?

In short, no.

Don’t believe me? Here’s what the CDC has to say:

EVD poses little risk to the U.S. general population at this time. However, U.S. healthcare workers are advised to be alert for signs and symptoms of EVD in patients…who have a recent (within 21 days) travel history to countries where the outbreak is occurring.

In other words, unless you have recently been to Guinea, Liberia, or Sierra Leone, the CDC does not think you are at risk from this epidemic.

The reason is that EVD makes it difficult for patients to move around, and it has a limited window of time during which it can spread to other people. That is not good news for the people who are already infected, but it does mean that those of us in countries that are far from the epidemic are not really at risk.

That does not mean it would be impossible for EBOV to escape these countries and spread across Africa or even to other continents. Viruses are capable of that. However, preventing that sort of thing is why the CDC and WHO exist, and both organizations, as well as Doctors Without Borders and other groups, have sent teams to the epidemic region who are helping to contain the virus. In Liberia, travelers exiting the epidemic region are being screened at the airport to prevent the disease from spreading abroad.

While Dr. Monroe of the CDC maintained that EBOV is not likely to come to the US and poses little risk to US citizens, he went further to say that the US would be better equipped to contain a case of EVD if one were to come to the United States, and it would be unlikely to develop into an epidemic even then:

While it’s clear there is an increased risk for working with patients with Ebola, we’re confident that the standard of care in the U.S. would prevent much of the transmission if a case were to show up here.

At this time, CDC has issued a Level 3 Travel Advisory for the countries that have cases of EVD in this epidemic. There are three levels of travel advisory, as explained by Marty Cetron of the CDC:

…level one is a “watch,” which is “practice usual precautions around outbreaks.” Level two is an “alert,” which is enhanced precautions…to populations at risk, such as health care workers or humanitarian aide workers…level three is a “warning,” which is to avoid nonessential travel if you don’t have a reason to be there.

Right now, the CDC seems to think that the epidemic is contained, and the only people who need to worry about it are those who are in the epidemic zone, or who may be traveling to that zone in a capacity that would cause them to interact with sick people. At the end of the day, this epidemic is a tragedy for patients and their families, but most people, even within the affected countries, are safe and are likely to remain so.

This is a major event in the history of global health, but just the same, it’s not the end of the world.

UPDATE: As of today, 1st August 2014, the CDC has raised its travel advisory for the epidemic area to Level Three. This is the highest level, and it says to avoid “nonessential” travel to the epidemic area. Think very carefully before you travel to this part of the world, until the travel advisory is lifted. I have updated the article to reflect this new condition.

UPDATE: 8 August 2014—New outbreak map added, new Nigeria cases included in article.

UPDATE: 19 August 2014—New outbreak map added, added reference to suggested patient zero from December 2013.

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