Can Ebola Become an Epidemic in the US?

Since the outbreak began in December 2013, friends have asked me about Ebola, including whether Ebola could become an epidemic in the US. Along with being scientifically fascinating, this is now a highly relevant topic. I’ve taken a few minutes to answer some of these questions:

What is Ebola?

First a very basic science lesson: Ebola is an “enveloped” RNA virus. An envelope is a lipid/fatty coating around a virus. Conceptually, enveloped viruses have a slimy coating. If that coat dries up, the virus dies. (Non-enveloped viruses have a ‘hard shell.’) This is important because it means Ebola viruses only survive in body fluids.

How does Ebola spread?

Because Ebola lives in body fluids, there needs to be direct contact with the blood, vomit, feces, saliva, or sweat of an infected patient to transmit infection. Once the fluids are touched, virus enters the healthy person’s body either by the person touching mucus membranes or through cuts on the skin (micro-abrasions such as little nicks around finger nails can let virus in). Miniscule amounts of virus can cause infection.

When Ebola first enters the body, it replicates before becoming widespread. In this incubation period (ranging from 2-21 days), a patient is not contagious. When replicating, Ebola disables the host immune system and is then free to replicate unchecked. The infected person initially develops fever, aches and chills. Symptoms progress to diarrhea and vomiting, which is often bloody because the virus also disables the host blood clotting system.

Importantly, Ebola does not pass through the air into the lungs of an uninfected person (it is not “aerosolized”). We know this for two reasons. First, the science shows it to be the case. Second, computer modeling of the spread of virus in epidemic regions provides robust ways of determining the infectiousness and routes of transmission. This means you do not need to worry about catching Ebola on a train or airplane from a person coughing two seats away.

Is there a cure?

Right now there are no established cures or therapies. However, there are several therapies being evaluated that are scientifically promising. One of these that has been tried in patients is ZMapp. This is a mix of antibodies (biological “guided missiles” specifically targeting Ebola that are produced by genetically modified tobacco plants).

Other drugs in development take advantage of the fact this virus is made of RNA. Virus use their own machinery to convert their RNA into new Ebola viruses (this is how the virus multiplies in the infected host). Fortunately, viral cellular machinery is different from human cellular machinery. In theory, we should be able to target viral proteins without impacting normal human proteins, but clinical trials are required to see the degree to which drugs are actually effective. There are already several drug candidates that look good in a Petri dish. That’s a good start.

The mortality rate of Ebola in West Africa appears to be about 70%. Hopefully it would be lower in developed nations where supportive care is better. The fact some patients live, coupled with the fact very few patients have been given experimental therapies, means we don’t yet know whether any experimental drugs work. Still, having several scientifically robust candidates is encouraging.

Is there a vaccine?

There is no vaccine . . . yet. The NIH has a vaccine in development that has protected chimps from infection after being exposed to otherwise lethal doses of Ebola. Another promising vaccine being developed by the Canadian government is being sent to the World Health Organization and may be given to some at-risk persons in West Africa. There are multiple others in development by private companies.

Vaccine (and drug) development can feel painfully slow in emergency situations. The first step is determining safety and then effectiveness in animals. Next, scientists must check safety in human volunteers. Finally, trials are needed to know whether there is efficacy in humans. The entire process takes 3-5 years even under accelerated conditions.

Several vaccine candidates are due to start the human safety trial stage in the next few months.

If Ebola infection becomes rampant, it is likely that early-stage vaccine candidates could be given to at-risk populations even in the absence of certain efficacy. The fact that there are promising vaccine candidates is a good start.

Where does Ebola come from?

Ebola virus is believed to live in fruit bats. These bats don’t get sick from the virus despite carrying it in their blood. Virus gets transmitted to humans who either eat the bats or eat pigs infected by these bats.

Why is this outbreak different?

Ebola is native to West Africa where people live in small clusters of isolated villages. Culturally, many such villages have the practice of isolating people infectious symptoms. These tribal customs protected the village. Villagers would build an isolated hut for the sick person and slide food under the door. Eventually the infected person would die, and the villagers would burn the hut. This proved to be an extremely effective method at containing Ebola.

Recently, outbreaks in villages with burial rituals that required cleansing the bodies resulted in close/dangerous contact leading to contamination and spread of infection. In addition, urbanization and improved transportation facilitated sick people getting to hospitals, where inadequate isolation procedures led to further spread of the disease.

What is the risk of outbreak in the US?

The short answer is that while this is scary stuff, the risk in the US is exceedingly low. Unless other cases begin to emerge, the risk is essentially de minimis in my humble opinion.

What could happen here in the US? Well, we need to look at what has transpired thus far and the degree to which virus has proven contagious. Two people became infected while treating Thomas Duncan, the Dallas patient who died of Ebola. These healthcare workers were aware of the risks and were supposed to be undertaking maximal precautions to avoid touching any body fluids, yet they still became infected. This highlights the risk of transmission is substantial. On the counter side, the outbreak in Africa, while terrifying, is still somewhat limited. The CDC (the Centers for Disease Control and Prevention) reports that there were 8,997 confirmed cases and 4,493 deaths as of October 12. This number can be viewed as small for an outbreak that began in December 2013. I take all this information to guide my thinking that the risk is probably manageable. The CDC is being vigilant about tracking potentially infected persons in order to prevent new cases and an epidemic.

One other element that will help contain Ebola is that the CDC, NIH (the National Institute of Health), and several private companies are developing a test for Ebola. Assuming it can be developed in the next few weeks, this would be huge. In theory, Ebola could be detected in the blood before a patient becomes contagious. This would go a long way toward containing diseases and lowering the risk of spread.

I am optimistic that Ebola in the US will be isolated to a few cases and not spread beyond this. Outbreak is not impossible but there is enough monitoring and flow of information that people will be alerted if the threat escalates.


Sources: World Health Organization “Ebola Factsheet“, CDC “About Ebola“, New York Times “Ebola Facts“, Wikipedia “Ebola Virus Disease“, CDC “2014 Ebola Outbreak“, Yahoo Finance “Canadian Ebola Vaccine“, New England Journal of Medicine “Ebola Virus Disease in West Africa“, New England Journal of Medicine “Emergence of Zaire Ebola Virus“, New England Journal of Medicine “Ebola Vaccine

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1 comment for “Can Ebola Become an Epidemic in the US?

  1. November 17, 2014 at 3:36 am

    I still believe that epidemic in US is virtually impossible. I’m more worried about economic consequences for developed world if Ebola becomes widespread in third world countries.

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