The current Ebola outbreak in the West African nations of Guinea, Liberia, and Sierra Leone has received a lot of media attention, with terrifying news headlines such as “Deadliest Ever Outbreak of Ebola” appearing across the globe . And this is indeed the largest outbreak recorded to date, with approximately 1,093 people infected and 660 deaths as of July 29, 2014 . But how exactly is this outbreak different from previous outbreaks, and what does that mean for the people of the three affected countries?
Origins of an epidemic
Ebolavirus (EBOV) is one of the deadliest viruses on the planet, with mortality rates as high as 90% in some outbreaks . EBOV causes Ebola Viral Disease (EVD), the symptoms of which typically include fever, body aches, vomiting, and diarrhea . Up to 50% of the patients with EVD eventually develop Ebola Hemorrhagic Fever (EHF) – a condition that involves internal and external bleeding followed by multiple organ failure . As if all that wasn’t scary enough, despite years of intense research, there are currently no treatments or vaccines available for EBOV.
Outbreaks begin when EBOV is transmitted from an infected animal to a human. Animal species carrying viruses that are capable of infecting humans are known as reservoir hosts . For EBOV, fruit bats are believed to be the reservoir [1, 4, 6]. Scientists studying the current outbreak don’t know yet whether the first infected human caught EBOV directly from a bat, or whether a second, intermediate animal host, such as chimpanzees and/or duikers (forest antelopes), was involved (Figure 1) [5, 10]. The virus can be transmitted from animal to human via contact with bodily fluids of infected animals. In West Africa, it’s not uncommon for people to come into contact with animals while hunting or preparing food.
In an outbreak, the first human infection is referred to as the source case . This person spreads the virus to others, beginning a human-to-human transmission chain . EBOV is spread via contact with an infected individual’s bodily fluids such as blood, sweat, urine, and breast milk . This means that touching an infected person, an object contaminated with their bodily fluid, or even handling their remains can spread EBOV.
Figure 1 ~ Possible EBOV animal-to-human transmission chains. Adapted from .
What made this outbreak the “deadliest ever”?
So why is this the worst outbreak of Ebola so far? What makes it different from previous outbreaks? One reason is that it was completely unexpected. Outbreaks have occurred in Central African countries such as the Democratic Republic of Congo, South Sudan, and Uganda (Figure 2) , but West African countries (over a thousand miles away from Central Africa) have never had Ebola outbreaks in human populations before. About 20 years ago, there were two documented outbreaks in chimpanzee populations in the Tai Forest of the Ivory Coast. One scientist researching the chimps also became infected, but she survived and did not pass the virus on to anyone else [1, 6]. But because there had been no news of EBOV anywhere in West Africa since the 1990s, nobody believed that countries like Guinea were at risk for an outbreak . In addition, many illnesses that are more widespread in West Africa have similar symptoms to EVD, including malaria and typhoid [1,5]. This means that EVD could have been initially misdiagnosed, allowing it to spread undetected.
Figure 2 ~ Past and Present EBOV Outbreaks. Yellow asterisks indicate countries where outbreaks have occurred in human populations in the past. Red asterisks mark countries affected by the current outbreak. The green asterisk indicates the location of chimpanzee outbreaks and single human infection. Note: Locations of asterisks are not precise geographical locations of outbreaks. Figure adapted from https://www.cia.gov/library/publications/the-world-factbook/geos/cv.html
Another difficulty with this outbreak is that certain cultural practices in the affected communities can actually promote the spread of EBOV. For example, families of deceased patients perform traditional burial practices, which involve transporting and touching the body of the deceased family member. Additionally, because the health care infrastructure in the affected regions is often minimal, families tend to care for sick loved ones at home rather than bringing them to the hospital. This makes it very likely that a sick person will infect family members that care for them during their illness. [7, 8]
Finally, in the current outbreak the virus has spread to several large cities, including Conakry and Monrovia, the capitals of Guinea and Liberia, respectively. The virus has an incubation period that varies between 2 and 21 days, meaning that a person can be infected with EBOV without experiencing any symptoms for up to 3 weeks [1, 3, 11], during which they may travel and bring the virus to previously unaffected areas. The movement of patients, together with home caregiving and traditional burial practices, impairs the ability of public health workers to track the spread of the virus, which, as we’ll see in the next section, is essential to suppressing the outbreak .
Racing to contain the virus
When an outbreak like this occurs, healthcare workers and scientists hurry to determine the animal source of the outbreak and the links between infected people. This information helps stem the spread of EBOV and has the potential to prevent future outbreaks. Recently, a team of scientists led by Dr. Stephan Günther reconstructed the path of the outbreak by looking through hospital records and interviewing community members in Guéckédou, Guinea, where the first cases were reported . Gunther’s team was able to identify the likely source case and then trace the virus’s spread from the first patient’s family members to their friends and other community members, and even into neighboring villages.
Why the detective work? Tracking the chain of transmission is essential to preventing further infections. If people who have been exposed can be identified and quarantined, they will be unable to pass the virus on to others, and the chain of transmission will come to a halt. In previous outbreaks, keeping track of who was infected and who was at risk for exposure was not as difficult. This time, however, due to the difficulties discussed in the previous section, health care workers have been unable to get ahead of EBOV’s spread.
Scientists are also hurrying to identify the animal source of this outbreak. Learning how the source patient was initially exposed will help to prevent further human exposure from the same source. A team of scientists is currently capturing bats in the forests of Guinea to test them for EBOV. They are also monitoring the chimpanzee, monkey, and duiker populations. An unusual decrease in their populations would indicate that EBOV is circulating in those species as well. This type of virus hunting can be extremely difficult, but if scientists are able to determine which animals are spreading the disease, they can advise the people living near the forests to avoid hunting these animals for food, which will reduce the likelihood of additional human exposure to EBOV. 
For now, medical workers will continue to try to identify, isolate, and care for infected people. The World Health Organization recently held a meeting with health officials from several West African nations in order to discuss the best ways to stop the spread of EBOV . The Centers for Disease Control and Prevention, which has also sent teams to West Africa to help control the outbreak, advocated for local community leaders to help educate people about EBOV spread and EVD symptoms . Hopefully the strategies agreed upon at the WHO meeting will lead to on-the-ground efforts that halt the outbreak quickly, saving as many lives as possible.
Alison Hill is a graduate student in the Virology Program at Harvard Medical School.
3. Dixon, Meredith G., and Schafer, Ilana J. “Ebola Viral Disease Outbreak – West Africa 2014.” Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report, 2014. Web, accessed 8 July 2014.
5. Bagcchi, Sanjeet. “Ebola haemorrhagic fever in west Africa.” The Lancet, 2014. Vol. 14: p.375.
6. Vogel, Gretchen. “Are Bats Spreading Ebola Across Sub-Saharan Africa?” Science, 2014. Vol. 344: p.140.
10. Baize, Sylvain, et al. “Emergence of Zaire Ebola Virus Disease in Guinea – Preliminary Report.” The New England Journal of Medicine, 2014 (NEJM.ORG).