By Shruti Tyagi
The Ebola virus disease (EVD) has been known as one of the deadliest infectious diseases in the world. Alternatively referred to as Ebola hemorrhagic fever, Ebola has been discovered in six unique strains that vary in their ability to affect human health. The six strains of Ebola include the Bombali ebolavirus, Reston ebolavirus, Zaire ebolavirus, Sudan ebolavirus, Taï forest ebolavirus, and the Bundibugyo ebolavirus. Nonetheless, only the latter four strains of the virus have caused infections in humans.
Although EVD was initially discovered in 1976 in bats in the Democratic Republic of Congo (DRC), various outbreaks of the disease have been occurring in humans ever since. The virus raises a major concern in the DRC, Liberia, Sierra Leone, Guinea, and many other areas of West Africa. Most notably, in the West African Epidemic of 2014, the Zaire ebolavirus infected 28,610 civilians and an estimated 11,308 civilians died. Overall, Ebola has been known to have mortality rates (death rates) ranging from 25% to nearly 90%, while the average remains 50%. It has most severely impacted communities that lack medical resources to combat it, but community cooperation has drastically improved the state of the epidemic.
In the region of the Equateur Province of the DRC, the 11th Ebola outbreak was officially over on November 18, 2020. This was largely due to supportive care, which involves giving patients fluids, oxygen, and medications to reduce diarrhea, and the introduction of the Ebola vaccine. Over 40,000 civilians have been vaccinated in the DRC thus far, with high-risk patients given first priority of access. The vaccine, like the newly developing COVID-19 vaccines, must be stored at extremely cold temperatures as low as -80 degrees Celsius. Its storage is maintained with the use of ARKTEK freezers, and such freezers are being considered as a possibility for storing arriving COVID-19 vaccines as well.
However, when it comes down to the etiology (the cause) of EVD, many people are stumped over the variances that Ebola virus has in terms of how it affects certain organisms. For example, Ebola is thought to originate in fruit bats such as the greater long-fingered bat species. In bats, however, Ebola does not produce disease. Conversely, in humans, the Ebola virus causes symptoms anywhere between 2 to 21 days after exposure, and common symptoms include diarrhea, vomiting, internal bleeding of the blood vessels, and severe headaches. The human immune system helps fight and protect the body from infectious diseases, but the Ebola virus blocks certain proteins of the immune cells that help fight against infections, thus weakening the immune response. Ebola viruses damage blood vessels and form clots, which ultimately cause the patient’s blood pressure to drop. Organ failure generally follows due to a lack of adequate blood supply and dehydration (lack of fluids). Organ failure is usually the cause of death of patients. Ebola’s effects are not limited to humans, however, as other primates, such as chimpanzees, can also die from the disease.
So why, then, does Ebola vary so much in its ability to harm the organism it infects? The answer lies in mutations, or changes in the genetic structure of the virus, as it is transmitted from one species to another. To make things a little more clear, let us delve into an example of Ebola being transmitted from a bat to another animal, then from an animal to a human.
Currently, researchers believe that bats are reservoir hosts of EVD, which means that the Ebola virus may naturally reside in and reproduce in these organisms. Therefore, bats will not experience symptoms of EVD like infected humans will. If a bat is carrying the Ebola virus, an animal such as a chimpanzee may become infected in what is known as a spillover event; this occurs through exposure from hunting or eating the meat of bats. From there, animals can spread EVD to humans through contact with infected feces, bodily fluids, and blood, and the human epidemic begins as the virus spreads across villages, countries, and even continents.
To explain this phenomenon, researchers at the University of Texas Medical Branch at Galveston have determined that as the virus is passed from bats to humans, certain adaptive mutations occur. Adaptive mutations mean that the genetic structure of the virus is changing so that it can survive in the human body. On the other hand, since bats are the hosts of Ebola virus, there are a variety of factors that allow them to control the virus and prevent illness. This allows the Ebola virus to be carried around by the bat, and thus give rise to more bacteria with similar properties. For example, a molecule known as ADAR1 is able to edit the virus, and it is more prevalent in cells of bats than in humans. Therefore, as ADAR1 edits the proteins of the virus, the proteins become more diverse because they are changed. Scientists believe these changes that ADAR1 induces causes the Ebola virus in bats to become less harmful. Since humans lack similar levels of ADAR1, the effect of EVD on their health is much more severe.
So there you have it: the reason why Ebola tends to affect humans and primates more than bats is because of a key difference in the amount of ADAR1 they have, which alters the virus to make it weaker. At the end of the day, it is astonishing how one small molecule can drastically change the course of disease in an organism!
Comprehension Questions
Out of the six strains of ebola virus, which four have caused disease in humans?
How many days after exposure to the ebola virus are symptoms generally witnessed in humans?
True/false: Only humans can die from an ebola virus infection.
Define a spillover event and give an example of its usage in context with the ebola virus.
What molecule found in bats is able to edit the virus to a less severe form?
Comprehension Question Answers
Zaire ebolavirus, Sudan ebolavirus, Taï forest ebolavirus, and the Bundibugyo ebolavirus
2-21 days
False
Transfer of a virus from the reservoir host to a susceptible host
Ex: bats transferring the ebola virus to chimpanzees who feed on them
ADAR1
Works Cited
"11th Ebola Outbreak in the Democratic Republic of the Congo Declared over." WHO |
Regional Office for Africa, 11 June-July 2020,
www.afro.who.int/news/11th-ebola-outbreak-democratic-republic-congo-declared-over.
"Ebola Report: Ebola by the Numbers." Centers for Disease Control and Prevention, 1 Nov.
2018, www.cdc.gov/about/ebola/ebola-by-the-numbers.html.
"Ebola Report: Ebola by the Numbers." Centers for Disease Control and Prevention, 1 Nov.
2018, www.cdc.gov/about/ebola/ebola-by-the-numbers.html.
"Ebola Virus Disease." WHO | World Health Organization, 10 Feb. 2020,
www.who.int/news-room/fact-sheets/detail/ebola-virus-disease.
Ebola virus. Photograph. Shutterstock, Accessed 3 Jan. 2021.
"Latest Ebola Outbreak in DR Congo is Declared Over, with Lessons for COVID-19." UN
News, 20 Nov. 2020, news.un.org/en/story/2020/11/1077912.
"Mode of transmission of Ebola virus infection." Diagram. researchgate.net, Accessed 3 Jan.
2021.
Parry, Wynne. "27 Devastating Infectious Diseases." Livescience.com, 10 Feb. 2020,
www.livescience.com/13694-devastating-infectious-diseases-smallpox-plague.html.
"Treatment." Centers for Disease Control and Prevention, 5 Nov. 2019,
www.cdc.gov/vhf/ebola/treatment/index.html.
TREMEAU, VINCENT. Ebola vaccine in Congo. World Bank, Accessed 3 Jan. 2021.
"What is Ebola Virus Disease?" Centers for Disease Control and Prevention, 6 Nov. 2019,
www.cdc.gov/vhf/ebola/about.html.
Whitfield, Zachary J., et al. "Species-Specific Evolution of Ebola Virus during Replication in
Human and Bat Cells." Cell Reports, vol. 32, no. 7, 2020, p. 108028.
"Why Doesn't Ebola Cause Disease in Bats, As It Does in People?" ScienceDaily, 2 Jan. 2021,
www.sciencedaily.com/releases/2020/08/200818142147.htm.
"Years of Ebola Virus Disease Outbreaks Error Processing SSI File." Centers for Disease
Control and Prevention, 14 July 2020, www.cdc.gov/vhf/ebola/history/chronology.html.