Ebola Virus Primer

Ebola Virus is in the news again. The latest Ebola outbreak in Africa has killed more than 670 people including a prominent Liberian doctor and two American doctors. World Health Organization (WHO) calls the latest outbreak the largest recorded outbreak to date.  Ebola virus always conjures up in my mind images of an inevitable and horrible death by bleeding out of every orifice. Despite being discovered in 1976, even before AIDS and HIV, I realized that I didn’t know very much about the virus or how it worked, other than the occasional blurbs that appear in the news at the time of an outbreak. So I decided to look into it. It’s not that the basic information is difficult to find—my primary sources were the WHO and CDC fact sheets about Ebola Hemorrhagic Fever, and of course, Wikipedia. I did, however, also look in the scientific literature for some review articles and to see what research is currently going on in the field. For this article, though, I will just present the basic facts about Ebola Virus and the disease it causes, and other information you might be interested in, such as how contagious is it, how lethal is it, and how do you treat it. I will follow up later with more scientific details for those who want to learn more.

Epidemiology

Ebola Virus  is endemic to Central and West Africa near tropical rainforest areas. It is initially spread to humans by contact with infected animals, most likely fruit bats, which are thought to be the most likely reservoir.

Disease

According to information published by The WHO and CDC, Ebola causes an acute and rapidly progressing hemorrhagic fever. The disease often begins with the sudden onset of fever, intense weakness, muscle pain, headache and sore throat. This is followed by vomiting, diarrhea, rash, impaired kidney and liver function, and in some cases, both internal and external bleeding (WHO). The incubation period, the time between infection and onset of symptoms, can be anywhere from 2 to 21 days, with an average of about 14 days. Disease progression and death are rapid, thus somewhat limiting the spread of an outbreak.

What is the treatment?

There is currently no treatment for Ebola hemorrhagic fever other than supportive therapy such as maintaining fluids and electrolytes. There are no drugs or anti-viral treatments.

How lethal is Ebola?

The mortality rate of Ebola hemorrhagic fever is 50-90%.

How contagious is it?

Ebola is highly contagious during outbreaks. It is spread by direct contact with  bodily fluids of an infected person. There is no evidence to date for natural spread of Ebola by an airborne route, but airborne transmission cannot be ruled out (4). People are infectious as long as their blood and secretions contain the virus. Ebola virus was isolated from semen 61 days after onset of illness in a man who was infected in a laboratory. (WHO).

Is there a vaccine?

There is no vaccine available yet, but work is in progress.

Virology

Family

Ebola Virus is a member of the Filovirus family, along with the Marburg and Cueva viruses. There are five known species of Ebola: Zaire ebolavirus (EBOV), Sudan ebolavirus (SUDV), Reston ebolavirus (RESTV), Côte d’Ivoire ebolavirus (TAFV), and Bundibugyo ebolavirus (BDBV). The Reston virus was isolated from cynomolgous monkeys imported from the Philippines. The Reston virus infects humans, but does not cause disease—so far. (CDC)

Genome

The genome of Ebola Virus is a 19 Kb (-) sense RNA, most similar  in genome size and organization to mumps, measles, and Respiratory Syncytial Viruses (RSV). The (-) or negative sense of the RNA strand means the virus genome must be transcribed into a (+) sense strand which then serves as messenger RNA for the production of virus structural proteins. The virus codes for, and carries with its virions, an RNA-dependent RNA polymerase to carry out the transcription and replication process.

Structure

Photo credit: "Ebola virus virion" by CDC/Cynthia Goldsmith - Public Health Image Library, #10816 This media comes  from the Centers for Disease Control and Prevention's Public Health Image Library (PHIL).
Photo credit: “Ebola virus virion” by CDC/Cynthia Goldsmith – Public Health Image Library, #10816 This media comes
from the Centers for Disease Control and Prevention’s Public Health Image Library (PHIL).

Ebola Viruses form long filamentous virions 80 nm in diameter, and  are highly variable in length, from 500-1400 nm. Ebola virions are usually seen with loops or knots at one end by electron microscopy (see photo).

Components of the Virus Particle (Virion)

Ebola virions consist of seven structural proteins: the nucleocapsid protein (NP), a polymerase co-factor VP35, a transcriptional activator VP30, two matrix proteins, VP40 and VP24, the envelope protein GP, and the RNA- dependent RNA polymerase L. The helical nucleocapsid core consists of the genomic (-) RNA surrounded by a sheath of nucleoproteins (NP), and is associated with the RNA-dependent RNA polymerase (L), with the other proteins present in the virion cores. The virions are surrounded by a lipid membrane derived from the host cell membrane as the particles bud from the cell. The membrane contains the envelope proteins GP which allow the newly formed viruses to recognize, bind to, and infect their target cells.

I will follow up later with more on the molecular biology of the virus and the mechanisms of pathogenesis, in particular, the interaction between the virus and the immune system.

References

  1. Ebola Virus Disease. WHO Fact Sheet. http://www.who.int/mediacentre/factsheets/fs103/en/
  2. Ebola Hemorrhagic Fever. CDC Fact Page.   http://www.cdc.gov/vhf/ebola/
  3. Ebola Virus Disease. Wikipedia:  http://en.wikipedia.org/wiki/Ebola_virus_disease
  4. Zumbrun EE, Abdeltawab NF, Bloomfield HA, Chance TB, Nichols DK, Harrison PE, Kotb M, and Nalca A. 2012. Development of a Murine Model for Aerosolized Ebolavirus Infection Using a Panel of Recombinant Inbred Mice.
  5. Viruses 4: 3468-3493.
  6. Sullivan N, Yang ZY, and Nabel GJ. 2003. Ebola Virus Pathogenesis: Implications for Vaccines and Therapies. J. Virol 77: 9733-9737.

Author: Steve Anderson, Ph.D.

Steve Anderson has a Ph.D. in Immunology with over 25 years experience in biomedical research. His scientific expertise includes immunology, immunological diseases, tumor immunology, virology, and HIV pathogenesis.