All About the Ebola Virus

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Ebola Virus

Ebola Virus
Ebola virus particles (green) attached to and budding from a chronically infected VERO E6 cell. Credit: NIAID

Ebola is the virus that causes Ebola virus disease. Ebola virus disease is a serious illness that causes viral hemorrhagic fever and is deadly in up to 90 percent of cases. Ebola damages blood vessel walls and inhibits the blood from clotting. This results in internal bleeding that can be life-threatening. Ebola outbreaks have garnered serious attention as there is no known treatment, vaccine, or cure for the disease. These outbreaks have primarily affected people in tropical regions of Central and West Africa. Ebola is typically transmitted to humans through close contact with the bodily fluids of infected animals. It is then transmitted between humans through contact with blood and other bodily fluids. It can also be picked up through contact with contaminated fluids in an environment. Ebola symptoms include fever, diarrhea, rash, vomiting, dehydration, impaired kidney and liver function, and internal bleeding.

Ebola Virus Structure

Ebola is a single-stranded, negative RNA virus that belongs to the virus family Filoviridae. Marburg viruses are also included in the Filoviridae family. This virus family is characterized by their rod-shape, thread-like structure, varied length, and their membrane enclosed capsid. A capsid is a protein coat that encloses the viral genetic material. In Filoviridae viruses, the capsid is also enclosed in a lipid membrane that contains both host cell and viral components. This membrane assists the virus in infecting its host. Ebola viruses can be relatively large measuring up to 14,000 nm in length and 80 nm in diameter. They often take on a U shape.

Ebola Virus Infection

The exact mechanism by which Ebola infects a cell is not known. Like all viruses, Ebola lacks the needed components to replicate and must utilize the cell's ribosomes and other cellular machinery to replicate. Ebola virus replication is thought to occur in the host cell's cytoplasm. Upon entering the cell, the virus uses an enzyme called RNA polymerase to transcribe its viral RNA strand. The viral RNA transcript synthesized is similar to messenger RNA transcripts that are produced during normal cellular DNA transcription. The cell's ribosomes then translate the viral RNA transcript message to create viral proteins. The viral genome instructs the cell to produce new viral components, RNA, and enzymes. These viral components are transported to the cell membrane where they are assembled into new Ebola virus particles. The viruses are released from the host cell through budding. In budding, a virus uses components of the host's cell membrane to create its own membrane envelope which encloses the virus and is eventually pinched off from the cell membrane. As more and more viruses exit the cell through budding, cell membrane components are slowly used up and the cell dies. In humans, Ebola primarily infects the inner tissue linings of capillaries and various types of white blood cells.

Ebola Virus Inhibits Immune Response

Studies indicate that the Ebola virus is able to replicate unchecked because it suppresses the immune system. Ebola produces a protein called Ebola Viral Protein 24 that blocks cell signaling proteins called interferons. Interferons signal the immune system to increase its response to viral infections. With this important signaling path blocked, cells have little defense against the virus. The mass production of viruses triggers other immune responses that negatively impact organs and causes a number of the severe symptoms seen in Ebola virus disease. Another tactic employed by the virus to evade detection involves cloaking the presences of its double-stranded RNA that is synthesized during viral RNA transcription. The presence of the double-stranded RNA alerts the immune system to mount a defense against infected cells. The Ebola virus produces a protein called Ebola Viral Protein 35 (VP35) that prevents the immune system from detecting the double-stranded RNA and thwarts an immune response. Understanding how Ebola suppresses the immune system is key to the future development of treatments or vaccines against the virus.

Sources:

  • Noda T, Ebihara H, Muramoto Y, Fujii K, Takada A, et al. (2006) Assembly and Budding of Ebolavirus. PLoS Pathog 2(9): e99. doi:10.1371/journal.ppat.0020099. Published 09/29/2006 (http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.0020099)
     
  • Mount Sinai Medical Center. "Ebola protein blocks early step in body's counterattack on virus." ScienceDaily. Accessed 08/15/2014 (http://www.sciencedaily.com/releases/2014/08/140813130044.htm)
     
  • Scripps Research Institute. "Scientists reveal key structure from Ebola virus." ScienceDaily. Accessed 08/15/2014 (http://www.sciencedaily.com/releases/2009/12/091208170913.htm)
     
  • Ebola Virus Disease Fact Sheet. World Health Organization. Updated 04/2014 (http://www.who.int/mediacentre/factsheets/fs103/en/)