Ebola, also known as Ebola hemorrhagic fever or Ebola viral disease, is a rare and deadly illness caused by one of the strains of Ebola virus. This viral agent is regarded as a prototype pathogen of viral hemorrhagic fever, with high fatality rates in humans and primates. The natural reservoir of Ebola virus remains unknown, although bats seem to be the most likely reservoir.
Ebola virus virion. Created by CDC microbiologist Cynthia Goldsmith, this colorized transmission electron micrograph (TEM) revealed some of the ultrastructural morphology displayed by an Ebola virus virion.
Infections with Ebola virus are characterized by immune suppression and a severe inflammatory response that damages vascular, coagulation and immune systems, subsequently resulting in bleeding, multiorgan failure and shock. Human-to-human transmission can lead to outbreaks, which are often initiated by a single introduction from the reservoir in nature or another end host.
Despite important scientific achievements on the biology and pathogenesis of Ebola virus during the past two decades, exact virulence factors and host responses are yet to be determined. Those unanswered questions have hampered the development of proper treatment methods and vaccines, which is a reason why there are currently no licensed prophylaxis or treatment measures for this infection.
Characteristics of the virus
Ebola virus belongs to the order Mononegavirales and the family Filoviridae, which is a taxonomic group of non-segmented, enveloped and negative-strand RNA viruses. Particles of these viruses have a characteristic filamentous appearance that gives the virus family its name. Their diameter is uniform at 80 nm, but particle length can be quite variable and up to 14 000 nm.
The genome of the virus consists of seven genes that code for nucleoprotein, glycoprotein, virion protein (VP) 24, VP30, VP35, VP40 and RNA-dependent RNA polymerase. With the exception of the glycoprotein gene, all aforementioned genes are monocistronic, which means that they encode for only one structural protein.
Production of a soluble glycoprotein (i.e. protein that contains covalently attached sugar residues) is an important distinction of Ebola virus from other viruses in the order. This major pathogenicity factor gets secreted from infected cells in large quantities, facilitating further viral entry by binding to the receptor present on the host cells.
According to the newest taxonomy of The International Committee on Taxonomy of Viruses (ICTV), in the genus Ebolavirus there are five recognized species: Zaire ebolavirus, Sudan ebolavirus, Taï Forest ebolavirus (formerly Côte d’Ivoire ebolavirus), Bundibugyo ebolavirus and Reston ebolavirus. Reston ebolavirus is the only species apathogenic for humans.
2014 Outbreak in West Africa
On March 21 2014, the Guinea Ministry of Health announced the outbreak of a disease manifesting with fever, vomiting, severe diarrhea and a high case-fatality rate of 59%. Specimens taken from sick individuals and tested at the Institute Pasteur in Lyon (France) were positive for an Ebola virus by polymerase chain reaction. Further viral sequencing revealed that the causative agent is a Zaire ebolavirus species, one of five viruses in the genus.
In order to implement prevention and control measures in affected countries, the governments collaborated with the World Health Organization, Médecins Sans Frontières and other organizations. Ebola treatment centers were established to provide better patient care and impede further virus transmission. Teams from Centers for Disease Control and Prevention played a significant role in characterizing and controlling the epidemic.
The 2014 outbreak in West Africa is definitely the largest and most complex Ebola outbreak since the discovery of the virus in 1976. There have been more cases and deaths in this outbreak than all others combined. In September 2014 there were a total of 5347 confirmed Ebola cases, 3095 laboratory confirmed cases and 2630 deaths, and the outbreak was still ongoing.