page_title: Western African Ebola virus epidemic
section_title: Virology
hierarchical_section_title: Western African Ebola virus epidemic / Virology
caption_attribution_description: English: Colorized transmission electron micrograph of Ebola virus particles (green) found both as extracellular particles and budding particles from chronically-infected African green monkey kidney cells (brown).
context_page_description: The Western African Ebola virus epidemic was the most widespread outbreak of Ebola virus disease in history—causing major loss of life and socioeconomic disruption in the region, mainly in Guinea, Liberia and Sierra Leone. The first cases were recorded in Guinea in December 2013; later, the disease spread to neighbouring Liberia and Sierra Leone, with minor outbreaks occurring elsewhere. It caused significant mortality, with the case fatality rate reported which was initially considerable, while the rate among hospitalised patients was 57–59%, the final numbers 28,616 people, including 11,310 deaths, for a case-fatality rate of 40%. Small outbreaks occurred in Nigeria and Mali, and secondary infections of medical workers occurred in the United States and Spain. In addition, isolated cases were recorded in Senegal, the United Kingdom and Italy. The number of cases peaked in October 2014 and then began to decline gradually, following the commitment of substantial international resources.
context_section_description: Ebola virus disease is caused by four of five viruses classified in the genus Ebolavirus. Of the four disease-causing viruses, Ebola virus (formerly and often still called the Zaire Ebola virus) is dangerous and is the virus responsible for the epidemic in Western Africa.
  Since the discovery of the viruses in 1976, when outbreaks occurred in South Sudan (then Sudan) and Democratic Republic of the Congo (then Zaire), Ebola virus disease had been confined to areas in Middle Africa, where it is native. With the current outbreak, it was initially thought that a new species native to Guinea might be the cause, rather than being imported from Middle to Western Africa. However, further studies have shown that the outbreak was likely caused by an Ebola virus lineage that spread from Middle Africa via an animal host within the last decade, with the first viral transfer to humans in Guinea.
  In a study done by Tulane University, the Broad Institute and Harvard University, in partnership with the Sierra Leone Ministry of Health and Sanitation, researchers provided information about the origin and transmission of the Ebola virus that set the Western African outbreak apart from previous ones, including 341 genetic changes in the virion. Five members of the research team became ill and died from Ebola before the study was published in August 2014.
  In a report released in August 2014,  researchers tracked the spread of Ebola in Sierra Leone from the group first infected—13 women who had attended the funeral of the traditional healer, where they contracted the disease—giving them a unique opportunity to track how the virus had changed.  This provided "the first time that the real evolution of the Ebola virus [could] be observed in humans." The research showed that the outbreak in Sierra Leone was sparked by at least two distinct lineages introduced from Guinea at about the same time. It is not clear whether the traditional healer was infected with both variants, or if perhaps one of the women attending the funeral was independently infected. As the Sierra Leone epidemic progressed, one virus lineage disappeared from patient samples, while a third one appeared.
  In January 2015, the media stated researchers in Guinea had reported mutations in the virus samples that they were looking at.  According to them, "we've now seen several cases that don't have any symptoms at all, asymptomatic cases. These people may be the people who can spread the virus better, but we still don't know that yet. A virus can change itself to [become] less deadly, but more contagious and that's something we are afraid of." A 2015 study suggested that accelerating the rate of mutation of the Ebola virus could make the virus less capable of infecting humans. In this animal study, the virus became practically non-viable, consequently increasing survival.
caption: Ebola virus particles
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