Ebola virus disease (EVD), formerly known as Ebola hemorrhagic fever, is a severe infectious disease illness caused by the Zaire ebolavirus. This viral agent is regarded as a prototype pathogen of viral hemorrhagic fever with high fatality rates in humans. Without treatment, between 70-90% of people infected will die. The natural reservoir of the Ebola virus remains unknown, although bats, porcupines and non-human primates are the most likely reservoir.
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. Symptom onset can vary between 2-21 days post-infection, with primary symptoms including fever, gastrointestinal disturbance, myalgia and unexplained bleeding or bruising.
Human-to-human viral transmission occurs after direct or indirect contact with contaminated body fluids, and can lead to outbreaks, which are often initiated by a single introduction from the reservoir in nature or another end host. Since the discovery of EVD in 1976, there have been 31 recorded outbreaks of the disease, with the majority occurring in Africa.
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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, Bombali ebolavirus and Reston ebolavirus. Reston ebolavirus is the only species apathogenic for humans.
On March 21st, 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 ebolavirus by polymerase chain reaction. Further viral sequencing revealed that the causative agent was a Zaire ebolavirus species, one of five viruses in the genus.
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 the Centers for Disease Control and Prevention played a significant role in characterizing and controlling the epidemic.
The 2014 outbreak in West Africa was the largest and most complex Ebola outbreak since the discovery of the virus in 1976. Initial weak surveillance and tracking of cases, paired with poor public health infrastructures, led to an unparalleled circulation of the virus. The outbreak was finally declared over in 2016, by which point there had been over 28,000 cases and 11,00 deaths globally.
At the outset of the 2014 outbreak, there were no available licensed vaccines against EBV, although early trials had delivered promising results. As earlier outbreaks had been sporadic, opportunities to conduct systematic later-stage trials were limited. In August 2014, following a World Health Organization declaration of the outbreak as a global public health emergency, an unlicensed vaccine was made available for use.
The vaccine was administered to over 7,000 people in Guinea, West Africa and found to be efficacious in over 75% of cases. By 2019, 12 clinical trials including more than 15,000 adults had demonstrated the safety and efficacy of the product. This one-dose vaccine was the first ebolavirus vaccine to be approved by the United States Food and Drug Administration (FDA), receiving approval in December 2019.
Ebola 101 | National Geographic
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