Ebola virus disease (EVD)
Ebola virus disease (EVD), Ebola hemorrhagic fever (EHF), or simply Ebola is a disease of humans and other primates caused by an ebolavirus. Symptoms start two days to three weeks after contracting the virus, with a fever, sore throat, muscle pain, and headaches. Typically, vomiting, diarrhea, and rash follow, along with decreased function of the liver and kidneys. Around this time, affected people may begin to bleed both within the body and externally.
Signs and symptoms of Ebola usually begin suddenly with an influenza-like stage characterized by fatigue, fever, headaches, joint, muscle, and abdominal pain.[9][10] Vomiting, diarrhea, and loss of appetite are also common.[10] Less common symptoms include the following: sore throat, chest pain, hiccups, shortness of breath, and trouble swallowing.[10] The average time between contracting the infection and the start of symptoms (incubation period) is 8 to 10 days, but it can vary between 2 and 21 days.[10][11] Skin manifestations may include a maculopapular rash (in about 50% of cases).[12] Early symptoms of EVD may be similar to those of malaria, dengue fever, or other tropical fevers, before the disease progresses to the bleeding phase.[9]
In 40–50% of cases, bleeding from puncture sites and mucous membranes (e.g., gastrointestinal tract, nose, vagina, and gums) has been reported.[13] In the bleeding phase, which typically begins five to seven days after first symptoms,[14] internal and subcutaneous bleeding may present itself in the form of reddened eyes and bloody vomit.[9] Bleeding into the skin may create petechiae, purpura, ecchymoses, and hematomas (especially around needle injection sites). Sufferers may cough up blood, vomit it, or excrete it in their stool.
Heavy bleeding is rare and is usually confined to the gastrointestinal tract.[12][15] In general, the development of bleeding symptoms often indicates a worse prognosis and this blood loss can result in death.[9] All people infected show some signs of circulatory system involvement, including impaired blood clotting.[12] If the infected person does not recover, death due to multiple organ dysfunction syndrome occurs within 7 to 16 days (usually between days 8 and 9) after first symptoms.
EVD is caused by four of five viruses classified in the genus Ebolavirus, family Filoviridae, order Mononegavirales. The four disease-causing viruses are Bundibugyo virus (BDBV), Sudan virus (SUDV), Taï Forest virus (TAFV), and one called, simply, Ebola virus (EBOV, formerly Zaire Ebola virus)). Ebola virus is the sole member of the Zaire ebolavirus
species and the most dangerous of the known Ebola disease-causing
viruses, as well as being responsible for the largest number of
outbreaks.[16] The fifth virus, Reston virus (RESTV), is not thought to be disease-causing in humans. These five viruses are closely related to the Marburg viruses.
One of the primary reasons for spread is that the health systems in the part of Africa where the disease occurs function poorly.[24] Medical workers who do not wear appropriate protective clothing may contract the disease.[25] Hospital-acquired transmission has occurred in African countries due to the reuse of needles and lack of universal precautions.[26][27] Some healthcare centers caring for people with the disease do not have running water.[22]
Airborne transmission has not been documented during EVD outbreaks.[2] They are, however, infectious as breathable 0.8– to 1.2-μm laboratory-generated droplets.[28] The virus has been shown to travel, without contact, from pigs to primates, although the same study failed to demonstrate similar transmission between non-human primates.[29]
Bats drop partially eaten fruits and pulp, then land mammals such as gorillas and duikers feed on these fallen fruits. This chain of events forms a possible indirect means of transmission from the natural host to animal populations, which has led to research towards viral shedding in the saliva of bats. Fruit production, animal behavior, and other factors vary at different times and places that may trigger outbreaks among animal populations.
Bats are considered the most likely natural reservoir of the EBOV. Plants, arthropods, and birds were also considered.[1][32] Bats were known to reside in the cotton factory in which the first cases for the 1976 and 1979 outbreaks were observed, and they have also been implicated in Marburg virus infections in 1975 and 1980.[33] Of 24 plant species and 19 vertebrate species experimentally inoculated with EBOV, only bats became infected.[34]
The absence of clinical signs in these bats is characteristic of a
reservoir species. In a 2002–2003 survey of 1,030 animals including
679 bats from Gabon and the Republic of the Congo, 13 fruit bats were found to contain EBOV RNA fragments.[35] As of 2005, three types of fruit bats (Hypsignathus monstrosus, Epomops franqueti, and Myonycteris torquata) have been identified as being in contact with EBOV. They are now suspected to represent the EBOV reservoir hosts.[36][37] Antibodies against Zaire and Reston viruses have been found in fruit bats in Bangladesh, thus identifying potential virus hosts and signs of the filoviruses in Asia.[38]
Between 1976 and 1998, in 30,000 mammals, birds, reptiles, amphibians and arthropods sampled from outbreak regions, no ebolavirus was detected apart from some genetic traces found in six rodents (Mus setulosus and Praomys) and one shrew (Sylvisorex ollula) collected from the Central African Republic.[33][39] Traces of EBOV were detected in the carcasses of gorillas and chimpanzees during outbreaks in 2001 and 2003, which later became the source of human infections. However, the high lethality from infection in these species makes them unlikely as a natural reservoir.[33]
Transmission between natural reservoir and humans is rare, and outbreaks are usually traceable to a single case where an individual has handled the carcass of gorilla, chimpanzee or duiker.[40] Fruit bats are also eaten by people in parts of West Africa where they are smoked, grilled or made into a spicy soup.
Like all mononegaviruses, ebolavirions contain linear nonsegmented, single-strand, non-infectious RNA genomes of negative polarity that possesses inverse-complementary 3' and 5' termini, do not possess a 5' cap, are not polyadenylated, and are not covalently linked to a protein.[42] Ebolavirus genomes are approximately 19 kilobase pairs long and contain seven genes in the order 3'-UTR-NP-VP35-VP40-GP-VP30-VP24-L-5'-UTR.[43] The genomes of the five different ebolaviruses (BDBV, EBOV, RESTV, SUDV, and TAFV) differ in sequence and the number and location of gene overlaps.
By Gaurav Singh
Signs and symptoms
Signs and symptoms of Ebola.[8]
In 40–50% of cases, bleeding from puncture sites and mucous membranes (e.g., gastrointestinal tract, nose, vagina, and gums) has been reported.[13] In the bleeding phase, which typically begins five to seven days after first symptoms,[14] internal and subcutaneous bleeding may present itself in the form of reddened eyes and bloody vomit.[9] Bleeding into the skin may create petechiae, purpura, ecchymoses, and hematomas (especially around needle injection sites). Sufferers may cough up blood, vomit it, or excrete it in their stool.
Heavy bleeding is rare and is usually confined to the gastrointestinal tract.[12][15] In general, the development of bleeding symptoms often indicates a worse prognosis and this blood loss can result in death.[9] All people infected show some signs of circulatory system involvement, including impaired blood clotting.[12] If the infected person does not recover, death due to multiple organ dysfunction syndrome occurs within 7 to 16 days (usually between days 8 and 9) after first symptoms.
Causes
Main articles: Ebolavirus (taxonomic group) and Ebola virus (specific virus)
Life cycles of the Ebolavirus
Transmission
Human-to-human transmission can occur via direct contact with blood or bodily fluids from an infected person (including embalming of an infected dead person) or by contact with objects contaminated by the virus, particularly needles and syringes.[17] Other body fluids with ebola virus include saliva, mucus, vomit, feces, sweat, tears, breast milk, urine, and semen. Entry points include the nose, mouth, eyes, or open wounds, cuts and abrasions. [18] The potential for widespread EVD infections is considered low as the disease is only spread by direct contact with the secretions from someone who is showing signs of infection.[17] The symptoms limit a person's ability to spread the disease as they are often too sick to travel.[19] Because dead bodies are still infectious, traditional burial rituals may spread the disease. Nearly two thirds of the cases of Ebola in Guinea during the 2014 outbreak are believed to be due to burial practices.[20][21] Semen may be infectious in survivors for up to 3 months.[22] It is not entirely clear how an outbreak is initially started.[23] The initial infection is believed to occur after ebola virus is transmitted to a human by contact with an infected animal's body fluids.One of the primary reasons for spread is that the health systems in the part of Africa where the disease occurs function poorly.[24] Medical workers who do not wear appropriate protective clothing may contract the disease.[25] Hospital-acquired transmission has occurred in African countries due to the reuse of needles and lack of universal precautions.[26][27] Some healthcare centers caring for people with the disease do not have running water.[22]
Airborne transmission has not been documented during EVD outbreaks.[2] They are, however, infectious as breathable 0.8– to 1.2-μm laboratory-generated droplets.[28] The virus has been shown to travel, without contact, from pigs to primates, although the same study failed to demonstrate similar transmission between non-human primates.[29]
Bats drop partially eaten fruits and pulp, then land mammals such as gorillas and duikers feed on these fallen fruits. This chain of events forms a possible indirect means of transmission from the natural host to animal populations, which has led to research towards viral shedding in the saliva of bats. Fruit production, animal behavior, and other factors vary at different times and places that may trigger outbreaks among animal populations.
Reservoir
Between 1976 and 1998, in 30,000 mammals, birds, reptiles, amphibians and arthropods sampled from outbreak regions, no ebolavirus was detected apart from some genetic traces found in six rodents (Mus setulosus and Praomys) and one shrew (Sylvisorex ollula) collected from the Central African Republic.[33][39] Traces of EBOV were detected in the carcasses of gorillas and chimpanzees during outbreaks in 2001 and 2003, which later became the source of human infections. However, the high lethality from infection in these species makes them unlikely as a natural reservoir.[33]
Transmission between natural reservoir and humans is rare, and outbreaks are usually traceable to a single case where an individual has handled the carcass of gorilla, chimpanzee or duiker.[40] Fruit bats are also eaten by people in parts of West Africa where they are smoked, grilled or made into a spicy soup.
Genome
Electron micrograph of an Ebola virus virion
By Gaurav Singh
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