Ebola: Filovirus
By AZAM on January 1, 2007Ebola Virus belongs to genus Ebolavirus, family Filoviridae, and for the disease which they cause, Ebola hemorrhagic fever. The viruses are characterised by a long, filamentous morphology surrounded by a lipid viral envelope. Ebola viruses are morphologically similar to the Marburg virus, also in the family Filoviridae, and share similar disease symptoms.
There are four identified subtypes of Ebola virus. Three of the four have caused disease in humans: Ebola-Zaire, Ebola-Sudan, and Ebola-Ivory Coast. The fourth, Ebola-Reston, has caused disease in nonhuman primates, but not in humans.
Ebola haemorrhagic fever (EHF) is one of the most virulent viral diseases known to humankind, causing death in 50-90% of all clinically ill cases. Several different species of Ebola virus have been identified.
The Ebola virus is transmitted by direct contact with the blood, body fluids and tissues of infected persons.Transmission of the Ebola virus has also occurred by handling ill or dead Infected chimpanzees.
The virus is named after the Ebola River Valley in the African nation-state of the Democratic Republic of the Congo (formerly Zaïre), near the site of the first recognized outbreaks in 1976
The Ebola virus was first identified in a western equatorial province of Sudan and in a nearby region of Zaire (now Democratic Republic of the Congo) in 1976 after significant epidemics in Yambuku, northern Zaire, and Nzara, southern Sudan.
Description
Structure
Electron Micrographs of members of Ebolavirus show them to have the characteristic thread-like structure of a filovirus.[5] EBOV VP30 is around 288 amino acids long.[5] The virions are tubular and variable in shape and may appear as a “U”, “6″, coiled, circular, or branched shape, however, laboratory purification techniques, such as centrifugation, may contribute to the various shapes seen.[5] Virions are generally 80 nm in diameter.[5] They are variable in length, and can be up to 1400 nm long. On average, however, the length of a typical Ebola virus is closer to 1000 nm. In the center of the virion is a structure called nucleocapsid, which is formed by the helically wound viral genomic RNA complexed with the proteins NP, VP35, VP30 and L. It has a diameter of 40 - 50 nm and contains a central channel of 20-30 nm in diameter. Virally encoded glycoprotein (GP) spikes 10 nm long and 10 nm apart are present on the outer viral envelope of the virion, which is derived from the host cell membrane. Between envelope and nucleocapsid, in the so-called matrix space, the viral proteins VP40 and VP24 are located.
Genome
Each virion contains one minor molecule of linear, single-stranded, negative-sense RNA, totaling 18959 to 18961 nucleotides in length. The 3′ terminus is not polyadenylated and the 5′ end is not capped. It was found that 472 nucleotides from the 3′ end and 731 nucleotides from the 5′ end were sufficient for replication.[5] It codes for seven structural proteins and one non-structural protein. The gene order is 3′ - leader - NP - VP35 - VP40 - GP/sGP - VP30 - VP24 - L - trailer - 5′; with the leader and trailer being non-transcribed regions which carry important signals to control transcription, replication and packaging of the viral genomes into new virions. The genomic material by itself is not infectious, because viral proteins, among them the RNA-dependent RNA polymerase, are necessary to transcribe the viral genome into mRNAs, as well as for replication of the viral genome.
Out Breaks
Ebola HF typically appears in sporadic outbreaks, usually spread within a health-care setting (a situation known as amplification). It is likely that sporadic, isolated cases occur as well, but go unrecognized. A table showing a chronological list of known cases and outbreaks is available.
Method of Transmission
Infections with Ebola virus are acute. There is no carrier state. Because the natural reservoir of the virus is unknown, the manner in which the virus first appears in a human at the start of an outbreak has not been determined. However, researchers have hypothesized that the first patient becomes infected through contact with an infected animal.
After the first case-patient in an outbreak setting is infected, the virus can be transmitted in several ways. People can be exposed to Ebola virus from direct contact with the blood and/or secretions of an infected person. Thus, the virus is often spread through families and friends because they come in close contact with such secretions when caring for infected persons. People can also be exposed to Ebola virus through contact with objects, such as needles, that have been contaminated with infected secretions.
Nosocomial transmission refers to the spread of a disease within a health-care setting, such as a clinic or hospital. It occurs frequently during Ebola HF outbreaks. It includes both types of transmission described above. In African health-care facilities, patients are often cared for without the use of a mask, gown, or gloves. Exposure to the virus has occurred when health care workers treated individuals with Ebola HF without wearing these types of protective clothing. In addition, when needles or syringes are used, they may not be of the disposable type, or may not have been sterilized, but only rinsed before reinsertion into multi-use vials of medicine. If needles or syringes become contaminated with virus and are then reused, numerous people can become infected.
Ebola-Reston appeared in a primate research facility in Virginia, where it may have been transmitted from monkey to monkey through the air. While all Ebola virus species have displayed the ability to be spread through airborne particles (aerosols) under research conditions, this type of spread has not been documented among humans in a real-world setting, such as a hospital or household.
Reservoir
Despite numerous studies, the wildlife reservoir of Ebolavirus has not been identified. Between 1976 and 1998, from 30,000 mammals, birds, reptiles, amphibians, and arthropods sampled from outbreak regions, no Ebolavirus was detected [19] apart from some genetic material found in six rodents (Mus setulosus and Praomys species) and a shrew (Sylvisorex ollula) collected from the Central African Republic in 1998.[20] Ebolavirus was detected in the carcasses of gorillas, chimpanzees and duikers during outbreaks in 2001 and 2003 (the carcasses were the source of the initial human infections) but the high mortality from infection in these species precludes them from acting as reservoirs.[19]
Plants, arthropods, and birds have also been considered as reservoirs, however bats are considered the most likely candidate[21]. Bats were known to reside in the cotton factory in which the index cases for the 1976 and 1979 outbreaks were employed and have also been implicated in Marburg infections in 1975 and 1980.[19] Of 24 plant species and 19 vertebrate species experimentally inoculated with Ebolavirus, only bats became infected.[22] The absence of clinical signs in these bats is characteristic of a reservoir species. In 2002-03, a survey of 1,030 animals from Gabon and the Republic of the Congo including 679 bats found Ebolavirus RNA in 13 fruit bats (Hyspignathus monstrosus, Epomops franquetti and Myonycteris torquata).[23] Bats are also known to be the reservoirs for a number of related viruses including Nipah virus, Hendra virus and lyssaviruses.
Symptoms of Viral Infection
The incubation period for Ebola HF ranges from 2 to 21 days. The onset of illness is abrupt and is characterized by fever, headache, joint and muscle aches, sore throat, and weakness, followed by diarrhea, vomiting, and stomach pain. A rash, red eyes, hiccups and internal and external bleeding may be seen in some patients.
Researchers do not understand why some people are able to recover from Ebola HF and others are not. However, it is known that patients who die usually have not developed a significant immune response to the virus at the time of death.
Clinical Diagnosis
Diagnosing Ebola HF in an individual who has been infected only a few days is difficult because early symptoms, such as red eyes and a skin rash, are nonspecific to the virus and are seen in other patients with diseases that occur much more frequently. However, if a person has the constellation of symptoms described above, and infection with Ebola virus is suspected, isolate the patient and notify local and state health departments and the CDC.
Diagnostic Test for Ebola
Antigen-capture enzyme-linked immunosorbent assay (ELISA) testing, IgM ELISA, polymerase chain reaction (PCR), and virus isolation can be used to diagnose a case of Ebola HF within a few days of the onset of symptoms. Persons tested later in the course of the disease or after recovery can be tested for IgM and IgG antibodies; the disease can also be diagnosed retrospectively in deceased patients by using immunohistochemistry testing, virus isolation, or PCR.
Treatment
There is no standard treatment for Ebola HF. Patients receive supportive therapy. This consists of balancing the patient’s fluids and electrolytes, maintaining their oxygen status and blood pressure, and treating them for any complicating infections.
Future Potential Therapeutic
Vaccines have been produced for both Ebola [15] and Marburg[16] that were 99% effective in protecting a group of monkeys from the disease. These vaccines are based on either a recombinant Vesicular stomatitis virus or a recombinant Adenovirus[17] carrying the Ebola spikeprotein on its surface. Early human vaccine efforts, like the one at NIAID in 2003, have so far not reported any successes.[18] The biggest problem with the vaccine is that unless the patient is given it near the onset of the virus (1-4 days after the symptoms begin) then there will be too much damage to the human body to repair, ie: ruptured arteries and capillaries, vomiting, and other symptoms which may still cause enough harm to kill or seriously traumatize the patient.
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