Ebola, or Ebola hemorrhagic fever (Ebola HF), is a disease whose etiological agent is the Ebola virus. The Ebola virus, along with the Marburg virus, belongs to the filovirus family, which is a group of RNA viruses.1 The filoviruses are classified at a biosafety level of 4 due to their severe pathogenicity and the lack of any vaccines or antiviral drugs to combat against them.2
The Ebola virus was first identified in 1976 when two outbreaks of Ebola HF occurred in Zaire and Sudan. These outbreaks were caused by two different strains of the virus, and thus were named according to the area where the outbreaks took place.3 The Ebola virus now has four subtypes: Ebola Ivory Coast, Ebola Sudan, Ebola Zaire, and Ebola Reston, all named for the areas where outbreaks occurred. All strains except Ebola Reston are capable of causing disease in humans.3
The Ebola virus is filamentous and pleomorphic, often assuming the shape of a "U." The viral particles can be up to 14,000 nm in length and are about 80 nm in diameter. The virus is made up of an RNA core surrounded by a nucleocapsid, which in turn is surrounded by a helical capsid. The entire virion is coated with a lipoprotein that is derived from the host cell. The lipoprotein coat has spikes that are about 7 nm in length.4 A study by Bavari et al suggests that the Ebola virus uses "lipid rafts" of the host cell in order to gain entry into that cell and to assemble new virus particles. Lipid rafts are tightly packed areas in the host cell membrane that are rich in cholesterol and fatty acids. These dense areas have receptors for several classes of enveloped viruses. Many enveloped viruses generate proteins that are targeted for these "lipid rafts" and thus help in virus assembly. Some proteins from the Ebola virus, such as matrix protein VP40, have been associated with these lipid rafts.5 The ability of the Ebola virus to use the host cell membrane for its own advancement is a possible source of this virus's virulence. Currently, Ebola hemorrhagic fever is one of the most virulent and deadly diseases, causing death in 50-90% of those that become infected with the virus.5
The natural reservoirs of the Ebola virus have yet to be determined, but the virus is believed to be a zoonotic, and thus maintained in an animal host.1 The virus can replicate in some species of bats, therefore bats that are native to the areas in Africa where Ebola is prevalent may in fact be the carriers and reservoirs of this virus.3 The virus can be transmitted from the animal carrier to humans and can also be spread from human to human. The disease is contracted when a person comes into contact with the blood or other body secretions of an infected person. The virus can also be spread by objects such as needles that may have been contaminated with the secretions of an infected host. Ebola HF is frequently transmitted nosocomially. In the lab, each Ebola strain has been successfully transmitted through the air, yet aerosol transmission has yet to be documented in a real world setting.1
The incubation period of the virus ranges from 2 to 21 days 6 and early symptoms include malaise, fatigue, headache, backache, vomiting, diarrhea, nausea, sore throat, and arthritis.7 After a week or so the patient develops a raised, hemorrhagic rash, and begins to bleed from the mucous membranes such as the eyes, nose, mouth, and rectum.8 Other late symptoms include chest pain, shock, blindness,1 conjunctivitis, genital swelling, depression, seizures, and coma.7
Several key tests are used to identify Ebola virus. Antigen-capture ELISA testing, IgG ELISA, PCR, and virus isolation can all be used to diagnose Ebola HF within a few days of the appearance of symptoms. Patients who have had the virus for longer or who have recovered from it can be tested for the presence of IgM or IgG antibodies. In deceased patients, diagnostic tests include immunohisto-chemistry testing, virus isolation, and PCR.1
Currently, there are no antiviral treatments for Ebola HF, nor does interferon treatment have any effect.9 Treatment is limited to supportive therapy, which includes balancing the patient's fluids and electrolytes, maintaining their oxygen level and blood pressure, and treating them for other infections. Eight patients of the Kikwit outbreak were given blood from Ebola survivors. Seven of these eight patients survived, but the efficiency of this type of treatment on a large scale remains unknown.1
Control of the virus depends on early recognition of Ebola HF in the patient and barrier isolation of that patient once Ebola HF is diagnosed.9 Attempts to control Ebola depend in part on learning more about the virus. Following a recent outbreak in Kikwit, Zaire in 1995, the CDC and other collaborators have begun trying to identify the natural reservoir of the virus.2 The CDC and other scientists are researching other facets of the virus as well. A vaccine for Ebola is being developed. The vaccine includes a primer DNA vaccine followed by a recombinant adenovirus booster.10 Hopefully some of these research efforts will shed more light on the activity of this mysterious virus.
End Notes
1 "Disease Information: Viral Hemorrhagic Fevers: Fact Sheets." http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola.htm April 30, 2002.
2 Sanchez, Anthony et al. "Reemergence of Ebola Virus in Africa." January 19, 1996. http://www.cdc.gov/ncidod/EID/vol1no3/sanchez.htm May 7, 2002.
3 "Disease Information: Viral Hemorrhagic Fevers; Fact Sheets." http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/filoviruses.htm May 6, 2002.
4 Crowley, J. and T. Crushberg. "Ebola and Marburg Virus: Genomic Structure, Comparative and Molecular Biology." May 18, 1995. http://www.bocklabs.wisc.edu/ebola/genomic-differences.html May 7, 2002.
5 Freed, E.O. Rafting with Ebola. (2002). Science 296 (5566), 279.
6 "Ebola Hemorrhagic Fever." December 2000. http://www.who.int/inf-fs/en/fact103.html May 6, 2002.
7 "Medical Encyclopedia: Ebola Hemorrhagic Fever." February 27, 2002. http://www.nlm.nih.gov/medlineplus/ency/article/001339.htm April 30, 2002.
8 "Medical Encyclopedia: Ebola Virus." http://www.nlm.nih.gov/medlineplus/ency/imagepage/17160.htm April 30, 2002.
9 Jacobson, A. "Emerging and Re-emerging Viruses: An Essay." May 18, 1995. http://www.bocklabs.wisc.edu/ed/ebolasho.html#EBOLA May 7, 2002.
10 "The Counter Bioterrorism Research Agenda of the NIAID for CDC Category A Agents. February 2002. http://www.niaid.nih.gov/dmid/pdf/biotresearchagenda.pdf May 7, 2002.