Ebola Virus, common name for several strains of virus, three of which are known to cause hemorrhagic fever in humans, which is characterized by massive bleeding and destruction of internal tissues. The Ebola virus belongs to the family Filoviridae. The viruses are long rods, 800 to 1000 nanometers (nm) long (1 nm equals one-billionth of a meter, or 4 x 10-8 in), but particles as long as 14,000 nm have been seen. Each virus consists of a coiled strand of ribonucleic acid (RNA) contained in an envelope derived from the host cell membrane that is covered with spikes. The virus is named for the Ebola River in the Democratic Republic of the Congo (DRC, formerly Zaire), Africa, where the virus was first identified.
Three strains of Ebola virus that are often fatal to humans have been identified. Named for the areas in which the first recognized outbreaks took place, these strains are referred to as Ebola-Zaire, Ebola-Sudan, and Ebola-Cote d'Ivoire. A fourth Ebola strain, called Ebola-Reston, has not been found to cause disease in humans. As outbreaks of Ebola hemorrhagic fever continue to occur, other strains may be identified.
Cases of Ebola hemorrhagic fever have occurred in isolated instances and in outbreaks in sub-Saharan Africa. A significant problem in diagnosing the disease is that the viruses often strike in remote areas of developing countries, where access to laboratories for specimen analysis is limited. Of all the hemorrhagic fevers, Ebola-Zaire hemorrhagic fever is the most dramatic and deadly.
II. Symptoms
The Ebola virus causes hemorrhagic fever, which is characterized by such symptoms as severe headache, weakness, and muscle aches, followed by vomiting, abdominal pain, diarrhea, inflammation of the throat (pharyngitis), inflammation of the mucous membranes in the eye (conjunctivitis), bleeding from body openings, and, often, destruction of internal tissues. Rapid viral replication in infected cells is the direct cause of cell and tissue destruction. The onset of illness is sudden, and the disease often progresses swiftly to extreme exhaustion (prostration), dehydration, and death. The time from exposure to the onset of illness is usually less than two weeks, and the time from onset of illness until death or improvement is usually seven to ten days. The mortality rates in the known outbreaks have been 60 percent with Ebola-Sudan virus and 77 to 88 percent with Ebola-Zaire virus.
Although it is believed that death results directly from the damage to internal tissues, it is not known why some patients manage to survive the disease. There are no proven therapeutic drugs to treat Ebola hemorrhagic fever, and treatment currently consists of preventing shock and providing supportive care. Medical care is complicated by the need to protect medical and nursing personnel. Convalescence is slow, often taking five weeks or more, and is marked by weight loss and amnesia in the early stages of recovery.
Currently, there is little hope of developing a vaccine against the Ebola virus. Near the end of one outbreak in Zaire during 1995, blood from convalescent patients was transfused into severely ill victims in an attempt to transfer antibodies and T-lymphocytes (one type of white blood cell) that might neutralize the Ebola virus and destroy infected cells. This procedure met with some success, but carefully controlled trials must be conducted to confirm the safety and effectiveness of this method.
III. History of Ebola Outbreaks
Ebola virus was identified for the first time in 1976, when two epidemics of hemorrhagic fever occurred, one in Zaire, the other 600 km (about 375 mi) distant in the Sudan. The combined outbreaks accounted for more than 550 cases and 430 deaths.
A third strain of the Ebola virus was identified in 1989 in a quarantine facility in Reston, Virginia, where hundreds of imported Philippine monkeys died. The Ebola-Reston virus seems not to cause disease in humans—although four laboratory technicians were infected with the virus, none of them became ill. Another large epidemic of Ebola hemorrhagic fever occurred in Zaire, this time in and around the city of Kikwit during the summer of 1995, infecting 315 people and killing about 250. The strains of Ebola virus isolated in Zaire in 1976 and 1995, 19 years and 500 km (about 310 mi) apart, are virtually identical.
A single nonfatal case of Ebola hemorrhagic fever occurred in late 1994 in Côte d'Ivoire. A Swiss zoologist who performed an autopsy on a chimpanzee was infected by the virus, which was subsequently identified as the fourth strain, Ebola-Côte d'Ivoire. Since the first episode there have been additional cases and fatalities caused by this virus, in Côte d'Ivoire, Liberia, and Gabon.
IV. Diagnosing the Virus
Each outbreak has been traced to an index case, an infected person who came into contact with a reservoir host, an animal or arthropod involved in the life cycle of the virus. Of all the disease-causing human viruses, the Ebola and its relative Marburg, which also causes hemorrhagic fever, are the only ones remaining for which the host and the natural transmission cycle remain unknown. It is not known whether monkeys serve as hosts or if other mammals, birds, reptiles, or even mosquitoes or ticks are involved.
From the index case, infection between humans is principally due to direct, close contact, such as that between a patient and nurses and doctors. Unhygienic hospital conditions also spread the virus.
The disease is diagnosed using a laboratory technique called ELISA (enzyme-linked immunosorbant assay) that searches for specific antigens (viral proteins) or antibodies made by the infected patient. A technique used to duplicate genetic material for study, called the polymerase chain reaction, is used to detect Ebola viral material in patient blood or tissues. When infection by the virus is suspected, local health officials institute strict barrier nursing procedures (such as the use of gowns, gloves, and masks) and usually call on experts from the World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), and the United States Army to provide help with diagnosis, patient care, and epidemic control measures.
The Ebola virus has been classified by the CDC as Biosafety Level 4, which requires the greatest safety precautions. To ensure maximum safety, virologists (scientists who study viruses) must work in special protective clothing, and their laboratories contain equipment that sterilizes air, and liquid and solid wastes.
Detailed studies comparing RNA sequences between the different viral strains are only now being performed. It is hoped that such genetic information will provide clues about the natural history and hosts of the viruses.