Ebola Virus

The Ebola virus is one of a number of viruses that cause a devastating disease in humans and closely related species such as monkeys, gorillas, and chimpanzees. The disease is known as a hemorrhagic fever because of the massive internal bleeding caused by the viral infection. Most strains of Ebola hemorrhagic fever progresses quickly from the initial appearance of symptoms to resolution, which is often death.

The name of the Ebola virus comes from a river located in the Democratic Republic of the Congo, where the virus was discovered in 1976. At that time, the country was known as Zaire. The virus was designated Ebola Zaire. Over 300 cases were associated with this initial outbreak. That same year, another type of Ebola virus, named Ebola Sudan, caused an outbreak in another African country, Sudan. Outbreaks also occurred in 1979, 1995, and 1996. In the latest outbreak, which has been ongoing in Gabon since late 2001, 54 people have died as of February of 2002.

The Ebola virus is a species in one group in a collection of viruses classified as Filoviridae. They were originally classified as rhabdoviruses. However, genome sequencing revealed more of a similarity to paramyxoviruses. However, filoviruses are sufficiently distinct from the other nonsegmented negative-stranded RNA viruses to warrant taxonomic status as a separate virus family. This family of viruses causes a disease that is typified by copious internal bleeding and bleeding from various orifices of the body, including the eyes. The disease can be swiftly devastating and results in death in about 90% of cases.

As of 2003, four species of Ebola virus have been identified. The species differ slightly in their genetic sequences and in the immune reaction they elicit in those who become infected. A different immune reaction means that the protein components that cause the immune reaction (antigens) are different.

Three of the Ebola virus species cause disease in humans. These are Ebola-Zaire (isolated in 1976), Ebola-Sudan (also isolated in 1976), and Ebola-Ivory Coast (isolated in 1994). The fourth species, called Ebola-Reston, causes disease only in primates.

Ebola Reston is named for the United States military primate research facility where the virus was isolated, during a 1989 outbreak of the disease caused by infected monkeys that had been imported from the Philippines. Until the non-human involvement of the disease was proven, the outbreak was thought to be the first outside of Africa. This incident brought the virus and the disease it causes to prominent public attention, particularly in the United States.

The appearance of the Ebola virus only dates back to 1976. The explosive onset of the illness and the fact that the outbreaks tend to occur in underdeveloped and remote regions of Africa have complicated both the treatment of the disease and the tracking of its habitat and origin. Indeed, the source of the Ebola virus is still unknown. However, the related filovirus, which produces similar effects, establishes a latent infection (one that does not always produce symptoms, but which is infectious and capable of being spread) in African monkeys, macaques, and chimpanzees. A credible theory, therefore, is that the Ebola virus normally resides in an animal that lives in Africa. As of 2002, the Ebola virus has not been isolated from live primates in the wild. Chimpanzees found dead and infected with the Ebola virus, however, were responsible for the initiation of Ebola outbreaks in 1996 in two regions of Gabon when local residents ate infected meat from the dead chimpanzees.

Almost all confirmed cases of Ebola from 1976 to 2002 have been in Africa. In the past, one individual in Liberia presented immunological evidence of exposure to Ebola, but had no symptoms. As well, a laboratory worker in England developed Ebola fever as a result of a laboratory accident in which the worker was punctured by an Ebola-contaminated needle. No human case has been reported in North America. A suspected case in Hamilton, Ontario, Canada in early 2001 turned out not to be Ebola, although the exact cause of this illness remains unresolved.

The Ebola virus. © Corbis Sygma/Corbis. Reproduced by permission.

Infection with the Ebola virus produces a high fever, headache, muscle aches, abdominal pain, tiredness and diarrhea within a few days. Some people will also display bloody diarrhea and vomit blood. At this stage of the disease some people do recover. But, for most of those who are infected, the disease progresses within days to produce copious internal bleeding, shock, and death.

The initial infection is presumably by contact between the person and the animal that harbors the virus. Subsequent person-to-person spread likely occurs by contamination with the infected blood or body tissues of an infected person in the home or hospital setting, or via contaminated needles. Following the initial human infection, spread occurs from direct contact with the blood and/or secretions of the infected person. Within a few days of infection, symptoms that typically develop include a fever, headache, muscle aches, fatigue, and diarrhea. Within one week of infection, hemorrhaging causes blood to flow from the eyes, nose, and ears. Death usually occurs within nine or 10 days of the onset of infection.

The severity and lethality of Ebola hemorrhagic fever effectively limit the outbreaks to a relatively few individuals. Outbreaks of infection with the Ebola virus appear sporadically, rapidly move through the population, and usually end just as suddenly. The high death rate associated with infection is likely the limiting factor in the infection's spread. Viruses require a host cell in which to reproduce. Killing the host eliminates the use of the host's cells as a reproductive factory for the virus. In other words, death is so rapid that the virus has a limited chance to spread to other victims.

That infected people tend to be in more under-developed regions, where even the health care facilities are not as likely to be equipped with isolation wards, furthers the risk of spread. The person-to-person passage is immediate. Unlike the animal host, people do not harbor the virus for lengthy periods of time.

Whether Ebola viruses can be transmitted through the air is not known. In the outbreak at the Reston facility, Ebola Reston may well have been transmitted from monkey to monkey via the air distribution system. Indeed, some of the monkeys that were infected were never in physical contact with the other infected monkeys. However, if the other species of the Ebola virus are capable of similar transmission has not yet been documented. Laboratory studies have shown that Ebola virus can remain infectious when aerosolized. But the current consensus is that airborne transmission, although possible, plays a minor role in the spread of the virus.

Similarly, the natural reservoir of the Ebola viruses is not known. Between the sporadic outbreaks, the Ebola virus probably is resident in this natural reservoir, whatever that reservoir may be.

Currently there is no cure for the infection caused by the Ebola virus. However, near the end of a 1995 outbreak in Kikwit, Africa, blood products from survivors of the infection were transfused into those actively experiencing the disease. Of those eight people who received the blood, only one person died. Whether or not the transfused blood conveyed a protective factor was not established, as resources were being devoted to bedside care rather than research. A detailed examination of this possibility awaits another outbreak. With the advent of recombinant DNA technology, the molecular structure of Ebola viruses, the details of their replication and interaction with the host are under intense study.

The molecular basis for the establishment of an infection by the Ebola virus is still also more theoretical than fact. One clue has been the finding of a glycoprotein in the circulating fluid of infected humans and monkeys that is a shortened version of a viral component. It may be that this protein acts as a decoy for the immune system, diverting the immune defenses from the actual site of viral infection. It has also been proposed that the virus suppresses the immune system, via the selective invasion and damage of the spleen and the lymph nodes (which are vital in the functioning of the human immune system).

The devastating infection caused by the Ebola virus is all the more remarkable given the very small size of the viral genome. Fewer than a dozen genes have been detected. How the virus establishes an infection and evades the host immune system with only the capacity to code for less than twelve proteins is unknown.