Ebola Virus

Since the mid-1970s, scientists from organizations that include the Centers for Disease Control and prevention (CDC) have periodically been pressed into action to help quell disease outbreaks caused by the Ebola virus. One facet of their responsibilities has been forensic investigations that involve determining the origins of the outbreaks.

Although naturally occurring, the swift and high lethality of the Ebola virus makes it an attractive potential bioterrorism agent. This high lethality characteristic can actually work against a large outbreak, as it limits the natural spread of the virus. Deliberate spread of the virus in multiple population areas could counteract the natural limiting factor.

The Ebola virus is one of two members of a family of viruses that is designated as the Filoviridae. The name of the virus comes from a river located in the Democratic Republic of the Congo. It was near this river that the virus was discovered.

The species of Ebola virus are among a number of viruses that cause hemorrhagic fever, which is typified by copious internal bleeding and bleeding from various orifices of the body, including the eyes. The disease can be swift and devastating, resulting in death in up to 90% of cases.

To date, four species of Ebola virus have been identified, based on differences in their genetic sequences and in the immune reaction they elicit in infected individuals. Three of the 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 in primates. The latter species is capable of infecting humans but so far has not caused disease in humans. 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.

The explosive onset of the illness and the under-developed and wild nature of the African region of the virus's appearance has complicated forensic investigations into the origin and natural habitat of Ebola. The source of the Ebola virus is still unknown. However, given that filovirus, which produces similar effects, can establish a latent infection in African monkeys, macaques, and chimpanzees, it seems reasonable that the Ebola virus could reside normally in a similar host. However, direct evidence is so far lacking.

Almost all confirmed cases of Ebola from 1976 to 2003 have been in Africa. In the latest outbreaks, which persisted in Gabon through 2003, 122 people were known to have been infected and 96 died, according to data from the CDC. A smaller outbreak killed 7 of 17 infected people in 2004 in Sudan. In the past, one individual in Liberia presented immunological evidence of exposure to Ebola, but had no symptoms. As well, in 1976 a laboratory worker in England developed Ebola fever as a result of a laboratory accident in which the worker was punctured by an Ebola-containing needle.

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

Outbreaks of infection with the Ebola virus appear sporadically and suddenly. The outbreak rapidly moves through the local population and often just as quickly ends. The initial infection is presumably by contact between the person and an 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. The fact 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.

The possibility of air-borne transmission of the virus is debatable. Ebola-Reston may have been transmitted from monkey to monkey in the Reston military facility via the air distribution system, since some of the monkeys that were infected were never in physical contact with the other infected monkeys. However, if the other species of the virus are capable of similar transmission, this 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 is possible but plays a minor role in the spread of the virus.

In the intervening years between the sporadic outbreaks, the Ebola virus probably resides in its natural reservoir. Whether that reservoir is an animal or plant, or resides in the soil or other environment is unknown, although scientists suspect the reservoir is a mammal.

Currently there is no cure for the infection caused by the Ebola virus. However, near the end of an outbreak of the virus in 1995 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 protective factor was not ascertained. A detailed examination of this possibility awaits another outbreak.

The molecular basis for the establishment of an infection by the Ebola virus is still also more in the realm of proposal than fact. One clue has been the finding of a glycoprotein that is a shortened version of the viral constituent in the circulating fluid of humans and monkeys. This protein has been suggested to function as a decoy for the immune system, diverting the immune defenses from the actual site of viral infection. Another immunosuppressive mechanism may be the selective invasion and damage of the spleen and the lymph nodes, which are vital in the functioning of the immune system.

The devastating infection caused by the Ebola virus is all the more remarkable given the very small size of the viral genome, or complement of genetic material. 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.