Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Ebola virus

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To date, four species of Ebola virus have been identified,

based on differences in their genetic sequences and in the

immun reaction they elicit in infected individuals. Three of the

species cause disease in humans. These are Ebola-Zaire (iso-

lated 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 appearance of the Ebola virus only dates back to

1976. The explosive onset of the illness and the under-devel-
      oped and wild nature of the African region of the virus’s
      appearance, has complicated the definitive determinations of
      the origin and natural habitat of Ebola. The source of the Ebola
      virus is still unknown. However, given that filovirus, which
      produce similar effects, establish a latent infection in African
      monkeys, macaques, and chimpanzees, scientists consider the
      possibility that the Ebola virus likewise normally resides in an
      animal that lives in Africa. A search for Ebola virus in such
      primates has so far not revealed evidence of the virus.
      Almost all confirmed cases of Ebola from 1976 to 2002
      have been in Africa. In the latest outbreak, which has been
      ongoing since late in 2001, 54 people have died in the Gabon as
      of February of 2002. In the past, one individual in Liberia pre-
      sented immunological evidence of exposure to Ebola, but had
      no symptoms. As well, a laboratory worker in England devel-
      oped 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, mus-
      cle 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 presumable by contact between the
      person and the animal that harbors the virus. Subsequent per-
      son-to-person spread likely occurs by contaminationwith 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 well 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 capa-

ble 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 is resident in the natural reservoir.

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 sur-

vivors 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 trans-

fused blood conveyed protective factor was not ascertained.

A detailed examination of this possibility awaits another

outbreak.

The molecular basis for the establishment of an infec-

tion by the Ebola virus is still also more in the realm of pro-

posal than fact. One clue has been the finding of a

glycoprotein that is a shortened version of the viral constituent

in the 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 complementof 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.

See alsoHemorrhagic fevers and diseases; Zoonoses

Negative stain electron micrograph of an Ebola virus.

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