Ebola Virus


The Ebola Virus is the 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.
Named for the Ebola River in Zaire, Africa, where the virus was first
identified, the Ebola virus belongs to the family Filoviridae. 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 (EBOZ),
Ebola/Sudan (EBOS), and Ebola/Tai Forest (EBOT). A fourth Ebola strain,
called Ebola/Reston(EBOR), has not been found to cause disease in
humans. As outbreaks of Ebola hemorrhagic fever continue to occur,
other strains may be identified. 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 7 nm
spikes placed 10 nm apart visible on the surface of the virion (Figure
1). When magnified several thousand times by an electron microscope,
these viruses have the appearance of long filaments or threads but the
particles are pleomorphic, meaning they can exist in many shapes. Their
basic structure is long and filamentious, essentially bacilliform, but
the viruses often takes on a "U" shape (Figure 2). They contain a
unique single-stranded molecule of noninfectious (negative sense ) RNA.
The virus is composed of 7 polypeptides, a nucleoprotein, a
glycoprotein, a polymerase and 4 other undesignated proteins. Proteins
are produced from polyadenylated monocistronic mRNA a species
transcribed from vi

genomes. As the infection progresses the cytoplasm of the infected cell
develops "prominent inclusion bodies" which contains the viral
nucelocapsid, which will become highly structured. The virus then
assembles, and buds off the host cell, attaining its lipoprotein coat
from the infected cell's outer membrane. The replication in and
destruction of the host cell is rapid and produces a large number of
viruses budding from the cell membrane. Symptoms Cases of Ebola 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 Ebola
strains, Ebola/Zaire is the most dramatic and deadly. 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), and
bleeding from body openings. The virus spreads through the blood and is
replicated in many organs. The histopathologic change is focal necrosis
in these organs, including the liver, lymphatic organs, kidneys,
ovaries and testes. The central lesions appear to be those affecting
the vascular endothelium and the platelets. The resulting
manifestations are bleeding, especially in the muc 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.
Evolution Besides morphological and biochemical similarities, all
nonsegmented negative-strand RNA viruses share several features in
their mechanisms of transcription and replication: similar genome
organization, complementarity of the genome extremities, homologous
sequences in the 3' untranslated region, conserved transcriptional
signals, interruption of genes by intergenic sequences, possession of a
virion-associated polymerase, helical nucleocapsid as the functional
template for synthesis of replicative and messenger RNA, replication by
synthesis of a full-length antigenome, transcription of messenger RNAs
by sequential interrupted synthesis from a single promotor,
transcription and replication in the cytoplasm, and maturation by
envelopment of independently assembled nucleocapsids at membrane sites
containing inserted viral proteins. These data suggest that all
nonsegmented negative-strand RNA viruses are derived from a common
progenitor and support the classification of the families Filoviridae,
Paramyxoviridae and Rhabdoviridae in the order Mononegavirales . In
addition, comparative amino acid sequence analyses of nucleoproteins
and polymerase proteins suggest that filoviruses are more closely
related to paramyxoviruses than to rhabdoviruses. 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 distant in 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 242. The strains of Ebola virus
isolated in Zaire in 1976 and 1995, 19 years and 500 km apart, are
virtually identical. A single nonfatal case of Ebola hemorrhagic fever
occurred in late 1994 in Cte 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/Tai Forest, named
for the Tai Forest in the Cte d'Ivoire. Since the first episode there
have been additional cases and fatalities caused by this virus, in Cte
d'Ivoire, Liberia, and Gabon. 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 original 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. The test is performed on a monolayer of infected
and uninfected cells fixed on a microscopic slide. IgG- or IgM-specific
immunoglobulin assays are performed. These tests may then be confirmed
by using western blot or radioimmunoprecipitation. Virus isolation is
also a highly useful diagnostic method, and is performed on suitably
preserved serum, blood or tissue specimens stored at -70oC or freshly
collected. 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 Diseas The Ebola virus has been classified by the CDC as Biosafety
Level 4, which requires the greatest safety precautions. To ensure
maximum safety, virologists 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. Natural Reservoir The
natural reservoir of the Ebola virus is not entirely known. Serological
studies suggest that Ebola or related viruses are endemic in Zaire,
Sudan, the Central African Republic, Gabon, Nigeria, Ivory Coast,
Liberia, Cameroon and Kenya. The geographic range of Ebola strains may
extend to other African countries, for which adequate survey is
lacking. Extensive ecological studies are currently underway in Cte
d'Ivoire, Gabon and Zaire to pinpoint the reservoir. Ebola-related
filoviruses were isolated from cynomolgus monkeys (Macacca
fascicularis) imported into the United States of America from the
Philippines in 1989. A number of the monkeys died and at least four
persons were infected, although none of them suffered clinical
illness. Annalysis of Outbreaks Serologic evidence has suggested the
presence of Ebola virus in Gabon since 1982. Since late 1994, three
apparently independent outbreaks of Ebola virus hemorrhagic fever have
occurred among humans in northeastern Gabon, in the forested areas of
equatorial Ogoou-Ivindo province. The first, which started in December
1994 in gold-panner encampments of far northeastern Gabon, in the
Minkouka area (Figure 3) near the Nouna River, had several
laboratory-confirmed cases. The second, which began in early February
1996 in Mayibout village (Figure 3) on the Ivindo River, resulted in 37
Ebola hemorrhagic fever cases. The only means of transportation between
these two areas is by boat; Makokou, the closest town to them (Figure
3), has the provincial hospital to which patients and contacts were
transferred. The third outbreak, started in July 1996 in the village of
Boou (Figure 3), where most of the cases occurred; however, scattered
cases have been diagnosed in surrounding villages and towns. Some

have even been transported to Libreville, probably during the
incubation period of the disease. One patient was treated in South
Africa, where a fatal nosocomial infection was subsequently reported in
a health care worker; over 43 deaths due to Ebola hemorrhagic fever
were reported during this prolonged outbreak. There were many gene
sequences obtained from human samples during each of the three Gabonese
epidemics. Some was obtained from blood collected 1 day before the
death of a patient, from the Nouna area, during the 1994 outbreak.
Other sequences were derived from blood collected during the spring
1996 outbreak, from two primary patients who were infected while
butchering a chimpanzee they found dead in the forest. One sequence was
derived from blood collected from what appears to have been a secondary
case during the same outbreak; the patient was probably infected by
contact with one of the index patients while visiting a traditional
doctor who lived near Mayibout village. The isolation of Ebola virus
in a cell culture from human blood samples collected during the three
different outbreaks was easily accomplished in a single passage. RNA
was extracted from blood or primary tissue culture samples by using a
commercial kit. Viral sequences were amplified from RNA by using the
reverse transcriptase-polymerase chain reaction technique. Briefly,
amplified products were subjected to agarose electrophoresis and were
stained and visualized with ethidium bromide; DNA bands were then
excised and extracted. In some cases, nested PCR with internal primers
was performed, using first-round products. Amplified products were
directly sequenced by using an automated nonisotopic method. Excess
dye-labeled dideoxynucleotide terminators were removed, and reaction
products were analyzed. A consensus sequence was established by
aligning all the Ebola from Gabon, the Zaire 1976 and 1995 Ebola virus
sequences, as well as the sequence of the virus obtained from a nurse
in South Africa who was infected
 of the three different outbreaks in Gabon. Although the viruses
 causing the Gabonese outbreaks clearly belong to the Zaire subtype,
 they were distinct from viruses that had caused disease in Zaire. No
 differences were observed between tissue-culture-passaged and
 clinical-material-derived sequences or between primary or secondary
 case sequences. RNA extracted from a single representative of each
 outbreak was then used to generate the entire gene sequence for the
 Gabon Ebola viruses. The gene sequence from the Gabon spring 1996
 viruses differed from the sequence of the Gabon fall 1994 viruses by
 four nucleotides. The genetic sequence from the Gabon fall 1996
 viruses differed from the sequence of the Gabon spring 1996 virus by
 four additional nucleotides. A single most parsimonious tree was
obtained (Figure 4), and bootstrap analysis strongly supports a common
evolutionary origin for the viruses associated with disease in Gabon
and Zaire. Overall, these data indicate that the three Gabon outbreaks
should be considered independent events, likely originating from
different sources. The presence of stable virus sequences and the lack
of genetic variability between strains isolated within an outbreak was
previously seen during the outbreak of Ebola hemorrhagic fever in
Kikwit, Zaire, in 1995, and despite the small number of isolates
tested, is again suggested in Gabon. During a 20-month period, Gabon
had three different outbreaks of Ebola virus hemorrhagic fever. The
first and the second episodes apparently started during the rainy
season (December and February), while the third began during the dry
season (July). The deaths of nonhuman primates were associated with all
three outbreaks. Minkouka area inhabitants reported finding dead
chimpanzees and gorillas in the forest during the fall of 1994. All the
primary human patients in the spring 1996 outbreak were infected while
butchering dead chimpanzees. For the third outbreak, the investigation
has indicated an index patient who was a hunter, living in a forest
camp in the Boou area. During the same period, an Ebola virus-infected
dead ch In Cte d'Ivoire in 1994, an investigator was infected with
Ebola virus while performing necropsy on a dead chimpanzee. Primates
are unlikely to be the reservoir of Ebola virus since experimental or
natural infection is quickly fatal. A better knowledge of the ecology
of great apes, particularly their food preferences and habitats, may
lead to the identification of the virus reservoir. Gabon's equatorial
forests, where three independent outbreaks have occurred in less than 3
years, offer an excellent opportunity for these investigations. It has
now become apparent that the only solution to this problem, which
society is increasingly becoming aware of, is diligent research and
experimentation. The CDC continues to inject infant mice and guinea
pigs with the virus and document the details of either their deaths or
recoveries. It is the hopes of both the scientific community and the
rest of the world that some tangible solution be found as soon as

Figure 1. Spikes surrounding the virus.

Figure 2. The "U" Shape of the virus.

Figure 3. Geographic distribution of the three Ebola virus
hemorrhagic fever epidemics and site of the infected
chimpanzee in Gabon.

Figure 4. Phylogenetic tree showing the relationship
between the Ebola viruses that caused outbreaks of disease
in Gabon and previously described filoviruses. The
entire coding region for the glycoprotein gene of the viruses
shown was used in maximum parsimony analysis, and a
single most parsimonious tree was obtained. Numbers in
parentheses indicate bootstrap confidence values for
 branch points and were generated from 500 replicates
Branch length values are also shown.

Figure 5. Immunostaining of Ebola virus antigens (red) within vascular
endothelial cells in skin biopsy of the chimpanzee found dead in the
forest near Boou. Note also the presence of extracellular viral


bjach.polk.amedd.army.mil/B8/Ebola.htm. Ebola


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