Post on 09-Apr-2023
Ebola and Marburg haemorrhagic fever viruses: major scientific
advances, but a relatively minor public health threat for Africa
E. M. Leroy1,2, J-P Gonzalez1 and S. Baize3
1) Centre International de Recherches Medicales de Franceville, Franceville, Gabon, 2) MIVEGEC (IRD 224, CNRS 5290, Universites Montpellier 1), Institut
de Recherche pour le Developpement, Montpellier, France and 3) Unite de Biologie des Infections Virales Emergentes, Institut Pasteur, IFR128-Biosciences
Gerland-Lyon Sud, Lyon, France
Abstract
Ebola and Marburg viruses are the only members of the Filoviridae family (order Mononegavirales), a group of viruses characterized by
a linear, non-segmented, single-strand negative RNA genome. They are among the most virulent pathogens for humans and great
apes, causing acute haemorrhagic fever and death within a matter of days. Since their discovery 50 years ago, filoviruses have caused
only a few outbreaks, with 2317 clinical cases and 1671 confirmed deaths, which is negligible compared with the devastation caused by
malnutrition and other infectious diseases prevalent in Africa (malaria, cholera, AIDS, dengue, tuberculosis …). Yet considerable human
and financial resourses have been devoted to research on these viruses during the past two decades, partly because of their potential
use as bioweapons. As a result, our understanding of the ecology, host interactions, and control of these viruses has improved consid-
erably.
Keywords: Ebola, immunity, Marburg, outbreak, reservoir, vaccine
Clin Microbiol Infect
Corresponding author: E. M. Leroy, Centre International de Re-
cherches Medicales de Franceville, Franceville, Gabon
E-mail: eric.leroy@ird.fr
Introduction
Ebola (EBOV) and Marburg (MARV) are the only members
of the Filoviridae family and are among the most virulent
pathogens for humans and great apes. The most virulent spe-
cies of these viruses induce acute haemorrhagic fever and
death within a few days in up to 90% of symptomatic individ-
uals [1,2]. The explosive disease course, high case fatality
rate, and lack of specific treatments or vaccines make EBOV
and MARV a major public health issue for Africa, and a cate-
gory A biothreat [3,4]. In the field, clinical specimens must
be handled according to WHO guidelines on viral haemor-
rhagic fever agents in Africa [5], and analyzed in biosafety
level 4 laboratories.
Research on filoviruses has been closely tied to outbreak
activity. Some advances in the biology and pathogenesis of
MARV and EBOV were made just after the discovery of these
agents, in 1967 and 1976, respectively. The two viruses were
then largely neglected until EBOV re-emerged in 1995 in Kik-
wit, Democratic Republic of Congo (DRC), where it caused a
large outbreak that received considerable media attention
worldwide. The extraordinarily high lethality, multifocal haem-
orrhaging, and specificity to African ecosystems enhanced the
fascination of Ebola for the international community, which
saw this disease as a potential threat in the post-Cold War
context [6]. This partly explains why research on the biology,
epidemiology, ecology and pathophysiology of these viruses
has advanced markedly over the past 15 years, leading
recently to the development of potential antiviral drugs and
candidate vaccines. Yet the overall disease burden of filovirus-
es over the past 45 years is minimal compared with major
infectious diseases such as AIDS, malaria, cholera, dengue and
tuberculosis [7,8]. This paper reviews research advances in
Ebola and Marburg ecology, virus–host interactions and dis-
ease control during the past 15 years.
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Filoviruses as Prototypes of EmergingPathogens
An emerging pathogen is defined as either a truly novel or a
hitherto unknown agent, or a pathogen whose incidence has
increased considerably in susceptible populations. EBOV and
MARV fulfil one criterion of this definition, in that they were
identified relatively recently. The first reported cases of
human filovirus infection occurred in 1967 in Marburg, Ger-
many, and in Belgrade, Serbia (former Yugoslavia), when lab-
oratory technicians were infected while manipulating African
green monkeys (Cercopithecus aethiops) imported from
Uganda [9–12]. An unknown infectious agent was recovered
from the victims’ blood and organs and was named ‘Marburg
virus’ after the outbreak location. EBOV first emerged in the
form of two near-simultaneous outbreaks in 1976 in Sudan,
near the border with DRC, and in DRC, near the border
with Sudan [13,14]. An unknown causative agent was isolated
from patients in the DRC outbreak and was named for the
river Ebola, which flows past Yambuku, the outbreak epicen-
tre (Fig. 1).
The morphology of MARV and EBOV is unique among
viruses (Fig. 2a,b). Using electron microscopy, they often
appear as long filamentous particles about 14 000 nm long
and 80 nm wide, whereas other particles assume the shape
of the number 6 or a hairpin [15–17]. MARV and EBOV are
the only members of the Filoviridae family. This family belongs
to the order Mononegavirales, which also includes Rhabdoviri-
dae, Paramyxoviridae and Bornaviridae, a group of viruses
characterized by a linear, non-segmented, single negative-
stranded RNA genome [18–23].
These viruses cause an explosive disease with multiple
haemorrhaging occurring within a few days and a high fatality
rate (up to 90%), which was rarely observed in viral diseases
[24–29]. Generally, after an incubation period ranging from 2
to 21 days (mean 4–9 days), non-specific symptoms such as
fever, headache, nausea and muscle pain occur abruptly.
There are rapidly followed by gastrointestinal problems
(stomach pain, vomiting, diarrhoea), respiratory disorders
(throat and chest pain, cough) and neurological manifesta-
tions (prostration, confusion, delirium), indicating systemic
viral dissemination and multi-organ failure. Haemorrhagic
manifestations vary in severity and location, and often include
skin rash, conjunctival injection, nosebleeds, melaena, hae-
matemesis and bleeding at venepuncture sites. In fatal cases
the symptoms and haemorrhaging rapidly become severe,
and death generally ensues in a context of tachypnoea,
coma and shock. The virus disappears rapidly from the
bloodstream after the resolution of systemic symptoms in
survivors, and prolonged convalescence is frequent, with epi-
sodic non-specific symptoms such as asthenia, myalgia and
fever. More specific signs, associated with transient persis-
tence of the virus in immunologically protected sites such as
the eyes and testicles have been reported during the early
stages of recovery. The virus has been isolated for up to
2 months in semen [30–32]. Biological parameters are not
specific for filovirus infection and most data come from
experimentally infected animals [33,34]. They include mild
and early leucopenia, associated with lymphopenia, thrombo-
cytopenia (< 100 000 platelets/mm3), hyperproteinaemia, and
marked transaminase elevation. The prothrombin time is
always prolonged and fibrin split products are often
detected, indicating disseminated intravascular coagulation
[35,36].
(a)
(b)
Republic
of Congo
2001–02
2003
2005
Democratic
Republic of Congo
1976
1995
2007
2008
Sudan
1976
1979
2004
Uganda
2000
Ivory Coast 1995
Uganda
2007
Gabon
1994
1996
1996–97
2001–2002
Kenya
1980
1987
South Africa
1975
Democratic
Republic of Congo
1998–2000
Angola
2005
Uganda
2007
2008
CIEBOV
BEBOV
SEBOV
ZEBOV
Marburg
Ebola
FIG. 1. Filovirus outbreaks in Africa. Reported outbreaks or isolated
cases of haemorrhagic fever caused by Marburg virus (MARV) (a)
and Ebola virus (EBOV) (b) are indicated with the corresponding
year and colour-coded according to virus species.
COLOR
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Filoviruses: a Questionable Public HealthBurden for Africa
Since their discovery 45 years ago, filoviruses have been
responsible for only a few outbreaks, resulting in 2317 clini-
cal cases and 1671 confirmed deaths (reviewed in Fig. 1 and
Table 1, [37]). Hence, the disease burden of filovirus infec-
tion in Africa is extremely small compared with other infec-
tious diseases and malnutrition.
As said previously, the first MARV outbreak occurred in
Europe and was associated with 31 cases, including 25 pri-
mary cases (i.e. infected directly by animals) with seven
deaths [9–11]. Since then, only sporadic cases have been
reported, in South Africa in 1975 (three cases, one death)
and in Kenya in 1980 (two cases, one death) and 1987 (one
fatal case) [31,38,39]. MARV subsequently caused two large
outbreaks, in 1998–2000 in DRC and in 2004–2005 in
Angola. The longest MARV outbreak hit the Durba area
(eastern DRC) between 1998 and 2000, affecting 154 people
and killing 83% of victims [40]. Most patients worked in
underground gold mines. The Durba outbreak was charac-
terized by the circulation of multiple viral strains, pointing to
multiple independent introductions from the unknown
(a) (b)
(c)
VP40
VP24
NPVP30RNA
VP35LMembrane
GP1,2
trimer
(d)
FIG. 2. 10Ultrastructure and structure of filovirus virons. Particles of various forms are visualized by negative staining by uracyl acetate (a, b). Part
of a rod-shaped particle showing the tubular nucleocapsid and the surface spikes (c). Schematic representation of a filovirion indicating protein
composition, the tubular nucleocapsid and the surface spikes (d). (From Kuhn JH 9. Filoviruses: a compendium of 40 years of epidemiological, clini-
cal, and laboratory studies. In: Calisher CH editor. Springer, Wien, NewYork; 2008).
TABLE 1. List of filovirus outbreaks with the corresponding
location, number of clinical cases and fatality rate
Virus Date Location CasesFatality(%)
Marburg virus 1967 Germany – Yugoslavia 31 231975 South Africa 3 331980 Kenya 2 501987 Kenya 1 100
1998–2000 DRC 154 832005 Angola 252 902007 Uganda 4 252007 Uganda 1 02008 Uganda 1 100
Ebola virusZaire
1976 DRC 318 881977 DRC 1 1001994 Gabon 49 651995 DRC 315 881996 Gabon 37 57
1996–1997 Gabon 60 752001–2002 Gabon and
Republic of Congo123 79
2003 begin Republic of Congo 143 902003 end Republic of Congo 35 83
2005 Republic of Congo 11 752007 DRC 264 712008 DRC 32 47
Ebola virusSudan
1976 Sudan 286 531979 Sudan 34 652000 Uganda 425 532004 Sudan 17 42
Ebola virusCote d’Ivoire
1994 Ivory Coast 1 0
Ebola virusBundibugyo
2007 Uganda 102 42
DRC, Democratic Republic of Congo.
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natural reservoir [29,40,41]. The largest recorded MARV
outbreak occurred in Uige province of northern Angola in
2004–2005 [42]. It was the first recorded outbreak in West
Africa, causing 252 cases and 227 deaths (fatality rate 90%).
Complete genomic characterization showed that the culprit
strain was closely related to previous East African isolates
[42]. Finally, small MARV outbreaks occurred in western
Uganda in 2007 and 2008 among workers in lead and gold
mines of the Kitaka Cave near Ibanda village [43,44]. Two of
the six victims died.
Ebola virus first emerged in the form of two near-simulta-
neous outbreaks in 1976 caused by two different species,
namely Sudan Ebola virus (SEBOV) in Sudan and Zaire Ebola
virus (ZEBOV) in DRC [13,14]. In Sudan, the outbreak was
centred in the Nzara and Maridi and was responsible for 284
cases. The mortality rate was 53%, characteristic of SEBOV
infection. The epicentre of the DRC outbreak was in Yamb-
uku, about 800 km from Maridi. This outbreak resulted in
318 cases with a mortality rate of 89%, characteristic of
ZEBOV infection. Later there was an unconfirmed lethal case
involving a 9-year-old girl living in Tandala, DRC, [45], fol-
lowed by another SEBOV outbreak in 1979, again in Nzara
and Maridi, with 34 cases and 22 deaths [46]. After a 15-year
period in which no further cases were recorded, EBOV
re-emerged in 1994 for a 3-year period. This new phase was
marked by the identification of a new species, Cote d’Ivoire
Ebola virus (CIEBOV), and by four ZEBOV outbreaks. The
only case caused by CIEBOV occurred in 1994, when an eth-
nologist sickened a few days after autopsying a chimpanzee
found dead in the Tai national park in Ivory Coast [47–49].
A large outbreak then occurred in 1995, in and around the
town of Kikwit, south of DRC, with 315 cases and a mortal-
ity rate of 81% [50]. Despite the deployment of more
sophisticated scientific and medical resources than those
available in 1976, this outbreak was as large as the 1976 out-
break, probably because it affected a town with several hun-
dred thousand inhabitants. Three other outbreaks, all as the
result of ZEBOV, struck northeast Gabon between 1994 and
1997 [51–54]. In total, these outbreaks involved 140 cases
and caused 75 deaths. The first occurred in gold-digger
camps located in the heart of the forest. The second out-
break hit the village of Mayibout, located south of Mekouka,
among children who had carried and butchered a chimpan-
zee carcass found in the forest. The third outbreak occurred
in 1996–1997, with 60 cases and 45 deaths over a 6-month
period. Fifteen cases and 11 deaths were recorded in Libre-
ville, the capital of Gabon, and a South African nurse was
infected by a Gabonese physician who had travelled to
Johannesburg. The period 2000–2008 was marked by
repeated ZEBOV outbreaks, SEBOV resurgence, and the dis-
covery of a new species of Ebola virus, Bundibugyo Ebola
virus (BEBOV). Between 2001 and 2005, Gabon and Republic
of Congo (RC) were hit by five ZEBOV outbreaks [55–58].
The first outbreak occurred within the cross-border area
between northeast Gabon and northwest RC, and resulted
in 143 cases and 128 deaths. RC was again affected twice in
2003 (respectively 143 and 35 cases, and 128 and 29 deaths),
and then in 2005 at Etoumbi, where only 11 cases (nine fatal-
ities) were reported. Two outbreaks of Sudan Ebola virus
also occurred during this period. One hit Uganda in 2000,
comprised three foci and was the largest of all recorded
EBOV outbreaks, causing 173 deaths among its 425 victims
[59,60]. A second SEBOV outbreak occurred in Sudan in
2004 in the town of Yambio, located near Nzara and Maridi.
There were 17 cases and seven deaths [61–63]. The latest
species to be discovered, Bundibugyo Ebola virus (BEBOV),
was discovered in 2007 in Uganda, where it was responsible
for a large outbreak [64] with 116 cases and 30 deaths (fatal-
ity rate 26%).
These four EBOV species are all pathogenic for humans
and arose in sub-Saharan Africa. An additional species, Res-
ton Ebola virus (REBOV), was isolated in 1989 from Asian
cynomolgus monkeys (Macaca fascicularis) imported from the
Philippines and housed in a quarantine facility in Reston, Vir-
ginia, USA. The monkeys developed a haemorrhagic disease
associated with high lethality, and subsequent outbreaks
among cynomolgus monkeys were reported in animal facili-
ties in Texas, USA, Italy and the Philippines [39,65–68].
Although no humans fell sick during these episodes, several
monkey handlers in the United States and in the Philippines
were shown to have seroconverted [69,70]. Recently, REB-
OV was also isolated from domestic Philippino swine with a
severe respiratory syndrome and co-infected by porcine
reproductive and respiratory syndrome virus [71].
Filoviral Genes and Proteins
The MARV and EBOV genomes are about 19 000 nucleo-
tides long and are transcribed into eight major subgenomic
mRNAs, which encode seven structural proteins: 3¢ leader,
nucleoprotein (NP), virion protein 35 (VP35), VP40, glyco-
protein (GP), VP30, VP24, and RNA-dependent RNA poly-
merase (L)-5¢ trailer; and one non-structural protein (sGP),
shown in Fig. 2. The central core of the virion is composed
of the ribonucleoprotein complex, which consists of the
genomic RNA molecule encapsulated by NP linked to the
inner matrix proteins VP30 and VP35 and the RNA-depen-
dent RNA polymerase (Fig. 2d). This complex is involved in
transcription and replication [72–74]. The matrix proteins
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VP40 and VP24 are linked to the ribonucleoprotein complex
at the inner surface of the lipid bilayer of the viral envelope,
which is derived from the host cell. VP40 and VP24 are
involved in viral nucleocapsid formation, viral budding or
assembly, and host range determination [72,75–82]. The viral
envelope contains only the glycoproteins, organized as tri-
meric spikes (Fig. 2c,d) consisting of two fragments (extracel-
lular protein, GP1, and membrane-anchored protein, GP2)
which result from furin-like enzyme cleavage of precursor
polyproteins and remain linked by a disulphide bond [74,83–
85]. GP binds preferentially to endothelial and monocytic
cells via GP1, mediating target cell entry, inducing endothelial
cell disruption and cytotoxicity in blood vessels in vitro, and
causing immunosuppression in vitro via a retrovirus-like pep-
tide [74,86–94]. EBOV differs from MARV and from other
members of the Mononegavirales in that infected cells
secrete large amounts of a non-structural glycoprotein
(sGP), the primary product of the GP gene, which is
expressed from non-edited mRNA species [93,95,96]. The
role of sGP is poorly understood. Finally, the matrix proteins
VP35 and VP24 seem to play a key role in the pathogenicity
of EBOV and MARV, by inhibiting antiviral responses and
particularly type 1 interferon (IFN) synthesis [97–102].
Filoviruses Genomics
The Filoviridae family comprises two genera, Marburgvirus and
Ebolavirus, and belongs to the order Mononegavirales, a
group of viruses characterized by a genome consisting of a
linear, non-segmented, single negative strand of RNA [18–
23]. Molecular evolutionary analyses of the glycoprotein
genes suggests that EBOV and MARV diverged several thou-
sand years ago [103]. The genus Marburgvirus consists of a
single species, Marburgvirus, comprising five viruses differing
from one another by up to 21% at the nucleotide level, while
there are five known species of Ebolavirus that have different
geographic locations, case fatality rates, and nucleotide
sequence divergences of about 32–41% [16].
Our knowledge of filovirus distribution and diversity may
be limited because EBOV and MARV have only been identi-
fied and characterized in sick or dead humans and animals
[40,42,43,53–56,64,68,71,104–107]. Numerous studies have
attempted to determine the regions of EBOV and MARV cir-
culation. Remotely sensed data and niche ecologic modelling
suggest that filoviruses may circulate within a much wider
region than their outbreak areas [108–111]. Using a sensitive
and specific ELISA method, an overall ZEBOV-specific IgG se-
roprevalence of 15.3% was found in Gabon, the highest ever
reported [112]. The seroprevalence was significantly higher
in forested areas (19.4%) than in grassland (12.4%), savannah
(10.5%) and areas rich in lakes (2.7%). These results are con-
sistent with previous reports of seroprevalence rates ranging
from 1.8% to 21.3% throughout central Africa using poorly
specific immunofluorescence antibody testing 2methods [113–
117]. More convincingly, two recent small serosurveys, based
on the same ELISA as the one we used, have shown high
antibody prevalence in some forested regions of Central
Africa. Indeed, a 9.3% (15/161) prevalence of ZEBOV-specific
IgG was found in unaffected villages surrounding Kikwit a few
weeks after the 1995 outbreak [118], and a 13.2% (25/190)
seroprevalence was found in the Aka Pygmy population in
the Central African Republic, where no ZEBOV outbreaks
have been reported [119]. Such high seroprevalence rates
indicate a high level of ZEBOV circulation in the tropical rain
forest environment and suggest that human rural populations
are highly exposed to the virus, with mild or asymptomatic
infection by outbreak strains [120] or less pathogenic strains,
or simple exposure to inert viral particles.
Filoviruses in Wildlife
Filovirus haemorrhagic fevers are typical zoonotic diseases
transmitted accidentally by direct contact with live or dead
animals. The role of wildlife species in the human epidemiol-
ogy of ZEBOV is only partly understood (Fig. 3). In addition
to its high pathogenicity for humans, ZEBOV has caused
massive outbreaks among gorillas and chimpanzees, killing
thousands of animals during the last decade in parts of Ga-
bon and RC [56,121–125]. Another study conducted in the
Tai forest of Ivory Coast showed that the disappearance of
11 members (26%) of a group of 43 chimpanzees during
November 1994 may have been the result of CIEBOV [126].
Such massive outbreaks in great apes could have major impli-
cations for animal conservation. The identification of multiple
strains during the 2001 Gabon/RC outbreak and the recent
identification of two phylogenetically divergent lineages sug-
gest independent introductions into great ape and human
populations following multiple viral spill-overs from a reser-
voir host [56,107,127]. In this ‘multi-emergence’ hypothesis,
Ebola outbreaks would occur episodically in certain ecologi-
cal conditions caused by habitat disturbances or climatic phe-
nomena [108,109]. Although the idea of multi-emergence
makes no reference to a particular timescale, this theory also
implicitly assumes that ZEBOV was present in Equatorial
Africa long before the first documented outbreak in 1976, as
supported by various serosurveys [128–130].
Although many field and laboratory studies have been
conducted since the first recorded outbreak in an attempt
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to identify the reservoir species of MARV and EBOV, conclu-
sive evidence that bats are natural hosts for filoviruses was
obtained only recently [14,45,131–138]. Antibodies and
nucleotide sequences specific for ZEBOV [127,139] were
detected in the liver and spleen of three fruit bat species in
Gabon and RC (Hypsignathus monstrosus, Epomops franquetti
and Myonycteris torquata), and antibodies and nucleotide
sequences specific for MARV [140,141] were found in a fruit
bat species in Gabon (Rousettus aegyptiacus) and in two insec-
tivorous bat species in DRC (Rhinolophus eloquens and Mini-
opterus inflatus). More recently, MARV was isolated from
cave-dwelling Rousettus aegyptiacus bats in Uganda [43].
Finally, a recent study showed that the 2007 Luebo outbreak
in DRC was linked to massive fruit bat migration, strongly
suggesting that humans could be infected directly by bats
(Fig. 3) [142].
Pathogenesis of Ebola Virus Infection andImmune Responses
Antigen-presenting cells such as dendritic cells and macro-
phages are major EBOV targets [143,144]. Indeed, by their
presence in mucosal tissues and skin, these cells are infected
early and serve as the substrate for the first replicative viral
cycles. Because of their extensive distribution among the dif-
ferent organs and tissues, and their mobility, dendritic cells
and macrophages are probably responsible for viral dissemi-
nation and systemic infection [145]. Intense viral replication
then occurs in secondary lymphoid organs and liver, and the
virus subsequently spreads to hepatocytes, endothelial cells,
fibroblasts and some epithelial cells [146,147]. The pathogen-
esis of filovirus infection is reviewed in Fig. 4.
However, despite widespread viral replication, the
observed damage to endothelia, liver and other organs is not
sufficiently severe to cause terminal shock and death. Endo-
thelial infection is infrequent, occurring in the terminal stages
with no clear evidence of cytopathic effects [36]. Similarly,
although multifocal hepatic necrosis may be linked to viral
replication in liver, the observed hepatocellular lesions can-
not directly lead to death [146,147]. A pathogenic role of
the host response is therefore suspected.
Extensive infection of monocytes and macrophages leads to
aberrant release of inflammatory mediators and chemokines
such as interleukin-1b (IL-1b), tumour necrosis factor-a (TNF-
a), IL-6, IL-15, IL-16, IL-1 receptor antagonist, soluble TNF
receptor, IL-10, NO), IL-8, growth regulated oncogene-a,
CCL3, CCL4, CXCL10, monocyte chemotactic protein-1, and
eotaxin [144,148–151]. This ‘cytokine storm’, particularly
prominent in the terminal stages of the disease in humans and
non-human primates, has dramatic consequences. It is proba-
ble that TNF-a, NO), and other vasoactive compounds play a
crucial role in the associated vascular leakage, by increasing
endothelial permeability, reducing vascular tone, and altering
endothelial cell functions [144,152–154]. In addition, some of
these mediators may induce the expression of adhesion mole-
cules at the surface of endothelial cells, so allowing neutroph-
ils and monocytes to invade sites of infection where they may
cause bystander tissue damage [155]. Infected macrophages
are involved in the coagulopathy and in the induction of dis-
seminated intravascular coagulation by their abundant expres-
sion of tissue factor [35,156,157]. Other consequences of the
massive antigen-presenting cell infection are lymphoid deple-
tion in the spleen, lymph nodes and thymus, and extensive
apoptosis of T lymphocytes and natural killer cells, as
observed in blood and tissues of human and non-human pri-
mate fatalities [145,150,154,158–161]. T-cell death cannot be
the result of a direct viral cytopathic effect, as lymphocytes
are not infected by EBOV, but would rather result from inter-
actions with infected antigen-presenting cells and soluble medi-
Reservoirs of ZEBOV & MARV:
fruit bats species
Intermediary susceptible species:
virus amplification in great apes and duikers
Outbreak
Suspected
Direct
contact
FIG. 3. 11Model of the natural cycle of filovirus. The diagram shows animal-to-human transmissions leading to outbreak appearance. Several Zaire
Ebola virus (ZEBOV) outbreaks appeared when hunters handled the infected carcasses of chimpanzee, gorilla and duiker. Several ZEBOV and
Marburg virus (MARV) outbreaks were suspected to be associated with exposure to fruit bats.
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ators [36,150,158]. A viral protein with superantigen activity
might also be involved [162]. Although T-cell depletion proba-
bly induces profound immunosuppression, the consequences
for disease progression are unclear [163,164]. Fatal Ebola
haemorrhagic fever is also associated with defective humoral
responses, with absent specific IgG and barely detectable IgM
[158,165]. This impairment of adaptive immunity may result
from extensive infection of dendritic cells. Indeed, EBOV-
infected dendritic cells produce only a limited panel of cyto-
kines without full activation and maturation, which may impair
their ability to trigger adaptive immunity [143,145,166,167].
Some viral factors are also able to counteract innate
immunity and thereby impede the control of viral replication.
Indeed, VP35 blocks IFN-a/b synthesis by preventing the acti-
vation of interferon regulatory factors 3 and 7
[97,99,101,102,168]. VP35 also interferes with the activation
of dsRNA-dependent protein kinase [169], and, like VP30
and VP40 [170], is a suppressor of RNA silencing [171].
VP24 impairs the nuclear accumulation of tyrosine-phosphor-
ylated signal transducer and activator of transcription 1 and
thereby inhibits IFN-a/b and IFN-c signalling [172,173]. Inhibi-
tion of type I IFN responses seems to be crucial for EBOV
virulence, as viruses that have lost this property by VP35
mutation are attenuated in vitro and in vivo [174,175].
In striking contrast with fatal outcome, effective control of
EBOV infection is associated with balanced immune
responses. A transient and well-regulated inflammatory
response is observed early in the disease not only in patients
that survive, but also in asymptomatic subjects [120,149].
This response may help to control the infection and to
induce specific immunity. An early and robust humoral
response is strongly correlated with survival in symptomatic
patients [158,165]. In asymptomatic EBOV-infected subjects,
antibodies are detected about 3 weeks after infection and in
moderate amounts [120]. In survivors and asymptomatic sub-
jects, high antibody titres persist for years [176]. Although
FIG. 4. A model of pathogenesis of filovirus infection, based on findings with Zaire Ebola virus. Dendritic cells and macrophages are early targets
of filoviruses. The virus spreads from the initial site of infection to secondary lymphoid organs and liver where intense replication takes place.
Then, other cells are infected such as hepatocytes, endothelial cells, fibroblasts and some epithelial cells. The inhibition of type I interferon (IFN)
production by viral proteins leads to relentless viral replication in most organs. The extensive infection of antigen-presenting cells (APC) leads to
altered inflammatory response and uncontrolled release of mediators. This ‘cytokine storm’ contributes to the pathogenesis by attracting inflam-
matory cells towards infected tissues, inducing coagulopathy and increasing endothelial permeability and vascular leakage. In addition, infected
APC and/or soluble factors are responsible for the defective adaptive immunity and the massive apoptosis of T lymphocytes. Multifocal necrosis
of hepatocytes, liver failure, destruction of secondary lymphoid organs and other tissue damage may result from direct cytopathic effects but also
from the host response. Together, these events lead to multi-organ failure, impairment of the vascular system, terminal shock and death.
COLOR
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antibodies probably participate in the control of infection,
their precise role is unclear. Importantly, control of EBOV
infection is also characterized by a lack of T-cell apoptosis
and by the circulation of activated T cells (probably including
cytotoxic T lymphocytes), coinciding with a fall in viraemia
and the disappearance of symptoms [158]. A similar
response also occurs in asymptomatic individuals [177], sug-
gesting that T-cell responses are crucial for controlling the
infection. The factors underlying the radically different out-
comes of EBOV infection are unclear, but may include the
route of infection, the size of the inoculum, the cell types
that are initially infected, previously acquired immunity [112],
heterologous immunity, and MHC status [178]. However, it
is unlikely that outcomes were related to different virus
strains, as no difference was found in the coding sequences
of viruses isolated from survivors and fatalities and different
outcomes can be observed in patients infected with the same
viral source [51,105].
Treatment and Prevention
Although several advances in the treatment and prevention
of EBOV infection have recently been described, there is no
licensed vaccine, and the only way to limit outbreaks is to
isolate patients. The development of therapeutic measures is
hampered not only by the need to manipulate EBOV in bio-
safety level 4 facilities, but also by the limited number of
people affected since the discovery of filoviruses. However,
the increasing frequency of EBOV outbreaks in humans and
great apes [56,122–124] and the recent emergence of new
EBOV species [47,64], give evidence for the intense circula-
tion of EBOV in the endemic area.
The main challenge is to develop a vaccine conferring
cross-protection against heterologous EBOV species, includ-
ing emerging strains, ideally after a single dose. Several vac-
cine candidates have shown good efficacy in non-human
primates over the last 10 years. The use of live viral vectors
or virus-like particles to produce EBOV GP is a promising
approach, providing sterilizing immunity after a single dose
[179–184]. Recent studies have demonstrated the feasibility
of a multivalent vaccine able not only to protect against the
filovirus species used to formulate the candidate
[180,185,186] but also to cross-protect against an emerging
species [187]. However, several concerns must be dealt with
before human use, such as the safety of the viral vector, pos-
sible pre-existing immunity to the vector, and formulation of
a single-dose cross-protective vaccine. Results obtained with
candidate vaccines confirm the major role of T-cell
responses, and of CD8+ T cells in particular, in the control
of filovirus infections, as well as the presence of cross-react-
ing immunogenic epitopes on the viral GP.
A vesicular stomatitis virus-based candidate also has thera-
peutic potential, conferring substantial post-exposure protec-
tion when administered very rapidly to non-human primates
[188,189]. Passive immunization with convalescent plasma or
monoclonal antibodies seems to be ineffective [190–192],
tending to confirm that humoral responses are not primarily
responsible for the control of EBOV infection. Similarly,
recombinant type I IFN administration fails to protect non-
human primates [193]. One promising approach is to manip-
ulate the coagulation system by inhibiting the tissue factor
pathway with a factor VIIa/tissue factor inhibitor [156] or by
activating the natural anticoagulant protein C pathway with
recombinant activated protein C [194]. However, the sur-
vival benefit is limited. Another approach is to target viral
replication with small interfering RNA or antisense oligonu-
cleotides [195–198], although the latter is limited by the
need to use virus species-specific sequences, which are not
known in the early stages of an outbreak.
Conclusions
Substantial advances in our understanding of the ecology of
EBOV and MARV, as well as host–virus interactions and dis-
ease control, have been made during the past two decades.
This period was marked by the first recorded MARV out-
break in West Africa, resurgence of Ebola Sudan in Sudan
and Uganda, the discovery of two new species (Cote d’Ivoire
and Bundibugyo), and escalation of ZEBOV outbreaks in the
border region of Gabon and the Republic of Congo. Bats
have been identified as a major filovirus reservoir, and many
human outbreaks have been shown to have arisen through
the handling of infected chimpanzee and gorilla carcasses.
The 2007 ZEBOV outbreak was linked to fruit bats, although
the precise mechanism of transmission to humans was not
identified. MARV and ZEBOV induce profound suppression
of adaptive immunity, characterized by massive B-lymphocyte
and T-lymphocyte apoptosis largely mediated by the TRAIL
(TNF-related apoptosis-inducing ligand) and Fas pathways. A
recent study suggests that superantigenic activity may be
involved and numerous studies have shown that VP35 and
VP24 play an essential role in ZEBOV suppression of IFN-a/
b production.
Transparency Declaration
Conflicts of interest: nothing to declare.
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