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Ebolavirus (EBOV)

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EBOV taxonomy and phylogeny

Viruses - ssRNA viruses - ssRNA negative-strand viruses - Mononegavirales - Filoviridae - Ebolavirus -

Synonyms

Ebola and Marburg viruses belong to the Filoviridae family (filoviruses). The Filoviridae family is classified within the order Mononegavirales, which also includes members of the Bornaviridae, Rhadoviridae, and Paramyxoviridae families. Filoviruses are enveloped, non-segmented, negative-stranded RNA viruses of varying morphology.

Filovirus virions have been visualized by electron microscopy as long filamentous particles, short bacillus-like particles, U-shaped, and even circular. The name "Filovirus" (filo- meaning "thread" in Latin) is derived from this unique morphology.

Filament length varies among viruses, with Marburg virions beingS shorter on average than Ebola (800 nm and 1000 nm, respectively); the diameter of filoviruses approaches 80 nm.

The Filoviridae family consists of two genera: Ebolavirus (EBOV) and Marburgvirus (MARV), which likely diverged from a common genetic ancestor several thousand years ago. There is only one known species of MARV and five identified species of EBOV: Zaire (ZEBOV), Sudan (SEBOV), Reston (REBOV), Tai Forest (TAFV, which was also known as Cote d'Ivoire Ebolavirus until 2010), and Bundibugyo (BDBV).

SEBOV and ZEBOV, which are the predominant EBOVs associated with known outbreaks. TAFV has only caused a single non-fatal human infection, REBOV has caused fatal infection in non-human primates and the only EBOV originated in Asia (Philippines).

The genomes of Marburg and Ebola are approximately 19 kilobases in length. Sequencing analysis reveals 55% homology between Ebola and Marburg at the nucleotide level. Homology between strains and subtypes within each species is high, especially for Marburg isolates. Each negative sense RNA genome consists of seven genes. Ebola has three overlap areas (sequences that are part of two different transcriptional products) and Marburg has a single overlap area. Areas involved in overlap are highly conserved. Gene products between the two species are similar and appear to have the similar functions.

The Glycoprotein (GP) differences between any two species range from 37% to 41% at the nucleotide level and from 34% to 43% at the amino-acid level. However, variations within EBOV-Z species are very low (∼2–3%). Thus, GP nucleotides are usually used in the phylogenetic analysis of EBOV.

Examination of the whole family suggests that members of the Filoviridae, including the recently described Lloviu virus, shared a most recent common ancestor approximately 10,000 years ago. With regard to the common ancestor of EBOV, it was estimated to be 1000–2000 years old or less. The ancestral viruses probably had been circulating in small mammals without causing fatal illness. It was also estimated that around 1900 EBOV experienced a genetic bottleneck. As a result, only those lineages with broader tropism and higher fitness could survive to infect primates, which caused the outbreaks reported since 1976.

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Introduction

Ebola and Marburg viruses are the causative agents of severe filoviral hemorrhagic fever (FHF) that occurs in sporadic outbreaks on the continent of Africa.

Filoviruses are classified as Biosafety Level 4 (BSL-4) pathogens because they cause a rapid, severe disease with high mortality for which there is no effective treatment or licensed vaccine.

EBOV disease was first identified in two separate large outbreaks occurring simultaneously in 1976, one in the Democratic Republic of the Congo and the other in Sudan. EBOV derives its name from the Ebola River located in northwestern Democratic Republic of the Congo where the outbreak occurred. These two initial outbreaks resulted in identification of the Zaire ebolavirus (ZEBOV) and Sudan ebolavirus (SEBOV) species. The Zaire and Sudan species of Ebola have been responsible for most Ebola outbreaks since 1976.

The remaining three species of EBOV (Cote d'Ivoire, Reston, and Bundibugyo ebolavirus) have occurred less frequently. The Cote d'Ivoire ebolavirus species has only been known to cause a single nonfatal infection acquired during the necropsy of a dead chimpanzee. The Bundibugyo ebolavirus species was responsible for an outbreak in 2007 to 2008 in Uganda resulting in over 100 cases with a fatality of 42%.

The Reston ebolavirus was discovered during an investigation of hemorrhagic fever deaths in primates in a quarantine facility in Reston, Virginia, in 1989. Primates imported from a single export facility in the Philippines were responsible for this and several subsequent outbreaks in primate facilities in the United States and Italy. Nine workers in the affected United States and Philippines facilities were identified as having been infected with the virus, but none were associated with disease, suggesting that Reston ebolavirus may be nonpathogenic in humans, or at least less virulent than the other viruses in the genus. Reston ebolavirus has recently been detected in pigs on commercial pig farms in the Philippines.

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Genomic organization of a representative member of the Filoviridae family of viruses

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EBOV virion structure

Secreted glycoprotein

EBOV genomes encode an additional protein, the nonstructural soluble form of the glycoprotein, sGP. As GP, sGP is encoded by the fourth gene, but is translated from non-edited mRNA species, while the membrane-bound GP is the result of mRNA editing during transcription. sGP is not incorporated into viral particles, but is secreted from infected cells. Although the function of the protein is not fully understood, there is evidence that it acts as an anti-inflammatory factor by protecting the endothelial cell barrier function during infection. Besides sGP, a second soluble GP variant generated by mRNA editing, the small soluble protein (ssGP) has been identified. A nonstructural MARV protein comparable to EBOV sGP is not expressed. Pre-sGP is the primary product and is processed by signalase and furin cleavage into the 291 amino-acid long N-terminal fragment, sGP, and the 41 amino-acid long C-terminal fragment, D-peptide. Both sGP and D-peptide are secreted from infected cells and sGP has been detected in the serum of infected individuals. sGP is secreted as a disulfide-linked parallel homodimer, containing Cys53-Cys530 and Cys306-Cys3060 disulfide bonds and five glycosylated sites. Disulfide bridges are critical for the proposed anti-inflammatory function of sGP.

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Life cycle

Entry

Filoviruses enter the target cells by different uptake mechanisms including lipid raft-dependent and receptor-mediated endocytosis and macropinocytosis. Receptor binding and attachment to the target cells is mediated by the glycoprotein subunit GP1. A number of cellular proteins have been implicated in filovirus entry. The different co-receptors likely provide access for the virus into different target cells.

After uptake, the virus particles are internalized into the endosomes, where fusion takes place. Fusion of the viral and cellular membrane is mediated by the fusogenic cleavage product GP2. To initiate fusion, the proteolytic cleavage of GP1 by the endosomal proteases cathepsin L and/or cathepsin B is mandatory depending on the cell type.

Replication

Fusion of the viral and cellular membrane leads to the release of the viral nucleocapsids into the cytoplasm of the infected cell where transcription and replication of the viral genome take place. The genome cannot be transcribed or copied by host-cell enzymes, all the molecular pattern required for viral replication is carried within the virion. This is why the nucleocapsid, rather than the naked RNA, serves as the template for both transcription and replication. During transcription, the seven viral genes are sequentially transcribed into monocistronic mRNAs which are capped and polyadenylated and are used for the production of viral proteins. During replication, the encapsidated RNA is copied into full-length positive-sense replicative intermediates, the RNA antigenomes, which are enwrapped by the nucleocapsid proteins. In turn, the antigenomes are used as templates for the synthesis of progeny genomes. The nucleocapsid proteins do not only encapsidate the RNA genomes, they are also essential for replication and transcription. The viral polymerase, consisting of L and VP35, catalyzes replication as well as transcription, including polyadenylation and capping.

Assembly

New genomes associate with nucleoprotein and VP30 to form nucleocapsids, which accumulate in inclusion bodies. Meanwhile, newly synthesized viral glycoprotein becomes glycosylated during its transit through the host-cell Golgi apparatus and is cleaved by a furin-like enzyme before transfer to the cell surface, producing extracellular GP1 and transmembrane GP2 segments that remain linked by a disulphide bond. The assembly of new virions takes place on the inner surface of the plasma membrane, when nucleocapsids associate with matrix proteins linked to the cytoplasmic tail of membrane-bound GP1/2.

Exit

Negative-strand RNA virus particles are formed by a process that includes the assembly of viral components at the plasma membranes of infected cells and the release of particles by budding.

Although all the structural information to build a viral particle is encoded in the viral genome, filoviruses (like other enveloped viruses such as HIV-1) hijack cellular protein machines to mediate assembly and budding from cellular membranes. The recruitment of cellular factors serves potentially two major purposes. Firstly, it needs to recruit factors that help to initiate the assembly process mediated by the matrix protein. Secondly, recruitment of endosomal sorting complexes required for transport may be required for the last step of the budding process, the release of the fully assembled virus particle from cellular membranes. Therefore, selective employment of cellular proteins by enveloped viruses may provide specialized fine tuning accessories for virus assembly and release.

Welsch and colleagues showed that the budding process is initiated by the lateral association of the intracellular nucleocapsids with the plasma membrane. Starting from one end, the nucleocapsids are then subsequently wrapped by the plasma membrane until the viral particles protrude vertically from the cell surface. The formation of such long filamentous particles (700–900 nm or more) is certainly challenging for the infected cells and may lead to membrane perturbation in the cells and in the released viruses. Notably, the release of infectious filamentous MARV from cultured cells has been reported to peak at early time points post infection (1–2 days p.i.), when the cells were still intact. At late time points (4 d p.i.), most of the infected cells were vesiculated, the released virions were morphologically different, being round or bent, and coincidentally infectivity was decreased.

As a whole:

  1. The transport of the matrix protein VP40 to the plasma membrane occurs by the retrograde late endosomal pathway via MVBs. Then, VP40 re-localizes the cellular budding machinery to the site of virus assembly and budding. VP40 was also found in association with viral inclusions containing assembled nucleocapsids, probably in a conformation different from the monomeric form such as the hexameric form or the octamers in complex with RNA.
  2. GP expression follows the regular secretory pathway and localizes to the late endosome, after proteolytic cleavage in the trans-Golgi network, and accumulates in multi-vesicular bodies (MVBs) together with VP40. Both are thus co-transported to the site of assembly and budding.
  3. NP–RNA interactions are sufficient for nucleocapsid assembly that recruits VP30, VP35, and L. Nucleocapsids accumulate in cellular inclusions that co-localize with small amounts of VP40. These complexes are then transported to VP40-and GP-containing MVBs and to the plasma membrane that leads to virus particle assembly and release.

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Clinical presentations

The data collected from a number of human outbreaks of EBOV and MARV suggest the incubation period (asymptomatic period) can be as short as 2 days or as long as 21 days (mean 4-10).

Schematically, clinical manifestations occur in three successive phases and include general, gastrointestinal, and mucocutaneous disorders. Death usually results from hemorrhagic complications. Cutaneous manifestations rarely make a major contribution to disease severity but can assist with the diagnosis. Rash, the main cutaneous disorder, is nonspecific and cannot guide the differential diagnosis. Immunohistochemical examination of skin biopsy or necropsy specimens can confirm the diagnosis.

It is virtually impossible to distinguish between Ebola and Marburg fever on clinical grounds alone. Presumptive diagnosis during outbreaks is based on a combination of epidemiological information and clinical manifestations, which may differ from one viral species to another. After an incubation period of 1–21 days, signs and symptoms evolve in three successive phases, which sometimes overlap. The first phase is characterized by nonspecific symptoms; the second by multiple organ involvement; and the third, terminal phase, by recovery or death, depending on both host and viral factors.

Phase I

Symptoms typically begin with a flu-like syndrome with abrupt-onset fever (39–40 °C), violent headache (primarily in the occipital region but spreading to the parietal and frontal regions), and generalized fatigue with myalgia and arthralgia (always present). Sometimes, myalgia with concomitant neck muscle tension occurs, mimicking meningitis. Marburg fever can be associated with prostration, malaise, ocular pressure sensitivity, photophobia, and vertigo.

Phase II

The second phase begins with severe visceral disorders, on day 2–4 after symptom onset, and lasts for 7–10 days. Severe abdominal pain, nausea, watery diarrhea, and loss of appetite lead to weight loss and marked dehydration. Patients have "a ghostly appearance" with deep-set eyes, an expressionless face, and extreme lethargy. Respiratory symptoms appear at the same time, with violent sore throat accompanied by chest pain and dry cough, resulting from pharyngeal damage; this is highly evocative of filovirus infection (patients describe having a "ball" in the throat). Swallowing is hindered, further aggravating nutritional status. A non-itchy macular or maculopapular rash appears between the second and seventh day, followed by fine scaling of the skin. Cutaneous manifestations of filovirus hemorrhagic fever syndrome (FHF) are not in themselves life threatening. They provide diagnostic pointers during early outbreaks, especially in light-skinned patients. Cutaneous manifestations usually appear in phase II, 4–5 days after initial symptom onset. They occur in more than half of patients. They are more frequent and more characteristic in MARV infection than EBOV infection. Bleeding is frequent, with petechiae at various sites, an ocular burning sensation, reddening, melena, hematemesis, hematuria, epistaxis, hemoptysis, and bleeding at puncture sites. Hemorrhages can be severe but are only present in fewer than half of patients and associated with fatal outcome. Pregnant women often develop uterine bleeding that results in miscarriage. Ultimately, the nervous system can be involved, leading to behavioral disorders (aggressiveness, confusion, delirium), paresthesia (tingling sensations), hyperesthesia (increased sensitivity to sound, light or tough), and seizures.

Phase III

The terminal phase results in recovery or death.

Early symptoms are similar between survivors and non-survivors. Testing of blood samples from infected patients has shown a dramatic difference in the levels of virus in the blood in patients who die versus patients who survive. Non-survivors have 100- to 1000-fold higher levels of viremia than survivors. Survivors also have higher levels of virus-specific antibodies compared with fatal cases, which have little or no detectable antibody responses. Fatal cases progress to more severe symptoms by days 7 to 14 after the onset of disease; death is generally imminent shortly after the onset of coma and shock. Tachypnea (rapid respiration rate) is one characteristic of late-stage disease that differentiates fatal from nonfatal cases.

Fatal disease

If illness is to be fatal, most patients will die by the end of the first week. Patients may become normothermic just before death due to shock or massive hemorrhage.

Tachypnea may occur with hiccup, followed by anuria. Death occurs after 2–3 days due to plasmatic shock with capillary extravasation and multiorgan failure.

Pathology performed on patients with EBOV and MARV infection all reveal extensive necrosis in a variety of organs including the liver, spleen, kidney, thymus, lymph nodes, and reproductive organs. In the liver, hepatocellular necrosis is widespread and an exceedingly large number of virus particles are present.

Based on the known outbreaks, the case fatality ratios range from 42% to 90%, depending on the viral species: Zaire ebolavirus 1976-2008 (12 outbreaks, ~1400 people) - 50-80%; Sudan ebolavirus 1976-2004 (4 outbreaks, 760 people) - 45-60%; Bundibugyo ebolavirus 2007-2008 (1 outbreak, 102 people) - 42%.

Convalescence

In patients who survive infection begin to show a decrease in amounts of circulating virus and clinical improvement around day 7–10. In most cases, this improvement coincides with the appearance of ebolavirus-specific antibodies. The humoral response could result in formation of antigen–antibody complexes, and some recovering patients develop acute arthralgia and other symptoms consistent with autoimmune disease.

Survivors experience a lengthy period (one month or more) of painful convalescence with intense fatigue, loss of appetite, profound prostration, weight loss, and migratory arthralgia. Some survivors are unable to recall the most severe period of illness. Bacterial infections and psychoses have been reported. Sequelae may include orchitis (inflammation of the testes), recurrent hepatitis, transverse myelitis (inflammation of the spinal cord), and uveitis (inflammation of the uvea, the middle layer of the eye). Psychiatric sequelae include confusion, anxiety, and restless and aggressive behavior.

In addition to these manifestations, survivors may experience hair loss at the end of convalescence.

During early convalescence, virus can be detectable in immunologically protected sites of the body, particularly the uveal and seminal tracts. Although long-term viral shedding is not thought to occur in FHF patients, infectious virus can be detected in genital secretions for up to 80 days post-onset of illness.

Long-term immunity

Total IgG levels appear to be a more reliable predictor of protection compared with neutralizing antibody levels. The prevalence of SEBOV-specific IgG antibodies among the survivors declined over time. During the original Gulu outbreak in 2000, 61% of survivors (33 individuals/54 total) initially tested positive for SUDV-specific antibodies, but when retested after 6 months, 2 years and 10 years, the seroprevalence was 73% (29/40), 66% (32/48) and 41% (13/32), respectively. More studies are needed to determine whether these observed sustained humoral and cell-mediated immune responses will be sufficient for protection from disease in the case of potential re-exposure.

Signs and symptoms of FHF caused by Ebola and Marburg viruses
General fever; chills; myalgia; general malaise; lethargy; fatigue
Gastrointestinal nausea; vomiting; diarrhea; abdominal pain
Respiratory non-productive cough; shortness of breath; chest pain
Vascular hypotension; conjunctivitis; edema
Neurologic headache; confusion; seizure; coma
Hemorrhagic petechiae; rash (trunk and shoulders); erythema and desquamation; bruising; bleeding from venipuncture sites; nosebleeds; mucosal hemorrhage, esp. GI/GU; visceral hemorrhagic effusions
Late stage shock (resembling septic); convulsion; delirium; coma; diffuse coagulopathy; increased respiration rate; anuria; metabolic abnormalities
Convalescence myalgia; arthragia; muscle weakness; hepatitis; ocular disease; myelitis; hearing loss; psychosis
Clinical manifestations in non-human primates (NHPs)

Laboratory studies of the filovirus hemorrhagic fever syndrome have been pursued most extensively with the cynomolgus macaque model of Zaire ebolavirus infection, which resembles an accelerated form of the lethal human disease. In these animals, the introduction of a small quantity of virus by injection or by droplets into the mouth or eye is followed within 3–4 days by illness with fever and a petechial hemorrhagic rash. By day 5–6, all monkeys are severely ill, and the development of shock and progressive hypothermia leads to death on day 7–8.

Brief course of the uniformly fatal infection in macaques is not accompanied by the development of antigen-specific immunity. By contrast, human beings show a variety of outcomes, which appear to be related to patterns of innate and adaptive immune responses.

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Pathogenesis

Six pathogenic mechanisms that appear, largely from studies of Zaire ebolavirus infection act in combination to cause filoviral hemorrhagic fever (FHF).

The mechanisms can be roughly classified as following:

Rapid viral replication

Filoviruses are pantropic, meaning that they can infect a wide variety of cell types. In-situ hybridization and electron microscopic analyses of tissues from patients with fatal disease or from experimentally infected non-human primates show that monocytes, macrophages, dendritic cells, endothelial cells, fibroblasts, hepatocytes, adrenal cortical cells, and several types of epithelial cells support replication of these viruses. The cellular receptor for filoviruses is currently not known, but is likely a lectin or other ubiquitously expressed protein. Macrophages and dendritic cells (DCs) are highly susceptible to infection by EBOV and MARV and are likely the primary cellular targets at the site of infection (e.g., a mucosal surface). They play a pivotal role in virus dissemination by trafficking the virus to draining lymph nodes, where the virus gains access to a larger pool of susceptible cells as well to the blood, where it travels to the liver and spleen, two key infected organs in FHF patients. Macrophages residing in the liver and spleen secrete soluble factors that recruit more macrophages. The macrophages, DCs, and surrounding parenchymal cells become virus factories and cause extensive focal necrosis. Fatal cases of FHF can have greater than 108 copies of viral RNA per milliliter of blood. Elevated liver enzymes, hepatocellular degeneration, and necrosis are prominent in many FHF cases. Decreased liver function may result in decreased synthesis of coagulation factors, which may contribute to the vascular and coagulation dysfunction.

Host immunosuppression

Immunosuppression is a hallmark of fatal filovirus infections. Filoviruses inhibit the Type I Interferon system through a number of mechanisms involving the VP35 and VP24 proteins. The interferon response is responsible for the early control of virus replication; by impairing this cellular mechanism, EBOV can replicate unchecked during the critical early stages of infection.

The ability of ZEBOV to disseminate rapidly from its site of entry suggests that infected cells are unable to produce sufficient amounts of interferon INF-α/β or respond adequately to exogenous types I or II IFN. There is good evidence that the ZEBOV VP35 protein blocks IFN production by virus-infected cells in a manner similar to the influenza virus NS1 protein, by preventing the recognition of dsRNA that normally leads to phosphorylation of IRF-3 and its translocation to the nucleus. Preliminary findings suggest that a second viral protein, VP24, contributes to this process by blocking responses to exogenous IFN. Such inhibition would profoundly impair anti-viral defenses, since types I and II IFN are needed to activate NK cells, assist adaptive immunity through up-regulation of major histocompatibility complex (MHC) molecules and activate macrophages and DC for effective anti-microbial function.

Survivors have demonstrable levels of virus-specific antibody responses.

Deregulation of immune response

Abnormal responses of macrophages and DCs to viral infection may result in immune dysregulation. Infected macrophages secrete high levels of inflammatory cytokines, which may lead to recruitment of more susceptible cells and uncontrolled inflammation.

One of the intriguing features of fatal filovirus infection in animals and humans is that little or no inflammatory cellular response occurs at the sites of viral replication. Accumulation of neutrophils, monocytes, and lymphocytes around infected cells has been rarely observed in infected tissues. The minimal inflammatory cellular response is considered to be a distinctive feature of fatal filovirus infection and may represent a part of the deregulated immune response observed in fatal cases of EBOV and MARV infection. In non-fatal EBOV infections of guinea pigs, in contrast, a prominent inflammatory response is observed, and infected cells are tightly surrounded by leukocytes forming a substantial barrier which could impair viral dissemination.

The resulting syndrome (mixed anti-inflammatory response syndrome (MARS)), which resembles septic shock resulting from infection with Gram-negative bacteria, can also be deadly.

Apoptosis of lymphocytes

Even though ZEBOV does not replicate in lymphocytes, large numbers of these cells undergo apoptosis in infected macaques, explaining the progressive lymphopenia observed over the course of illness. Blood samples from fatally infected African patients also show reduced lymphocyte counts and biochemical markers of apoptosis, suggesting that a similar process occurs in humans.

Like coagulation abnormalities, lymphocyte apoptosis begins early in infection. A number of mediators produced by virus-infected macrophages, including TNF-α, Fas and its ligand, TNF-α-related apoptosis-inducing ligand (TRAIL) and NO, may be capable of inducing apoptosis. Impaired DC function may also contribute to the elimination of lymphocytes. Recent studies indicate that NK cells and CD4+ and CD8+ lymphocytes are the principal cell types affected in ZEBOV-infected macaques. Lymphopenia is also seen in other viral hemorrhagic fevers, and a massive apoptotic loss of lymphocytes occurs in septic shock, suggesting that similar host responses occur in these conditions.

Vascular dysfunction

Vascular impairment is another hallmark of filovirus pathogenesis. At end-stage disease, there is a significant loss of vascular integrity resulting in bleeding and leakage of fluid into tissue spaces. Virus infection of macrophages results in expression of tissue factor on the surface of the cells. Tissue factor initiates the coagulation cascade leading to microthrombosis and disseminated intravascular coagulation (DIC). DIC is activation of the coagulation system resulting in development of microthrombi (locally or systemically) that then hamper blood supply leading to multiple organ failure. During DIC, platelets and clotting factors are consumed, leading to hemorrhage elsewhere in the body.

The binding of double-strand RNA or other viral products to pattern recognition molecules triggers cytoplasmic-signaling pathways that bring about the migration of NF-κB and other transcriptional activators to the nucleus, resulting in release of proinflammatory cytokines, such as TNF-α and IL-1, chemokines, such as MIP-1, and nitric oxide (NO) and other vasoactive molecules. These mediators attract additional monocytes/macrophages to the site of infection, mobilize immature neutrophils from blood vessel walls and the bone marrow and facilitate the exit of inflammatory cells and proteins from the circulation by causing vasodilatation, increased endothelial permeability and expression of endothelial cell-surface adhesion molecules. Although these changes in vascular function may be beneficial in resolving a localized infectious lesion, their occurrence throughout the body as a result of the systemic spread of filovirus leads to catastrophic circulatory collapse.

Virus-infected macrophages also play an important role in initiating DIC by synthesizing cell-surface tissue factor (TF), which interacts with circulating factors VIIa and X to trigger the extrinsic coagulation pathway, leading to deposition of fibrin on the surface of infected cells and on membrane microparticles released into the bloodstream. Binding of coagulation factors to cell-surface TF also alters macrophage function by exciting intracellular signaling pathways, through the phosphorylation of the cytoplasmic tails of TF and associated membrane-bound protease-activated receptors (PARs). Additional factors contributing to severe coagulopathy may include the release of additional TF in areas of necrosis, increased initiation of clotting on altered endothelial surfaces, and the release of fibrin degradation products, such as D-dimers, into the plasma as thrombi are broken down by plasmin and other enzymes. In ZEBOV-infected macaques, D-dimers are detectable on the first day post-infection. Thrombocytopenia, by contrast, does not become evident until days 3–4, as platelets attach to activated endothelium or become part of nascent thrombi.

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Diagnosis

Because local hospitals and health departments in the United States do not have the technology or resources to routinely test for filovirus infections, the clinical assessment is critical for the initial diagnosis of a filovirus infection. Local and state health departments should be notified of suspected FHF cases, but diagnostic confirmation can only be obtained by sending samples to the Centers for Disease Control and Prevention (CDC) in Atlanta. Information for sample submission in the United States can be found at the following website: The Viral Special Pathogens Branch (VSPB) Samples from suspected FHF patients should be handled minimally and with the utmost caution. A class II biosafety cabinet should be used to process clinical specimens and BSL-3 practices should be used. This includes use of barrier gowns, two pairs of gloves, eye protection, and a mask or N95 respirator.

Hartman AL, Towner JS, Nichol ST. Ebola and Marburg hemorrhagic fever. Clin Lab Med. 2010 Mar;30(1):161-77.

Test Target Source Advantages Disadvantages Useful in Field Lab?
Indirect immunofluorescence assay (IFA) Antiviral antibodies Serum Rapid and easy Nonspecific positives; subjective No
Enzyme-linked immunosorbent assay (ELISA) Antiviral antibodies Serum Rapid, specific, sensitive, high throughput Slower than IFA Yes
Immunoblot Antiviral antibodies Serum Protein specific Interpretation sometimes difficult No
Antigen detection ELISA Viral antigen Blood, serum, tissues, oral/nasal washes, breast milk Rapid, sensitive, high throughput None Yes
Immunohistochemistry Viral antigen Tissue Shows histologic location of viral antigen Slow, requires formalin fixation No
PCR Viral nucleic acid Blood, serum, tissues, oral/nasal washes, breast milk Rapid, sensitive, can distinguish between strains Requires specialized equipment Yes
Electron microscopy Viral particle Blood, tissues Distinguishes unique morphology Insensitive; requires specialized equipment No
Virus isolation Viral particle Blood, tissues Isolate is available for study Slow (~1 wk) No

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Treatment

Present treatment strategies are mainly symptomatic and supportive. In developing countries with minimum health-care provision, these strategies should include isolation, malaria treatment, broad spectrum antibiotics, and antipyretics before diagnosis. Fluid substitution, preferentially intravenous administration, and analgesics should be provided as needed. In developed health-care systems with appropriate isolation units and other medical resources (drugs, equipment, etc.), proper intensive care should be directed towards maintenance of effective blood volume and electrolyte balance. Shock, cerebral edema, renal failure, coagulation disorders, and secondary bacterial infection have to be managed and can be life-saving. Organ failure should be addressed appropriately - e.g., dialysis for kidney failure and extracorporeal membrane oxygenation for lung failure. At present, no strategy has proved successful in specific pre-exposure and post-exposure treatment of Ebola virus infections in man.

Although no controlled trials on various types of routine interventions have been conducted, it is interesting to note that the case-fatality rate of the only human filovirus outbreak (excluding isolated cases) to occur in an area where aggressive supportive care was routinely possible – Marburg hemorrhagic fever in Germany and Yugoslavia in 1967 – was 22%, compared to 87% for all other large outbreaks in more remote and undeveloped areas of sub-Saharan Africa (Anon., 2005; Bausch et al., 2006). Whether this difference can truly be attributed to the quality of care or is influenced by differences in virus strain, route and dose of infection, underlying prevalence of immunodeficiency and co-morbid illnesses, different intensity of surveillance and ability to detect mild cases, or genetic susceptibility in the populations under observation in Europe and Africa is unknown.

More details in Clark DV, Jahrling PB, Lawler JV. Clinical management of filovirus-infected patients. Viruses. 2012 Sep;4(9):1668-86.. Also see other related publications in PubMed.

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Piercy TJ, Smither SJ, Steward JA, Eastaugh L, Lever MS. The survival of filoviruses in liquids, on solid substrates and in a dynamic aerosol. J Appl Microbiol. 2010 Nov;109(5):1531-9.

The survival of filoviruses

This study has demonstrated that filoviruses are able to survive and remain infectious for cell culture, for extended periods when suspended within liquid media and dried onto surfaces. In addition, decay rates of a range of filoviruses, within small-particle aerosols, have been calculated, and these rates suggest that filoviruses are able to survive and remain infectious for cell culture for at least 90 min. Recovery of virus from liquid media (tissue culture media and sera) was significantly higher in samples stored at +4°C compared to room temperature. MARV and ZEBOV, dried onto solid substrates, were recovered in high titers from both plastic and glass surfaces. It has also been shown that low titers of virus could be recovered from samples suspended in tissue culture media and dried onto both plastic and glass until day 26, but only virus dried onto glass substrate was recovered by day 50, when stored at +4°C. The only significant differences that could be detected across the range of conditions were attributable to the suspending liquid dried onto glass, where virus in tissue culture media could be recovered for significantly longer than virus dried in guinea pig sera. Data from researchers of the FSU indicated that MARV in human blood was able to survive on steel, glass and cotton wool for at least 6 days.

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Transmission from human to human

Filoviruses spread through the human population by close contact with acutely infected patients. Virus-containing bodily fluids include blood, vomitus, saliva, stool, semen, breast milk, and tears. Virus transmission has only been documented between symptomatic patients and does not likely occur during the incubation period. Filoviruses retain infectivity while at room temperature on environmental surfaces; thus, fomites may be sources of transmission.

Percutaneous inoculation is also an important mode of transmission. The earliest outbreaks of ZEBOV were propagated through the reuse of unsterilized needles and syringes in those seeking care at an outpatient facility. Mortality was 100% for those inoculated by contaminated needles and syringes.

Sexual transmission of Ebola is theoretically possible and most experts advocate use of condoms during sexual activity for at least 3 months after recovery. Viable Ebola virus has been cultured from semen up to 82 days after illness onset. Moreover, Ebola RNA has been detected by RT-PCR from vaginal secretions and semen 33 and 91 days after initiation of clinical syndrome, respectively. Although sexual transmission of Marburg has been demonstrated in at least one patient during the original Marburg outbreak of 1967, there have been no clearly documented cases of Ebola transmission via sexual intercourse.

Overall, three distinct contact modalities account for most virus transmissions during outbreaks:

As a result, outbreak response involves three major components:

The ability to disseminate disease through aerosol or airborne small-droplet nuclei would render filoviruses a potential biologic warfare threat. Both Marburg and Ebola have been weaponized in this manner. Aerosol transmission of Ebola has been demonstrated in experimental models involving nonhuman primates. During an experiment to evaluate therapeutic benefit of interferon following infection with ZEBOV, two of three control rhesus monkeys (Macaca mulatta) became ill. Control animals were caged 3 m from infected animals. Researchers were unable to document direct or percutaneous contact through injections, and speculated inoculation to the control animals occurred via pulmonary, nasopharyngreal, oral, or conjunctival routes. Definitive evidence that small-droplet nuclei pose a substantial transmission risk among humans during naturally occurring outbreaks is lacking.

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Ecology

Filoviruses appear to be endemic in Central Africa in an area between the 10th parallel north and south of the equator.

Scientists have long hypothesized that EBOV must persist in the endemic areas in a classical zoonotic reservoir in which it does not cause disease, or does so only infrequently. Given that in apes and non-human primates EBOV infection is so severe and even more lethal than in humans, these species are considered as accidental hosts and not as a classical zoonotic reservoir species.

Ebola as zoonosis

The intimate relationships between human societies, ecosystems and potential pathogens have, throughout history, given rise to complex challenges to human health. Zoonosis, the process whereby disease passes to humans from other species, now widely acknowledged as critical in the emergence and re-emergence of infectious disease. It has been suggested that all new infectious diseases of human beings to emerge in the past 20 years have had an animal source, while Jones et al. (2008) find more than 60% of emerging infectious disease events since 1940 to involve zoonoses, 72% of these with wildlife origins.

The impact of population growth interacts with patterns of human and animal population distribution and mobility. Rapid population growth in urban centers, especially in less developed economies, has resulted in overcrowded accommodation and highly congested transport systems which, combined with inadequate water and sanitation services, provide greatly increased opportunities for person to person disease transmission. With more than two billion air journeys a year globally, the isolation of a disease outbreak becomes an increasingly formidable task. Disease dynamics are also shaped by changing food production and livelihood systems that increase the intensity of contact people and animals that can be accidental as well as reservoir hosts. Where wildlife disease reservoirs and vectors are involved, environmental and land use changes that affect human contact with these become key. For instance hemorrhagic fevers such as Ebola, Lassa fever and Rift Valley fever in Africa have been linked to deforestation and population shifts, with contributing politico–economic dynamics varying from dam construction to diamond mining, logging and the bushmeat trade.

Reservoir

The reservoir(s) of the filoviruses has yet to be identified. The sporadic nature of outbreaks frequently takes local health care communities by surprise; therefore, extensive epidemiologic investigations are initiated retrospectively. In case of EHF, it can be assumed that the reservoir is either a rare species or that transmission within the reservoir species itself is less efficient. Since the first recorded human outbreak in 1976, several laboratory and field studies have been conducted to identify the animal(s) - vertebrate or invertebrate - that can harbor the virus asymptomatically. Thousands of vertebrates (mammals, birds, reptiles, and amphibians) and invertebrates (flies, mosquitoes, ticks, and other bugs) were tested and no viruses were isolated. Experimental inoculation of Ebola in various plants and animals collected from Zaire after the 1995 outbreak resulted in viral replication in fruit and insectivorous bats without apparent illness. The possibility of an airborne vector that sheds virus in bats' stool could explain the sporadic nature of the outbreaks.

The first evidence for the presence of Zaire Ebola virus in naturally infected fruit bats was documented by detection of viral RNA and antibodies in three tree-roosting species: Hypsignathus monstrosus, Epomops franqueti, and Myonycteris torquata but viabale ZEBOV has not been successfully isolated from naturally infected animals. However, the identification and successful isolation of Marburg virus from the cave-dwelling fruit bat Rousettus aegyptiacus further supports the idea of bats as a reservoir species for filoviruses.

High titers shown in the experimental bat model raise questions as to why virus isolation has not been achieved from any of the naturally infected bat species, particularly because virus isolation is usually easily achieved for filoviruses from clinical material. One suggestion has been that virus titers might be very low in these naturally infected animals (consistent with the need for nested RT–PCR to detect ZEBOV-specific nucleic acid) and that perhaps a specific physiological or environmental stimulus is needed to stimulate virus infection.

EBOV emergence in animals

Pourrut X et al. The natural history of Ebola virus in Africa. Microbes Infect. 2005 Jun;7(7-8):1005-14.

The chronological and geographic characteristics of the different outbreaks in Gabon and DRC between 1995 and 2003 point to a drift from northeast Gabon towards Democratic Republic of Congo (DRC). This raises the possibility that gorillas and chimpanzees are succumbing to a single outbreak that has been devastating animal populations for about 10 years, spreading along a northwest/southeast axis. The EBOV genome is particularly stable. For example, the rate of mutations between strains Booué 96 (Gabon) and Zaire 76 was only 1.7% in the membrane glycoprotein gene (GP), 1.3% in the nucleoprotein (NP), 1.2% in the 40-kDa structural protein VP40, and 0.9% in the 24-kDa structural protein VP24, even though these strains were isolated more than 1000 km, and 20 years apart. Similar genetic differences were found between the 2001 and 2003 outbreak strains and the strains Mekouka 94 (Gabon) and Zaire 76. Moreover, sequencing of a 249-nucleotide stretch in the most variable part of the GP gene showed no genetic differences between isolates recovered from nine patients (five of whom survived) who were sampled during the 1995 Kikwit outbreak in Zaire (DRC). In light of these results, and in order to understand how great apes are infected, authors amplified and systematically sequenced the coding part of the GP gene (the most variable gene of the EBOV genome) in samples from all gorilla and chimpanzee carcasses. A distinct viral sequence was found in each carcass, even in carcasses belonging to the same species (gorilla or chimpanzee) that had been discovered at the same time and only a few hundred meters apart. As it had been shown that a given outbreak strain undergoes no genetic variations, even after several human passages, these results argued against intra-species transmission. On the contrary, the observed genetic diversity in great apes suggests that the infection results from simultaneous but independent multiple inter-species transmission events from the reservoir species to primates. In this scenario the sequence variations probably result from viral adaptation to the natural host, which likely consists of several subspecies or genetically distinct groups. Thus, Ebola outbreaks in great apes appear to result not from the propagation of a single outbreak from one individual to another, but rather from massive simultaneous infection by the animal reservoir in particular environmental conditions (outbreaks always occur at the same times of the year, during the transitional periods between the dry and rainy seasons). Human infection occurs secondarily, and is generally linked to handling of infected animal corpses.

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Filoviruses as bioweapon

The increased threat of terrorism necessitates an evaluation of the risk posed by various microorganisms as biological weapons. This is especially important in the case of the filoviruses, Marburg and Ebola, because of their high virulence and lethality, demonstrated aerosol infectivity in the laboratory, capacity for inducing fear and anxiety, and their history of being weaponized.

US Centers for Disease Control and Prevention (CDC) classified filoviruses as "Category A" biological weapons.

Key characteristics that make filoviruses a serious risk if used as bioweapons:

  1. have low infectious doses and can be disseminated via aerosol;
  2. have no effective vaccine currently available;
  3. are available for procurement;
  4. can be produced in large quantities;
  5. are relatively stable in the environment;
  6. have been previously developed for use as a biological weapon

Work with all filovirus species requires Biosafety Level 4 (BSL-4) "space suit" containment.

As for all biowarfare agents, an effective defense against filoviruses require a comprehensive approach that includes the following elements:

Although a variety of mechanisms are in place to protect the military and civilian populations of industrialized countries against infectious agents in food and water, it is impossible to provide them with a constant supply of purified air. The most dangerous form of biological warfare exploits this vulnerability by delivering pathogens directly to the lungs, which have a huge interior surface area that is highly susceptible to microbial infection. To reach this target, particles must be small enough to remain suspended in inspired air until they arrive in the terminal branches of the airways. Most particles larger than 15–20  µm in diameter fall to the ground soon after leaving their source, while those in the range of 5–10 µm tend to become trapped in the upper airways. The most highly infectious particles are those in the 1–5  µm range, which settle out in terminal bronchioles and alveolar sacs.

Although filoviruses are highly infectious by the airborne route, major assault on an entire city seems inherently less likely than a small-scale effort, since the former would require a very large amount of highly infectious material, proper aerosolization equipment, and correct conditions of wind, temperature and humidity. Failing this, terrorists could choose the less ambitious option of releasing a smaller amount of aerosolized virus into an enclosed space.

Alternatively, non-aerosolized material could be used to produce a limited number of infections by contaminating surfaces, foods or beverages with a suspension of virus.

The greatest risk of transmission of a filovirus from sick to healthy people will occur during the opening phase of an outbreak, when patients are being cared for by family members and examined and treated by health care workers, but the nature of their illness is still not known. In African Ebola outbreaks, this situation has resulted in a large number of infections among doctors and nurses. However, failure to recognize the presence of a filovirus does not necessarily lead to the occurrence of numerous secondary infections. In the 1967 Marburg outbreak, physicians and nurses were completely ignorant of the nature of the disease agent and of the need for caution in handling contaminated materials, but only six secondary and no tertiary cases occurred.

Once a diagnosis of filovirus infection has been made, experience obtained by World Health Organization (WHO)/CDC teams in managing African Ebola outbreaks indicates that patients can be managed effectively in essentially any setting, without undue risk to the medical staff, by employing standard patient isolation and barrier nursing procedures. All workers handling infectious material must wear protective clothing and use respiratory protection, which may be either a properly fitted N-95 mask or a powered air-purifying respirator. If conditions permit, laboratory personnel should handle all specimens in a laminar flow biosafety cabinet.

Decontamination

If attack with a filovirus goes undetected, at least a week will elapse before the onset of the first illnesses. By that time, no infectious virus will remain in the environment, and there will be no need for surface decontamination.

However, as in all biowarfare situations, it would be prudent for people who may have been exposed to an aerosolized agent to take a full body shower with soap and to wash their clothing in hot water with detergent.

Decontamination becomes a very important concern when virus-containing liquids or other materials are present, either as body fluids from sick patients or as residues from liquid suspensions employed to carry out a terrorist attack. Filoviruses may survive at room temperature in liquid or dried material for a number of days. Steam sterilization is the most effective method of inactivating filoviruses and other infectious agents. For the disinfection of surfaces and objects that are contaminated with blood or other body fluids, but cannot be sterilized by steam, the CDC recommends treatment with either a 1:100 dilution of household bleach (one-fourth of a cup in 1 gallon of water) or with any of the standard hospital disinfectants registered with the US Environmental Protection Agency, such as those based on quaternary ammonium compounds or phenol.

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Filovirus outbreaks—controlled chaos

Bausch DG, Sprecher AG, Jeffs B, Boumandouki P. Treatment of Marburg and Ebola hemorrhagic fevers: a strategy for testing new drugs and vaccines under outbreak conditions. Antiviral Res. 2008 Apr;78(1):150-61.

Except for outbreaks related to the inadvertent import of infected monkeys to Europe, cases of FHF in humans have been exclusively noted in sub-Saharan Africa and are typically seen in the most remote and resource-poor areas of sub-Saharan Africa where the healthcare system and infection control standards have deteriorated in most cases due to years of civil unrest and violence. The patient-care setting often consists of multi-bed units, frequently without running water, a limited array of basic antimalarials, antibiotics, and analgesics, and an at-best rudimentary diagnostics and clinical laboratory. The physical infrastructure is often equally inadequate, with no stable electricity, refrigerators or freezers. Telecommunications and Internet access may be difficult or even non-existent, and transport to the distant major population centers limited and arduous.

Outbreaks begin when a human is infected through contact with the still unidentified primary reservoir (perhaps a bat), or an infected nonhuman primate, with amplification resulting from secondary transmission between humans, which typically takes place when family members or healthcare workers are exposed to the patient’s blood and bodily fluids. As the number of cases begins to mount, the community is confronted with a mysterious and fearful new scourge. Traditional belief systems common in Central Africa often result in the disease being attributed to sorcery or poisoning rather than to a virus. Deaths of healthcare workers often occur (and are often a decisive clue that a filovirus is circulating), sometimes prompting other workers to flee their posts, further destabilizing the already-fragile healthcare delivery system. Laboratory confirmation, which requires sending specimens to one of the few laboratories in the world with the necessary reagents and biosafety measures to perform filovirus diagnostics, usually lags. Consequently, a concerted international outbreak response is typically not mounted until months after the occurrence of the first case, which is almost always identified retrospectively. Finally, an outbreak control team composed largely of foreigners and government representatives from a distant capital descends upon the village.A flurry of activity ensues, with creation of committees and control programs. If clinical research on the filoviruses is to take place, it must occur in this tense and chaotic environment and be executed simultaneously and without compromising the multifaceted components of outbreak control.

Standard control measures for the filoviruses call for hospitalization of patients in an isolation ward. This is the setting where future clinical research would logically take place. Ironically, in addition to the challenges presented by the aforementioned limitations in physical infrastructure and personnel, recruitment of patients, even in large outbreaks, could be a problem. The inherent severity of filovirus infection, absence of specific antifilovirus therapies, limited availability of supportive measures, and, at times, the channeling of energies and resources into community control rather than patient-care have all contributed to the extremely high case-fatality rate. An unfortunate result of this situation is that the isolation ward is frequently considered as a place where one goes to die, not survive. Furthermore, the environment of the isolation ward is unfamiliar and even threatening, with treatment often being administered by mask-, gown-, and glove-clad foreigners and contact with family members extremely limited. Death brings the possibility of being buried in an unfamiliar setting without the traditional rites so important in African culture, while those who survive to return to their communities are often ostracized. Adding on the vestiges of colonial era suspicion of foreigners and present-day frictions between ethnic groups, it is not surprising that resistance to admission to the isolation ward is high, and sometimes even violent. In the worst case scenario, and if not conducted openly and with cultural sensitivity, clinical research in this charged setting could run the risk of meeting with the perception that foreigners and others in power are experimenting on vulnerable populations. However, measures now routinely implemented in the outbreak response to filoviruses, including the inclusion of social mobilization teams and concerted efforts to provide thorough explanation of the goals and movements of the outbreak response efforts to the local population, are the right steps to alleviating this concern.

The recent success in developing therapies and vaccines that protect laboratory primates against the filoviruses suggests that similar approaches may be effective in humans. However, the typical setting of a filovirus outbreak in Central Africa, the only place where human efficacy trials might be possible, presents an extreme contrast to the highly controlled, resource-rich environment of high-containment laboratories in industrialized countries. Although human efficacy trials might be avoided altogether through use of the Food and Drug Administration's (FDA) so called "animal rule", which allows for licensure of new drugs and biological products based on evidence of effectiveness derived from studies in well-characterized animal models when human efficacy studies are not ethical or feasible, it is likely that some evidence of efficacy in humans will still be needed before widespread use of any developed product. As discussed above, communities in Africa most affected by filoviruses and where treatments and vaccines are most needed, often harbor significant suspicion of outbreak response teams, including researchers. Distinctly American concepts such as FDA licensure and the animal rule are unlikely to mean much to these populations. Some degree of demonstrated efficacy in humans will likely be required to alleviate their suspicions. Furthermore, even in the United States, licensure based on the animal rule alone may not bring a sufficient degree of comfort to allow widespread use, even of a marketed product. Consequently, a strategy to translate discoveries from the laboratory to the field must be considered and take into account the challenges of conducting high quality and ethically sound research in some of the world’s poorest nations.

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Glossary

Adaptive immunity
Components of the immune system that are acquired after birth. These are characterized by specific immune responses to an antigen, including antibody production by B cells, selection of active T cells, T-cell apoptosis and development of immunological memory.
Animal model
Refers to an animal species that is sufficiently similar to humans in its response to an injury or disease so that it can be used in medical research to obtain information that can be extrapolated for human medicine. Endothelial barrier: the interface that is formed by a layer of endothelial cells that lines the interior surface of blood vessels and controls the passage of materials in and out of the circulating blood supply.
Glycoprotein
A protein conjugated with one or more carbohydrate (sugar) components. The sugar residue(s) are typically attached to the protein at either asparagine residues (N-linked glycosylation) or at serine or threonine residues (O-linked glycosylation).
Innate immunity
All non-specific immune mechanisms by which pathogens are recognized and responded to. These rapid responses are coordinated through receptors that recognize a wide spectrum of conserved pathogenic components.
Interferon (IFN) antagonism
The process of evading the host response to IFN, including both immune activation and induction of an antiviral state. This can be achieved either by inhibition of IFN production or by blockade of IFN signalling in target cells and is known to be mediated by many viral proteins.
Pathophysiology: the disturbance of normal mechanical, physical and biochemical functions leading to or resulting from disease or injury.
Bottleneck
A reduction in the size of a breeding population, which can result in the loss of genetic variability depending on its magnitude and its duration.
Endemic: the usual rate of a disease in a certain area as a result of the conditions constantly present within a community.
Epidemic: the occurrence of cases of an illness in a community or region in excess of the number of cases normally expected for that disease in that area at that time of year.
Host
An animal (including humans), arthropod or plant that supports an infectious agent under natural (as opposed to experimental) conditions.
Index case: the first person to be infected with a disease during an epidemic within a population.
Index case
The first person to be infected with a disease during an epidemic within a population.
Reservoir
An animal, arthropod or plant in which an infectious agent normally replicates usually without causing an overt disease in such a manner that it can be transmitted to a susceptible host.
Viral hemorrhagic fever
A group of illnesses caused by several families of viruses. They are associated with fever and gastrointestinal symptoms, often followed by hemorrhagic symptoms, such as internal and/or external bleeding and coagulation dysfunction.
The term viral haemorrhagic fever (VHF) classifies a number of virus-induced acute diseases that are typically associated with fever and, in severe cases, haemorrhage and shock. Currently, members of four virus families are known to cause VHF in humans:
  • arenaviruses,
  • filoviruses,
  • bunyaviruses,
  • flaviviruses.
The severity of VHF ranges from relatively mild illnesses to severe life-threatening cases characterized by virus-induced shock syndrome and multiorgan disease that is to some extent comparable to gramnegative bacteria-induced septic shock syndrome. Lethality likely results from overall impairment of cardiovascular regulation (e. g. blood pressure regulation, coagulation/anticoagulation balance and control of fluid distribution between intravascular and interstitial spaces) and the inability to a sufficient immune response due to the destruction of immune competent cells. VHFs are zoonotic diseases, which are characterized by pathogens capable to spread from animals or insects to humans or might spread from humans to humans, who are not their natural reservoir. Survival of zoonotic pathogens in a particular area depends on the presence of host organisms that serve as the natural reservoir. Primary human infection by VHF agents mostly occurs through close contact with an infected host (e. g. Ebola haemorrhagic fever occurs by close contact to gorillas or chimpanzees).
Zoonotic disease
An infection that occurs naturally in animals but can be transmitted to humans.
Apoptosis
Or programmed cell death is characterized by shrinking of the dying cell, plasma membrane blebbing, nuclear condensation, and final fragmentation of the cell in apoptotic bodies. Biochemically, apoptosis is characterized by activation of caspases. Caspase cleavage can be activated by extrinsic pathways via death receptor signaling induced by TRAIL, Fas/CD95 (Fas), or TNFα or by intrinsic pathways via regulation of cytochrome C efflux from the mitochondria.
Necrosis
Generally described as uncontrolled cell death, but recent findings suggest that it also might be regulated by conserved biochemical mechanisms. Necrosis is characterized by swelling of the cell and cellular organelles, membrane blebbing, vacuolization and results in rupture of the plasma membrane.
Autophagy
A conserved pathway of eukaryotic cells for recycling cellular components. However, extensive cellular stress can lead to autophagic or type II cell death. Autophagic cells feature vacuolization and formation of double membraned vesicles, the autophagosomes, for degradation of cellular content. Characteristic features of non-apoptotic cell death, such as vacuolization, swelling, and the lack of chromatin condensation.

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References

  1. Ebolavirus - free articles in PubMed
  2. Ebola - NCBI webpage
  3. Takada A. Filovirus tropism: cellular molecules for viral entry. Front Microbiol. 2012 Feb 6;3:34.

    Structure of Ebola virus particle and genome organization

    Structure of Ebola virus particle and genome organization. Electron micrograph of Ebola virus particle (A), its diagram (B), and negative-sense genome organization (C) are shown. Viral protein names and functions are described in the text. Transcribing the glycoprotein (GP) gene produces a soluble GP (sGP). Transcriptional editing accompanied by frame shifting is required to produce full-length, membrane-anchored GP, which shares its first 295 amino acid residues with sGP.

    Filovirus replication in a cell

  4. Roddy P, Howard N, Van Kerkhove MD, Lutwama J, Wamala J, Yoti Z, Colebunders R, Palma PP, Sterk E, Jeffs B, Van Herp M, Borchert M. Clinical manifestations and case management of Ebola haemorrhagic fever caused by a newly identified virus strain, Bundibugyo, Uganda, 2007-2008. PLoS One. 2012;7(12):e52986.

    Filovirus ward clinicians administering supportive treatment while concurrently recording clinical data during the Bundibugyo Uganda 2007–08 Ebola haemorrhagic fever outbreak.

    Filovirus ward clinicians administering supportive treatment while concurrently recording clinical data during the Bundibugyo Uganda 2007–08 Ebola haemorrhagic fever outbreak.

  5. Hong JE, Hong KJ, Choi WY, Lee WJ, Choi YH, Jeong CH, Cho KI. Ebola hemorrhagic Fever and the current state of vaccine development. Osong Public Health Res Perspect. 2014 Dec;5(6):378-82.

    Ebola virus virion under light microscopy. (Courtesy of CDC/Cynthia Goldsmith).

    Ebola virus virion under light microscopy. (Courtesy of CDC/Cynthia Goldsmith).

  6. Rouquet P, Froment JM, Bermejo M, Kilbourn A, Karesh W, Reed P, Kumulungui B, Yaba P, Délicat A, Rollin PE, Leroy EM. Wild animal mortality monitoring and human Ebola outbreaks, Gabon and Republic of Congo, 2001-2003. Emerg Infect Dis. 2005 Feb;11(2):283-90.

    Ebola cycle in equatorial forest

    Schematic representation of the Ebola cycle in the equatorial forest and proposed strategy to avoid Ebola virus transmission to humans and its subsequent human-human propagation. Ebola virus replication in the natural host (a). Wild animal infection by the natural host(s) (b), no doubt the main source of infection. Wild animal infection by contact with live or dead wild animals (c). This scenario would play a marginal role. Infection of hunters by manipulation of infected wild animal carcasses or sick animals (d). Three animal species are known to be sensitive to Ebola virus and to act as sources of human outbreaks, gorillas, chimpanzees, and duikers. Person-to-person transmission from hunters to their family and then to hospital workers (e). The wild animal mortality surveillance network can predict and might prevent human outbreaks. Medical surveillance can prevent Ebola virus propagation in the human population.

  7. Moshirfar M, Fenzl CR, Li Z. What we know about ocular manifestations of Ebola. Clin Ophthalmol. 2014 Nov 21;8:2355-7.

    Ebola virus outbreaks distribution map

    Past (1976–2007) and present (2014) maps of Africa demonstrating the Ebola virus outbreak distribution. Note: *Cases from January to August 2014.

  8. Zhang L, Wang H. Forty years of the war against Ebola. J Zhejiang Univ Sci B. 2014 Sep;15(9):761-5.

    The Present CIRMF High Security P4 Laboratory with  Glove Box

    The Present CIRMF High Security P4 Laboratory with "Glove Box".

  9. Leroy E, Gonzalez JP. Filovirus research in Gabon and equatorial Africa: the experience of a research center in the heart of Africa. Viruses. 2012 Sep;4(9):1592-604.

  10. Piercy TJ, Smither SJ, Steward JA, Eastaugh L, Lever MS. The survival of filoviruses in liquids, on solid substrates and in a dynamic aerosol. J Appl Microbiol. 2010 Nov;109(5):1531-9.
  11. Simmons G. Filovirus entry. Adv Exp Med Biol. 2013;790:83-94.
  12. Ascenzi P, Bocedi A, Heptonstall J, Capobianchi MR, Di Caro A, Mastrangelo E, Bolognesi M, Ippolito G. Ebolavirus and Marburgvirus: insight the Filoviridae family. Mol Aspects Med. 2008 Jun;29(3):151-85.
  13. Bausch DG, Sprecher AG, Jeffs B, Boumandouki P. Treatment of Marburg and Ebola hemorrhagic fevers: a strategy for testing new drugs and vaccines under outbreak conditions. Antiviral Res. 2008 Apr;78(1):150-61.
  14. Carroll SA, Towner JS, Sealy TK, McMullan LK, Khristova ML, Burt FJ, Swanepoel R, Rollin PE, Nichol ST. Molecular evolution of viruses of the family Filoviridae based on 97 whole-genome sequences. J Virol. 2013 Mar;87(5):2608-16.
  15. Kühl A, Pöhlmann S. How Ebola virus counters the interferon system. Zoonoses Public Health. 2012 Sep;59 Suppl 2:116-31.
  16. Eichner M, Dowell SF, Firese N. Incubation period of ebola hemorrhagic virus subtype zaire. Osong Public Health Res Perspect. 2011 Jun;2(1):3-7.
  17. Olejnik J, Ryabchikova E, Corley RB, Mühlberger E. Intracellular events and cell fate in filovirus infection. Viruses. 2011 Aug;3(8):1501-31.
  18. Ramanan P, Shabman RS, Brown CS, Amarasinghe GK, Basler CF, Leung DW. Filoviral immune evasion mechanisms. Int J Biochem Cell Biol. 2005 Aug;37(8):1560-6.
  19. Bray M, Geisbert TW. Ebola virus: the role of macrophages and dendritic cells in the pathogenesis of Ebola hemorrhagic fever. Int J Biochem Cell Biol. 2005 Aug;37(8):1560-6.
  20. Ebihara H et al. Host response dynamics following lethal infection of rhesus macaques with Zaire ebolavirus. J Infect Dis. 2011 Nov;204 Suppl 3:S991-9.
  21. Leach M, Scoones I., Stirling A. Governing epidemics in an age of complexity: Narratives, politics and pathways to sustainability. Global Environmental Change 20 (2010) 369–377.
  22. Leach M, Scoones I. The social and political lives of zoonotic disease models: narratives, science and policy. Soc Sci Med. 2013 Jul;88:10-7.
  23. Sobarzo A et al. Persistent immune responses after Ebola virus infection. N Engl J Med. 2013 Aug 1;369(5):492-3.
  24. Bornholdt ZA, Noda T, Abelson DM, Halfmann P, Wood MR, Kawaoka Y, Saphire EO. Structural rearrangement of ebola virus VP40 begets multiple functions in the virus life cycle. Cell. 2013 Aug 15;154(4):763-74.
  25. Wong G, Kobinger GP, Qiu X. Characterization of host immune responses in Ebola virus infections. Expert Rev Clin Immunol. 2014 Jun;10(6):781-90.
  26. Olival KJ, Hayman DT. Filoviruses in bats: current knowledge and future directions. Viruses. 2014 Apr 17;6(4):1759-88.
  27. O'Shea TJ, Cryan PM, Cunningham AA, Fooks AR, Hayman DT, Luis AD, Peel AJ, Plowright RK, Wood JL. Bat flight and zoonotic viruses. Emerg Infect Dis. 2014 May;20(5):741-5.
  28. Li YH, Chen SP. Evolutionary history of Ebola virus. Epidemiol Infect. 2014 Jun;142(6):1138-45.
  29. MacNeil A, Rollin PE. Ebola and Marburg hemorrhagic fevers: neglected tropical diseases? PLoS Negl Trop Dis. 2012 Jun;6(6):e1546.
  30. Aleksandrowicz P, Wolf K, Falzarano D, Feldmann H, Seebach J, Schnittler H. Viral haemorrhagic fever and vascular alterations. Hamostaseologie. 2008 Feb;28(1-2):77-84.
  31. Nkoghe D, Leroy EM, Toung-Mve M, Gonzalez JP. Cutaneous manifestations of filovirus infections. Int J Dermatol. 2012 Sep;51(9):1037-43.
  32. Takada A, Kawaoka Y. The pathogenesis of Ebola hemorrhagic fever. Trends Microbiol. 2001 Oct;9(10):506-11.
  33. Mahanty S, Bray M. Pathogenesis of filoviral haemorrhagic fevers. Lancet Infect Dis. 2004 Aug;4(8):487-98.
  34. Feldmann H. Ebola - A Growing Threat? N Engl J Med. 2014 May 7.
  35. Pourrut X et al. The natural history of Ebola virus in Africa. Microbes Infect. 2005 Jun;7(7-8):1005-14.
  36. Groseth A, Feldmann H, Strong JE. The ecology of Ebola virus. Trends Microbiol. 2007 Sep;15(9):408-16.
  37. Hartman AL, Towner JS, Nichol ST. Ebola and marburg hemorrhagic fever. Clin Lab Med. 2010 Mar;30(1):161-77.
  38. Bray M. Defense against filoviruses used as biological weapons. Antiviral Res. 2003 Jan;57(1-2):53-60.
  39. Diamond MS, Fremont DH. Ebola images emerge from the cave. Cell Host Microbe. 2008 Aug 14;4(2):87-9.
  40. Feldmann H, Geisbert TW. Ebola haemorrhagic fever. Lancet. 2011 Mar 5;377(9768):849-62.
  41. Turner C. Ebola virus disease: An emerging threat. Nursing. 2014 Sep;44(9):68-9.
  42. Ebola news
  43. Feldmann H, Wahl-Jensen V, Jones SM, Ströher U. Ebola virus ecology: a continuing mystery. Trends Microbiol. 2004 Oct;12(10):433-7.
  44. Carroll SA et al. Molecular evolution of viruses of the family Filoviridae based on 97 whole-genome sequences. J Virol. 2013 Mar;87(5):2608-16.
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