Ebola Virus---A Global Threat

Ebola virus is a filamentous, enveloped, non-segmented, single-stranded, negative-sense RNA virus. It belongs to the Filoviridae and was first recognized near the Ebola River valley in Zaire in 1976. Since then most of the outbreaks have occurred to both human and nonhuman primates in sub-Saharan Africa . Ebola virus causes highly fatal hemorrhagic fever in human and nonhuman primates. In addition to hemorrhagic fever, it could be used as a bioterrorism agent. Although its natural reservoir is yet to be proven, current data suggest that fruit bats are the possibility. Infection has also been documented through the handling of infected chimpanzees, gorillas, monkeys, forest antelope and porcupines. Human infection is caused through close contact with the blood, secretion, organ or other body fluids of infected animal. Human-to-human transmission is also possible. Ebola virus infections are characterized by immune suppression and a systemic inflammatory response that causes impairment of the vascular, coagulation, and immune systems, leading to multiorgan failure and shock. The virus constitutes an important public health threat in Africa and also worldwide as no effective treatment or vaccine is available till now DOI: http://dx.doi.org/10.3329/jemc.v5i1.21497 J Enam Med Col 2015; 5(1): 44-51

Ebola virus (EV) is the causative agent of the ongoing deadly epidemic in West Africa.It is one of the world's most dreadful pathogens, causing catastrophic clinical disease 1 and remains one of the most lethal transmissible infections with high fatality rates up to 90% and substantial morbidity during sporadic outbreaks. 2,3][6] EBOV together with Marburg virus comprise the family Filoviridae in the order Mononegavirales. 7][10] Among the five species Zaire, Sudan and Bundibugyo Ebolaviruses are responsible for most of the Ebola hemorrhagic fever (EHF) outbreaks. 11Reston Ebolavirus has caused disease in nonhuman primates but not in humans in the Philippines. 12The recent ongoing epidemic is caused by the ZEBOV and has been the most severe outbreak since Ebola virus was first identified in 1976. 13No previous outbreak has been as large or persistent as the current epidemic.To date, the number of cases now exceeds the number from all previous outbreaks combined.In addition to mortality, indirect effects include disruption of standard medical care, substantial economic losses, insecurity and social disruption in countries that were already struggling to recover from decades of war. 14On August Introduction 44

Ebola Virus ---A Global Threat
and at present no effective treatment or vaccine is available.Therefore, people should be aware of the threats from the Ebola virus in order to avoid infection and scientists should try their best to formulate a treatment and vaccine.

History and geographic distribution
Sporadic outbreaks of Marburg virus and Ebola virus infection have presumably occurred in central Africa for millennia, but the agents were not recognized by the scientific community until the late 20 th century. 16The cases of filovirus hemorrhagic fever were reported first in 1967 among workers in German and Yugoslavian vaccine plants who were processing tissues from monkeys imported from Uganda.The causative agent was identified as Marburg virus. 17,18Similar cases of hemorrhagic fever were described in 1976 from outbreaks in two neighboring locations: first in southern Sudan and subsequently in northern Zaire, now Democratic Republic of the Congo (DRC).An unknown causative agent was isolated from patients in both outbreaks and was named Ebolavirus after a small river in northwestern DRC. 19These two epidemics were caused by two distinct species of Ebolavirus, Sudan Ebolavirus and Zaire Ebolavirus.The third African Ebola virus species, Tai Forest Ebolavirus (Côte d'Ivoire Ebolavirus) was discovered in 1994 from an infected ethnologist who had worked in the Tai Forest reserve in Côte d'Ivoire and had done a necropsy on a chimpanzee.Bundibugyo Ebolavirus is the fourth species of Ebola virus found in equatorial Africa. 7,20An additional species, the Reston Ebolavirus, was first described in 1989 and isolated from Cynomolgus monkeys (Macaca fascicularis) housed at a quarantine facility in Reston, VA, USA.Subsequently, Reston Ebolavirus has been found in the Philippines on several occasions in pigs. 20,213][24] The frequency of recognized outbreaks has been increasing since 1990. 25After smaller outbreaks in Zaire and Sudan, some 15 years passed before Ebola virus reappeared in 1994 in Gabon.A large hospital-based epidemic in the Democratic Republic of Congo in 1995 brought the virus to worldwide attention and re-emergence of Sudan Ebolavirus in Uganda in late 2000 resulted in 425 cases and 225 deaths.Epidemics of Zaire Ebolavirus have increased in frequency in recent years. 16The virus has caused more than 20 outbreaks since its identification.In most instances, the virus emerged in geographically restricted rural regions. 26Outbreak of Ebola virus disease in West Africa is so large and severe; there are many factors behind it, but poverty is considered as the main reason.The hardest-hit countries Guinea, Liberia and Sierra Leone are among the poorest nations and have recently emerged from years of conflict and civil war.Their health systems are destroyed or severely disabled and in some areas war left a generation of children without education.Health care infrastructure is inadequate and health workers and essential supplies including personal protective equipments are scarce.Population movements across the porous borders are constant, so transmission is intense and people continue to reinfect each other. 27Traditional practices, such as bathing of corpses before burial, are also an important factor for disease transmission. 26

Current situation
The recent ongoing epidemic caused by the Zaire Ebolavirus started in Guinea in December 2013 and then spread to Liberia, Sierra Leone and Nigeria; it is the largest Ebola virus epidemic in history.It has been the most severe outbreak in terms of the number of human cases and fatalities since the discovery of the virus in 1976. 13The situation is changing rapidly and other countries might experience imported cases or outbreaks.Instances of civil unrest and violence against aid workers have been reported in West Africa as a result of the outbreak.The public health systems in the affected countries are being severely strained as the outbreak grows. 28On 10 December 2014, WHO reported a total of 17,942 suspected cases and 6,388 deaths. 29

Ebola virus and Bangladesh situation
Olival et al 19 conducted a study in Bangladesh during April 2010 to March 2011.They tested 276 bats of several species from Faridpur, Rajbari, Lalmonirhat, and Comilla districts.Among them five bats were positive for antibodies against Ebola Zaire and Reston viruses, but no virus was detected by PCR.There are reasons to speculate that bats might be a reservoir for Ebola or Ebola-like viruses and extend the range of filoviruses to mainland Asia.Failure to detect filovirus nucleic acid might reflect the relatively small sample size, low virus prevalence, or use of a PCR that has low sensitivity for filoviruses circulating in Bangladesh. 19The national disease monitoring arm Institute of Epidemiology, Disease Control and Research (IEDCR) is keeping a close watch on the current situation.Due to travel restrictions on Ebola patients and the absence of direct air links with the affected West African countries made the deadly virus making its way to Bangladesh "a remote possibility".Director of IEDCR considers Bangladesh a low-risk country and urged everyone not to spread panic.Bangladesh has also issued an alert against Ebola virus for three months, after the WHO declared the epidemic an international health emergency. 30

Viral structure
Ebola virus is filamentous, enveloped, non-segmented, single-stranded, negative-sense RNA virus. 31,32The EBOV genome is about 19000 nucleotides long that encodes seven structural proteins, nucleoprotein (NP), polymerase cofactor (VP35), matrix protein (VP40), glycoprotein (GP), replication-transcription protein (VP30), minor matrix protein (VP24) and RNA-dependent RNA polymerase (L). 33,34The structural proteins VP40 and VP24 represent viral matrix proteins connecting the nucleocapsid with the viral envelope.VP40 plays an essential role in assembly and budding of the virus. 35Ebola virus is susceptible to 3% acetic acid, 1% glutaraldehyde, alcohol-based products and dilutions of 5.25% sodium hypochlorite and calcium hypochlorite.The WHO recommendation for cleaning up spills of blood or body fluids is flooding the area with a 1:10 dilution of 5.25% sodium hypochlorite for 10 minutes.The virus is moderately thermolabile and can be inactivated by heating for 30 to 60 minutes at 60°C, boiling for 5 minutes or gamma irradiation combined with 1% glutaraldehyde.It has been reported that the virus is capable to survive for weeks in blood particularly at low temperatures (4°C).When dried in tissue culture media and stored at 4°C, Zaire Ebolavirus survived for over 50 days.This information is based on experimental findings and intended to be used to support local risk assessments in a laboratory setting. 36

Cell tropism and replication
Ebola virus is known to be pantropic in infection of human and can infect a wide variety of cell types.Though Ebola shows broad tissue tropism, hepatocytes, endothelial cells, dendritic cells, monocytes, and macrophages are thought to be their preferred target cells. 31Infection begins with the attachment of the virion to a receptor or lectin on the cell surface.Binding is followed by endocytosis, fusion of the viral envelope with the cellular endosomal membrane and release of the RNA genome and viral proteins into the cytoplasm.A replication complex made up of VP30, nucleoprotein, VP35 and large protein then generates mRNA transcripts.The 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 furinlike 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 GP.The nascent virions leave the cell through budding. 16

Natural reservoir
The first recorded human outbreak of Ebola virus was in 1976, but the wild reservoir of this virus is still unknown. 37Since the discovery of filoviruses more than 40 years ago, ostensibly random, sporadic and fatal outbreaks of disease in primates have evoked interest in delineation of host tropisms, potential reservoirs for disease transmission and persistence in nature. 8However, researchers have hypothesized that it is an animal origin virus. 38urrent data suggest that in Africa fruit bats are the possible natural reservoir hosts.As a result, the geographic distribution of Ebola viruses may overlap with the range of the fruit bats. 9Infection has also been documented through the handling of infected chimpanzees, gorillas, monkeys, forest antelope and porcupines. 13

Transmission
The exact mode of transmission of the virus from the natural reservoir to a human is not known.Evidence suggests that human infection is caused through close contact with the blood, secretion, organ or other body fluids of infected animal.Human-to-human transmission is also possible. 9BOV is shed in a wide variety of body fluids (saliva, stool, semen, breast milk, tears, nasal blood and skin swab of infected person) during the acute period of illness.However, the risk of transmission from fomite of a patient and during the convalescent period is low. 39t was found that men who have recovered from the disease can still transmit the virus through their semen for up to 7 weeks after recovery from illness.Healthcare workers may get infection through close contact with patients, when infection control precautions are not practiced properly. 9Although Ebola virus has been detected in breast milk, it is not known clearly whether Ebola virus can be transmitted through breastfeeding.Infected mothers may be critically ill and unable to breastfeed; but when they are able to breastfeed, decisions about whether or not to breastfeed may depend on the age of the infant, the availability and feasibility of safe nutrition and infant care and overall sanitary conditions.The recommendation of CDC is when safe alternatives to breastfeeding and infant care exist, mothers with probable or confirmed Ebola virus disease should not have close contact with their infants including breastfeeding. 40

Pathogenesis
EBOV is an aggressive pathogen that causes highly fatal hemorrhagic fever in human and nonhuman primates. 41The virus has the specialized mechanisms to evade the immune system and the course of illness results from a complex pathogenic mechanism. 16In a study it is shown that fatal outcome is associated with aberrant innate immune responses and suppression of the adaptive immunity.The innate immune responses are characterized by the hypersecretion of numerous proinflammatory cytokines (IL-1β, IL-1RA, IL-6, IL-8, IL-15 and IL-16), chemokines, growth factors (MIP-1α, MIP-1β, MCP-1, M-CSF, MIF, IP-10, GRO-α and eotaxin) and by the noteworthy absence of antiviral IFNα2.Suppression of adaptive immunity is characterized by very low levels of circulating cytokines produced by T lymphocytes and by massive loss of peripheral CD4 and CD8 lymphocytes. 37,42Viral replication, in conjunction with immune and vascular dysregulation, is thought to play the vital role in the disease process.Specific organ involvement includes extensive disruption of the parafollicular regions in the spleen and lymph nodes and proliferation of the virus in mononuclear phagocytic cells has been demonstrated. 43tudies in nonhuman primate models depicted monocytes, macrophages and dendritic cells are the major sites of initial viral replication.Virus is then distributed by the circulating phagocytic cells to a wide variety of organs and cells.Infected dendritic cell cultures supported exponential viral growth without releasing interferon (IFN-α). 44Two viral proteins (EBOV VP35 and EBOV VP24) are responsible for suppression of interferon responses.It seems that EBOV infection blocks the cellular production of IFNα/β and the ability to respond to IFN-α/β or IFN-γ.The VP24 is likely to be an important virulence factor that allows the virus to evade the antiviral effects of IFNs. 45,46In most instances, patients fail to produce antibodies against the virus and die with persistent high viremia.For initiation of an adaptive immune response presentation of viral antigens to lymphocytes is required.Phagocytic cells are the major sites of viral replication, which block their maturation and cause their death through necrosis.The system-wide release of proinflammatory cytokines and chemokines by these infected cells causes fever, disseminated intravascular coagulation, vascular instability, hypotension, shock and multi-organ failure.Although lymphocytes remain free of infection, they are destroyed in massive numbers over the course of illness through apoptosis. 47Massive apoptosis of natural killer and T cells further impairs immunity. 48Although some studies have shown that survival of the patient is associated with the ability of production of antigen-specific antibodies, a recent report from Sudan suggests that cell-mediated responses could also play an important role in protection. 49,50Blood samples obtained during several outbreaks in Gabon also suggested that survival is associated with the earlier appearance of proinflammatory cytokines in the blood. 51

Clinical features
Clinical findings are variable in Ebola infection.After an incubation period of around 2-21 days, 4 disease starts nonspecifically with the abrupt onset of fever, chills, headache, malaise, anorexia, sore throat, myalgia and joint pains. 20,46The initial features of the disease may mimic other tropical diseases and it is difficult to distinguish these features from other febrile illnesses.Conjunctival infection is seen in up to half of the patients. 46Respiratory symptoms include chest pain, shortness of breath, dry cough and nasal discharge. 20astrointestinal manifestations including nausea, vomiting, abdominal pain and diarrhea develop within the first few days of illness.In severe cases, vascular instability develops, usually 4-5 days after the onset of symptoms and may be evidenced by facial flushing, edema, proteinuria, bleeding, hypotension and shock. 46aculopapular rash associated with varying severity of erythema appears which desquamates by day 5-7 of the illness; this symptom is a valuable differential diagnostic feature and is usually followed by desquamation in survivors. 20Hemorrhage is most often gastrointestinal but vaginal bleeding, petechie, purpura, epistaxis and bleeding from the gums may be seen.Central nervous system manifestations including disorientation, gait anomalies, convulsions and hiccups may also be noted in end-stage disease. 46

Laboratory diagnosis
In the absence of effective intervention strategies, diagnosis becomes a key element in response to Ebola virus infection.Diagnosis of EHF must be sensitive, specific and reliable because misdiagnosis may bring huge turmoil to society.Therefore, the diagnosis of EHF must not rely on any single method.During outbreak, patients with EHF must be isolated and a false-positive result will put an individual at unnecessary risk of cross infection by placing the person in an isolation ward.A false-negative result will allow infected persons to be released into the community and may cause person-to-person transmission of the virus in the community. 52aboratory diagnosis of Ebola virus is achieved in two ways: measurement of host-specific immune responses to infection and detection of viral particles or particle components in infected individuals. 20Therefore, the diagnosis rests largely on molecular techniques utilizing multiple reverse-transcriptase-polymerase-chainreaction assays that can be used at remote outbreak sites.Antigen detection may be performed in parallel or serve as a confirmatory test for immediate diagnosis whereas assays for detection of antibodies (IgM and IgG) using unique virus antigens are secondary tests that are primarily important in surveillance. 53,54efinitive diagnosis is usually made by PCR and virus isolation on Vero cells.As a class-4 pathogen, Ebola virus culture requires a maximum containment facility.Additional immunological tests include ELISAs for the detection of Ebola IgG-and IgM-specific antibodies and virus antigens; more specialized molecular testing is also available but is not readily available in the usual clinical setting. 11Now-a-days, RT-PCR and antigen detection ELISA are the primary assays to diagnose an acute infection.Viral antigen and nucleic acid can be detected in blood from day 3 up to 7-16 days after onset of symptoms.For antibody detection the most generally used assays are direct IgG and IgM ELISAs and IgM capture ELISA.IgM antibodies can appear as early as 2 days postonset of symptoms and disappear between 30 and 168 days after infection.IgG-specific antibodies develop between day 6 and 18 after onset and persist for many years.An IgM or rising IgG titer constitutes a strong presumptive diagnosis.Decreasing IgM or rising IgG titers (four-fold), or both, in successive paired serum samples are highly suggestive of a recent infection. 20Histopathological techniques and antigen detection by immunohistochemical analyses are sensitive methods, particularly for postmortem diagnosis.Diagnosis by detection of virus antigens is suitable for patients in the early stage and detection of specific IgM and IgG antibodies is suitable for patients in a relatively late stage of illness. 48

Treatment
EBOV infections are a public health concern because of the high mortality rate and lack of prophylactic and therapeutic interventions as no specific antiviral treatment is available at present. 55Severely ill patients require intensive supportive care which includes oxygen, blood pressure medication, blood transfusions, rehydration with intravenous fluids containing electrolytes and treatment for other infections. 9A study done by Qiu et al in 2012 shows that a combination of three neutralizing monoclonal antibodies directed against the envelope glycoprotein resulted in complete survival of four of four cynomolgus macaques with no apparent side effects when three doses were administered 3 days apart beginning at 24 hours after a lethal challenge with EBOV.The same treatment initiated 48 hours after resulted in two of four cynomolgus macaques fully recovering. 56

Prevention
Although safe and effective vaccines or other medicinal agents to block Ebola infection are currently unavailable, a significant effort has been put forth to identify several promising candidates for the treatment and prevention. 2,57Some vaccines under trial have been shown to protect NHPs: a replication-incompetent adenovirus expressing the EBOV glycoprotein (29-31), a replication-competent vesicular stomatitis virus (VSV) expressing GP (7, 15), a recombinant paramyxovirus expressing GP (4), and virus-like particles (38, 41). 5 In the absence of effective treatment and vaccine, raising awareness regarding the risk factors and personal protective measures is the only way to reduce human infection and death. 9There are three key preventive interventions which have gained attention with encouraging outcome.The first is meticulous infection control in health care settings.Second is educating and supporting the community to avoid contact with body fluids of people who died from EVD, at least temporarily until the outbreak is controlled.And the third is avoiding handling of bush meat and contact with bats. 14

Conclusion
The main goal currently being addressed with Ebola virus is finding ways of treatment and effective vaccines that can be applied to humans.Although Ebola virus infection is not a problem right now for most populations outside Africa, it has the potential to be alarming from the point of view in global health in the future.