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“Filoviruses”: a real pandemic threat?

2009; Springer Nature; Volume: 1; Issue: 1 Linguagem: Inglês

10.1002/emmm.200900005

1757-4684

B Martina, Albert D. M. E. Osterhaus,

Viral gastroenteritis research and epidemiology

In Focus26 March 2009Open Access "Filoviruses": a real pandemic threat? Byron E.E. Martina Byron E.E. Martina Erasmus MC, Department of Virology, Rotterdam, The Netherlands Search for more papers by this author Albert D.M.E. Osterhaus Corresponding Author Albert D.M.E. Osterhaus [email protected] Erasmus MC, Department of Virology, Rotterdam, The Netherlands Search for more papers by this author Byron E.E. Martina Byron E.E. Martina Erasmus MC, Department of Virology, Rotterdam, The Netherlands Search for more papers by this author Albert D.M.E. Osterhaus Corresponding Author Albert D.M.E. Osterhaus [email protected] Erasmus MC, Department of Virology, Rotterdam, The Netherlands Search for more papers by this author Author Information Byron E.E. Martina1 and Albert D.M.E. Osterhaus *,1 1Erasmus MC, Department of Virology, Rotterdam, The Netherlands *Erasmus MC, Department of Virology, Rotterdam, The Netherlands. Tel: +31 10 7044066; Fax: +3110 7044760. EMBO Mol Med (2009)1:10-18https://doi.org/10.1002/emmm.200900005 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions Figures & Info Abstract Filoviruses are zoonotic and among the deadliest viruses known to mankind, with mortality rates in outbreaks reaching up to 90%. Despite numerous efforts to identify the host reservoir(s), the transmission cycle of filoviruses between the animal host(s) and humans remains unclear. The last decade has witnessed an increase in filovirus outbreaks with a changing epidemiology. The high mortality rates and lack of effective antiviral drugs or preventive vaccines has propagated the fear that filoviruses may become a real pandemic threat. This article discusses the factors that could influence the possible pandemic potential of filoviruses and elaborates on the prerequisites for the containment of future outbreaks, which would help prevent the evolution of filovirus into more virulent and more transmissible viruses. Introduction It is generally appreciated that infectious diseases have had a major impact on the population ecology and the course of history. For instance, smallpox, an acute human viral disease caused by variola virus, was one of the most devastating viral diseases known to have affected mankind and human ecology. Consequently, it has had a major influence on the course of history. The prevalence of smallpox around the previous turn of the 19th century was about 50 million people per year and the disease had a case–fatality rate of approximately 30%. Thanks to a world-wide vaccination campaign orchestrated by the World Health Organisation (WHO) using an effective vaccine that had been developed about 200 years earlier, the prevalence of smallpox was reduced to 10 million people by the year 1967. In 1979, WHO announced that the eradication of smallpox from the globe was a fact, the first infectious disease to be eradicated completely. This first success triggered optimism about the feasibility to eradicate all major infectious diseases of the mankind (Snowden, 2008). However, the discovery of the filoviruses, Marburg virus (MARV) and Ebola virus (EBOV) in 1967 and 1976 respectively, refuelled the fear that viruses of this family Filoviridae could, similar to smallpox virus, sweep around the globe in a matter of weeks, and kill millions of people. That fear provided the incentive for several countries to establish an infrastructure for studies aiming at the development of intervention strategies for such highly pathogenic (category 4) microorganisms. However, rather than witnessing such a pandemic filovirus outbreak, the world was instead confronted in the early 1980s with the more insidious pandemic viral outbreak of AIDS, caused by the human immunodeficiency virus (HIV). Although AIDS was identified only two and a half decades ago, the HIV pandemic belongs to the most devastating plagues in human history. Nevertheless, the high fatality rate of filovirus infections, the potential of filoviruses as bioterrorist agents and the high media attention for outbreaks caused by viruses of this family, have further increased the fear for outbreaks caused by these viruses. Furthermore, the increasing number of newly emerging and re-emerging virus infections in humans and animals in the past decades, as well as the general perception that the global capacity and infrastructure to adequately respond to such threats are insufficient, had had a negative impact on public confidence in our overall preparedness to combat such challenges. Obviously, the most important question in this regard is whether filoviruses do indeed pose a global threat and whether they may be the cause of future pandemics. Historical View The Filoviridae family consists of two genera, Marburgvirus and Ebolavirus, which harbour viruses that are morphologically identical but antigenically distinct. To date, only one subtype of MARV has been described. In 1967, MARV was first identified in patients with a severe febrile syndrome, with signs of haemorrhage and shock, who were admitted to University hospitals in Marburg and Frankfurt (Slenczka & Klenk, 2007). The patients had been working in a pharmaceutical company and the infection was traced back to contacts with African green monkeys (Cercopethicus aethiops) imported from Uganda (Bonin, 1969; Kissling et al, 1968; Kunz et al, 1968; Siegert et al, 1967). All the primary cases had been in contact with blood, organs or cell cultures from these animals. All secondary cases involved medical personnel and family members who had been in contact with body fluids of these patients. In 1976, EBOV was discovered as a second member of the family (Pourrut et al, 2005). In contrast to MARV, five subtypes of EBOV have been described; Zaire (ZEBOV), Sudan (SEBOV), Reston (REBOV), Cote d'Ivoire (CEBOV) and Bundibugyo (BEBOV). A summary of all filovirus outbreaks known to date is provided in Table 1 and Figure 1 depicts the locations in Africa from where filovirus outbreaks were reported. Several conclusions may be drawn from the different filovirus outbreaks: 1.. The cases of REBOV clearly illustrated that filoviruses could readily be imported into previously unaffected areas by means of international animal transports, nourishing the fear that filoviruses could spread worldwide with devastating consequences. 2.. The MARV outbreak in Democratic Republic of Congo (DRC) in 1998 was unique because of the high case–fatality rate, which had not been observed in previous MARV outbreaks. Furthermore, this outbreak strongly suggested that the reservoir host(s) for MARV would inhabit the caves and mines where the outbreaks started. 3.. The Angola outbreak in 2004 represented the first appearance of MARV in western Africa and the largest outbreak reported to date (Ligon, 2005). The epidemiology of this outbreak was different in that a high percentage of children were infected, and the estimated incubation time was shorter with an even higher case–fatality rate of up to 92%. 4.. Apparently, filovirus outbreaks are no longer restricted to remote, scarcely populated areas, but may strike in medium-size cities (e.g. Kikwit, 1995) and may be introduced in large cities (e.g. Johannesburg, 1975). Table 1. Outbreaks of filovirus infection in humans Year Location Number of human cases Case–fatality rate (%) Virus strain References 1967 Europe: Marburg, Frankfurt, Belgrade 31 26 MARV (Kissling et al, 1968) 1975 South Africa: Johannesburg 3 33 MARV (Gear et al, 1975) 1976 Africa: DRC 318 88 ZEBOV (Johnson et al, 1977) 1976 Africa: Sudan 284 53 SEBOV (WHO, 1978) 1977 Africa: DRC 1 100 ZEBOV (Heymann et al, 1980) 1979 Africa: Sudan 34 65 SEBOV (Baron et al, 1983) 1980 Africa: Kenya 2 50 MARV (Smith et al, 1982) 1987 Africa: Kenya 1 100 MARV (Johnson et al, 1996) 1989 USA: Virginia 0 0 REBOV (Jahrling et al, 1990) 1990 USA: Pennsylvania 0 0 REBOV (Groseth et al, 2002) 1992 Europe: Siena 0 0 REBOV (Rec, 1992) 1994 Africa: Gabon 49 65 ZEBOV (Georges et al, 1999) 1994 Africa: Cote dvoire 0 0 CEBOV (Formenty et al, 1999) 1995 Africa: DRC 317 77 ZEBOV (Khan et al, 1999) 1996 Africa: Gabon 37 57 ZEBOV (Georges et al, 1999) 1996 Africa: Gabon 62 74 ZEBOV (Georges et al, 1999) 1996 USA: Texas 0 0 REBOV (Rollin et al, 1999) 1998 Africa: DRC 154 83 MARV (Bausch et al, 2003) 2000 Africa: Uganda 425 53 SEBOV (Lamunu et al, 2004; Okware et al, 2002) 2001 Africa: Gabon 124 78 ZEBOV (Leroy et al, 2002b) 2002 Africa: Gabon 11 91 ZEBOV (Leroy et al, 2002b) 2003 Africa: DRC 143 90 ZEBOV (Formenty et al, 2003) 2004 Africa: DRC 35 83 ZEBOV (Leroy et al, 2004) 2004 Africa: Sudan 17 41 SEBOV (Towner et al, 2004) 2004 Africa: Angola 252 92 MARV (Ligon, 2005) 2005 Africa: DRC 11 82 SEBOV (Rec, 2005; Sanchez & Rollin, 2005) 2007 Africa: Uganda 3 33 MARV (Towner et al, 2007b) 2007 Africa: Uganda 29 36 BEBOV (Towner et al, 2008) DRC, Democratic Republic of Congo. Figure 1. Filovirus outbreaks reported in Africa. EBOV outbreaks are flagged in red and MARV in blue. Download figure Download PowerPoint These observations together with the increase in filovirus outbreaks in the last 15 years, point at a seemingly changing epidemiology, and prompt the question whether filoviruses are becoming a threat to the world at large. Is there a need to prepare ourselves for a more extensive spread of filoviruses, which might eventually even lead to a pandemic filovirus outbreak? Transmission For a virus to become a real pandemic virus, it must in principle comply with all of the four following criteria: i.. the population should be immunologically naive towards the virus; ii.. it should be pathogenic; iii.. it should have a short generation period; iv.. it should have a basic reproduction number (R0) greater than 1 (Box 1). Pending issues Identification of the animal reservoir(s) for EBOV and MARV. Understanding the transmission cycle of filovirus infections. Understanding the determinants of virulence of ZEBOV compared to REBOV. Understanding the immune response to the virus, viral induction of immune suppression and identifying the correlates of protection for vaccine testing. Further development of effective pan-filovirus preventive vaccines. Glossary Index case The first identified case in an outbreak. Nosocomial infection An infection acquired while in hospital. Sentinel animal An animal intentionally placed in a specific environment to assess the presence of an infectious agent. Haemostatic system A system composed of vessel walls, blood platelets and soluble factors responsible for blood coagulation and fibrinolysis. Parenteral exposure Exposure of the internal systems of the body via any route except the alimentary canal. Barrier precautions Any means used to reduce contact with potentially infectious body fluids. Generation period The interval between infection and transmission to another person. BOX 1: Basic reproduction ratio Basic reproduction ratio (R0) is a key concept in epidemiology and is widely used to study infectious diseases. R0 is defined as the average number of secondary infections produced by a single infected individual during the entire infectious period. This definition applies to a model where everyone in the population is susceptible. Therefore, determination of R0, assuming a SEIR (susceptible–exposed–infectious–recovered) model, is used to estimate the risk of an epidemic or pandemic. When R0 is 1, the virus is able to persist in the susceptible population and cause an epidemic. Virus transmissibility is an important parameter that affects R0. Transmissibility, which is affected by the route of transmission, is the product of infectiousness and generation time (time between infection and excretion). SARS–corona virus emerged in 2003 as the cause of a fatal respiratory syndrome. The R0 for SARS was determined to be between 2 and 4 in community-based settings (Lipsitch et al, 2003). The variation of the effective reproduction number reflects the control measures implemented during the SARS outbreak, such as early case detection, prompt contact tracing, strict isolation and quarantine and timely treatment, indicating that the response time and the strength of control measures, have significant effects on the scale of an outbreak and the lasting time of an epidemic. Interestingly, R0 of the 1918 pandemic influenza virus spread has also been estimated between 3 and 4 for the community-based setting (Vynnycky et al, 2007). Therefore, the rapid spread of pandemic influenza in 1918 as well as in other pandemics is more likely the result of a short generation time of influenza virus in humans (about 4 days) rather than a high R0. Influenza virus is thought to be infectious before the onset of symptoms whereas transmission of SARS coronavirus is thought to occur more than a week after infection and several days after the onset of symptoms (Lipsitch et al, 2003). An important concept in understanding outbreaks and determining the risks of becoming a pandemic is the minimum population size needed to maintain an infection in the population. Highly transmissible, acute virus infections with R0 >1, require a large number of susceptible individuals to persist in the population. For example, it has been estimated that for measles to persist, the threshold population size must exceed 100,000 (Keeling & Grenfell, 1997). On the other hand, vector-borne infections can be maintained in populations with much smaller numbers of susceptible individuals. For newly emerging zoonotic infections with an R0 1, and the infection is self-sustaining, understanding the ecology of the disease is less important than implementing control measures. Furthermore, it is important to realize that when appropriate control measures are not activated, zoonotic infections with an R0 1. BOX 2: Preparedness plan As most filovirus outbreaks are the result of nosocomial infections or infections of family members who nurse patients without using protection measures, targeted plans need to be in place to contain filovirus outbreaks. CDC and WHO have published guidelines on how to contain viral haemorrhagic fever in African health care settings (Centers for Disease Control and Prevention & World Health Organization, 1998), which may be adopted according to other settings. Essentially, the actions to be implemented in an outbreak response plan include: (i) direct notification of a suspected case to the authorities, (ii) intensify surveillance to identify cases as early as possible, (iii) active listing, tracing and follow-up of contact cases, (iv) patient isolation and barrier nursing, (v) inactivation of virus on contaminated materials with, for example, sodium hypochlorite and incineration of clinical waste, (vi) training of health care workers and provision of personal protective equipments, (vii) funerals to be performed by specialized teams that understand the risks involved with body preparation, but are familiar with the local beliefs and customs during burial proceedings, (viii) assignment of a taskforce entrusted with reviewing the outbreak response efforts. The first two criteria are certainly applicable to filoviruses. The last two criteria are slightly more difficult to apply to filoviruses. Filoviruses are unlikely to be transmitted during the incubation period and transmissibility is generally highest late in the clinical course of infection. Most individuals who have acquired infections in the last few decades were infected by needle-stick injuries or reuse of unsterilized medical devices like needles and syringes. Furthermore, direct contact with blood, body secretions or tissues of infected humans and non-human primates have posed the main risk for virus transmission (Bausch et al, 2007). High numbers of filovirus particles can be found in sweat glands and the human skin (Zaki et al, 1999), suggesting that transmission may occur through direct contact and that nursing patients and preparing bodies for burial without practicing the appropriate barrier precautions, represent an important factor contributing to the spread of the infection. However, it remains unclear how the virus enters the body upon direct contact. It has been shown that administration of filoviruses into the mouth, nose, or conjunctiva of non-human primates resulted in infection (Jaax et al, 1996; Schou & Hansen, 2000). Therefore, it is conceivable that human infections occur through indirect contact of, for example, contaminated fingers with oral mucosa or conjunctiva. The REBOV outbreak and experimental infections carried out with ZEBOV have raised concerns that EBOV may be naturally transmitted by aerosol (Jahrling et al, 1990; Johnson et al, 1995). There is also circumstantial evidence that during the EBOV outbreak in DRC in 1995, some patients became infected through aerosol transmission (Roels et al, 1999). However, although aerosol transmission cannot be completely ruled out, the primary mode of transmission is through direct or indirect contact with an infected body and thus transmission of filoviruses is an inefficient process. Given the relatively limited transmissibility, eventually, the overall R0 will not be greater than 1. Although initially several patients may become infected by an index case even resulting in a R0 as high as 2.7 (Legrand et al, 2007), usually R0 readily drops below 1 when people realize what kind of actions predispose for transmission and relevant measures to prevent further transmission are taken. Obviously, a filovirus may reach any place in the world within days through modern transportation. However, the extent to which the virus will spread after introduction in a new area will largely depend on the preparedness plan (Box 2) that will be executed upon its introduction and the ability of the health care system to deal with infected patients and prevent further transmission. Genetic Stability and Virulence The question remains, whether there is a risk that filoviruses could mutate and become efficiently transmitted from person-to-person, e.g. by aerosol, and therefore may become a real pandemic threat. It has been postulated that if ever ZEBOV would cause an outbreak of significant size in a densely populated urban environment, the evolution towards an airborne variant could occur. Specifically, the argument has been put forward that a large enough epidemic would provide sufficient evolutionary pressure to the virus to give rise to an airborne variant. Since there is some evidence that REBOV infections may be airborne, a variant with an intrinsically high mutation rate could evolve towards an airborne virus. In general, RNA viruses encode error-prone polymerases that lack proofreading mechanisms, allowing these viruses to mutate and evolve under the appropriate selection pressures. It has been hypothesized that MARV strains that were involved in the DRC and Angola outbreaks in 1998 and 2004 respectively, were more virulent than the strains involved in the outbreak in Germany in 1967. Analysing the nucleotide sequences of the glycoprotein gene of both MARV and EBOV from different outbreaks indicates however that isolates from the same outbreak are almost identical in nucleotide sequences, whereas viruses recovered from different outbreaks may vary up to 20%, the majority of the mutations being silent (Towner et al, 2006). The genetic stability observed during the Angolan outbreak of MARV infection indicates that the outbreak was probably caused by a single introduction of the virus by one index case, followed by spread to community members and further amplification by nosocomial transmission (Towner et al, 2006). This sequence of events may explain the relatively limited accumulation of mutations observed during this outbreak. In contrast, the MARV outbreak of 1998 was characterized by circulation of multiple genetic lineages, as well as identical lineages within, but not across clusters of epidemiologically linked cases. This suggested that multiple introductions of MARV in the community had occurred (Bausch et al, 2006). These sequence data are in agreement with the relatively limited number of secondary infections that were found. Taken together, there is no convincing data to suggest that genetic variation and selection has resulted in more virulent MARV strains during the DRC and Angolan outbreaks. Actually the virulence of the strains was comparable to that of those identified in 1967. More likely factors such as nutrition status, underlying co-infections, and quality of health care were involved in the different case–fatality rates observed among the different outbreaks. Future studies using reverse genetics technology will help us understand the possible biological significance of the few mutations observed in the different strains from different outbreaks. The initial high rate of infection among children in Angola was probably related to needle use and the high mortality recorded in that outbreak may have been related to the parenteral exposure as well, which has been associated with higher mortality rates (Emond et al, 1977). The same degree of genetic stability is observed among the respective EBOVs (Leroy et al, 2002a; Rodriguez et al, 1999). When the geographic distance between the different ZEBOVs is considered, it is estimated that the virus spreads at a constant rate of 50 km per year, and diversity between strains increases with distance (Walsh et al, 2005). Overall, filoviruses evolve slowly, but little is known about the biological consequences of accumulation of mutations. It is interesting to note that REBOV is less virulent than ZEBOV and SEBOV, and several factors may be involved in this difference (Morikawa et al, 2007). For instance, ZEBOV encodes a 17 amino acid peptide in the surface glycoprotein, which is involved in the apoptosis of lymphocytes and induction of immunosuppression (Yaddanapudi et al, 2006). The corresponding peptide in REBOV lacks this immunosuppressive effect. More studies are needed to understand the difference in virulence between viruses like ZEBOV and REBOV, which will help us to understand better the risk of filoviruses to eventually evolve, through mutation or recombination, to become more virulent and perhaps more importantly airborne. Reservoirs of Filoviruses It is still a mystery as to which animal species constitute the reservoir host or hosts of filoviruses, and how they spread the virus geographically. Despite a lot of efforts to identify the natural reservoir of filoviruses in the last decade, little progress has been made. RNA of ZEBOV was detected in rodents and shrews captured in the Central African Republic, although serology and virus isolation were negative. In several outbreaks of filovirus infections, index cases were associated with visiting a cave or cave-like environments inhabited by bats. Experimental infections of fruit and insectivorous bats showed that filoviruses replicate to high titres in several organs (Swanepoel et al, 1996). Up to three weeks after infection, ZEBOV RNA could still be recovered from faecal samples. Furthermore, infected bats did not develop disease signs, which is an important condition to function as a natural host in which the virus might persist. Recently, evidence was presented that three species of fruit bats caught near affected villages at the Gabon–Congo border, harboured RNA sequences of ZEBOV (Leroy et al, 2005). The three bat species have a broad geographical range that includes the regions where filovirus outbreaks had occurred. The sequences recovered cluster with strains found during human outbreaks, providing strong indications for the involvement of these bats in either transmission of EBOV to humans or to other types of introduction of the virus into new areas. There is evidence to suggest that the reservoir for MARV can be found in the same or related bat species (Towner et al, 2007a). However, it is not completely clear how humans and non-human primates may become infected by bats. Bat-scratch incidents and certain eating habits involving bat meat are possible risk factors. Clearly, more studies are needed to understand the role of bats in the ecology of filovirus infections of humans and animals. The outbreak in Angola in 2004 was surprising because MARV was not expected in that region, which was about 1,600 km far from the locations where previous MARV outbreaks had occurred (see Figure 1). If MARV would not have been present in Angola initially but was only recently introduced, it is possible that migratory animal species could have carried it over such a long distance, e.g. from Zaire to Angola. Birds have been proposed as possible hosts for filoviruses to accomplish such an introduction (Galat & Galat-Luong, 1997). However, there are no experimental studies to backup this hypothesis. It is not clear whether in this particular case bats could have played that role. Interestingly, epidemiological studies have suggested that EBOV has spread as a one-dimensional wave (Walsh et al, 2005). This has prompted some scientists to propose that rivers pose a barrier to the spread of filoviruses rendering both birds and bats unlikely candidate reservoirs. This theory has however been disputed by others (Lahm et al, 2007). Identifying the principle host species of filoviruses will hopefully contribute to new and more successful strategies for preventing and controlling future outbreaks. Treatment To date, there is no specific treatment for filovirus haemorrhagic fever and interventions are mainly supportive, aiming to maintain fluid and electrolyte balance, circulatory volume and blood pressure. Several studies suggested that both viral and immune-mediated factors are involved in the pathogenesis of filovirus infections. Development of fatal disease has been associated with high viral RNA copies (Towner et al, 2004). High levels of viral replication presumably lead to necrosis in cells of many organs, including liver and spleen, as can be inferred from microscopic examination of infected tissues (Zaki & Goldsmith, 1999). Therefore, use of effective antiviral drugs could reduce virus-induced pathology in infected individuals, thereby increasing the survival rates. The antiviral drug ribavirin, a nucleoside analogue that inhibits many RNA viruses, has been shown to be ineffective against filoviruses. Encouraging results are being reported with other candidate drugs (Bray et al, 2000; Huggins et al, 1999) and more efforts should be deployed in developing and testing new antiviral drugs against filoviruses. Alternatively, intervention strategies may be directed against the aberrant host response to filovirus infection. Upon infection with a filovirus, it is believed that the interplay between the immune and haemostatic systems results in increased expression of the tissue factor (TF). The development of disseminated intravascular coagulation might be crucial in the cascade of haemorrhagic complications produced by filovirus infections. Experimental studies conducted in non-human primates indicated that 33% of animals infected with a lethal dose of ZEBOV and treated within 24 h with the TF-specific hookworm-derived inhibitor nematode anti-coagulant peptide c2 (NAPc2), survived the infection (Geisbert et al, 2003). These results should encourage further studies aiming to understand the role of the coagulation system in filovirus pathogenesis that will provide important targets for designing intervention strategies. Preventive vaccines against filoviruses would also be useful, especially in endemic areas, laboratory settings and for primates whose populations are seriously threatened. Furthermore, the classification of filoviruses as potential bioterrorist agents makes development of a safe and effective vaccine a priority. However, the development of effective filovirus vaccines represents a challenge in the face of a lack of understanding of the correlatedness of protection. The role of antibodies in protection and recovery of the host from a filovirus infection is controversial. On the one hand it has been observed that failure of infected patients to develop filovirus-specific antibody response by the second week of onset of symptoms is associated with poor prognosis. However, in many patients, neutralizing antibodies cannot be detected, and therefore the biological function of filovirus-specific non-neutralizing antibodies remains unclear. Treatment of guinea pigs infected with ZEBOV using hyperimmune equine IgG has been shown to be protective if administered early after infection (Jahrling et al, 1999). Such preparations provided variable results when used on humans and non-human primates (Kuhn, 2008). Also the role of cell-mediated immune response in recovery from infection remains unclear. In mice, T-cells have been shown to play a role in protection against ZEBOV (Warfield et al, 2005). Some evidence exists that responses of CD8+ T-cells are elicited in macaques upon infection as well as by vectored vaccines such as adenovirus-based vaccines (Sullivan et al, 2003). Different vaccine platforms have been investigated and, in general, several vaccines have been shown to be effective in rodent models, but have failed to induce protective immunity in non-human primates (Kuhn, 2008). Surprisingly, most vaccine platforms studie