Chronological and spatial analysis of the 1996 Ebola Reston virus outbreak in a monkey breeding facility in the Philippines

Risk assessment of Ebola Reston virus in humans in the Philippines

Objective

There have been five documented outbreaks of Ebola Reston virus (RESTV) in animals epidemiologically linked to the Philippines. This assessment was conducted to determine the risk of RESTV occurring in humans in the Philippines and its potential pathogenicity in humans.

Methods

The World Health Organization Rapid Risk Assessment of Acute Public Health Events Manual was used for the assessment. A literature review was done and a risk assessment matrix was used for the risk characterization of the outbreaks in the Philippines. The risk assessment was conducted by the Philippines Field Epidemiology Training Program.

Results

The risk of RESTV occurring in humans in the Philippines and its potential pathogenicity in humans were both assessed as moderate. Animals involved in RESTV outbreaks in the Philippines were non-human primates and domestic pigs. The presence of RESTV in pigs poses a possibility of genetic evolution of the virus. Although RESTV has been identified in humans, there was no death or illness attributed to the infection. The Philippines Inter-agency Committee on Zoonoses oversees collaboration between the animal and human health sectors for the prevention and control of zoonoses. However, there is no surveillance of risk animals or previously affected farms to monitor and facilitate early identification of cases.

Discussion

The moderate risk of RESTV recurring among humans in the Philippines and its potential pathogenicity in humans reinforces the need for early detection, surveillance and continued studies of RESTV pathogenesis and its health consequences. The One Health approach, with the involvement and coordination of public health, veterinary services and the community, is essential in the detection, control and management of zoonosis.

Sequence of Reston Virus Isolate AZ-1435, an Ebolavirus Isolate Obtained during the 1989-1990 Reston Virus Epizootic in the United States

Reston virus (RESTV) was discovered in 1989-1990 during three connected epizootics of highly lethal viral hemorrhagic fever among captive macaques in primate housing facilities in the United States and Philippines. Currently, only one RESTV isolate from that outbreak (named Pennsylvania) has been sequenced. Here, we report the sequence of a second isolate, Reston virus/M.fascicularis-tc/USA/1990/Philippines89-AZ1435.

Neglected filoviruses

Eight viruses are currently assigned to the family Filoviridae Marburg virus, Sudan virus and, in particular, Ebola virus have received the most attention both by researchers and the public from 1967 to 2013. During this period, natural human filovirus disease outbreaks occurred sporadically in Equatorial Africa and, despite high case-fatality rates, never included more than several dozen to a few hundred infections per outbreak. Research emphasis shifted almost exclusively to Ebola virus in 2014, when this virus was identified as the cause of an outbreak that has thus far involved more than 28 646 people and caused more than 11 323 deaths in Western Africa. Consequently, major efforts are currently underway to develop licensed medical countermeasures against Ebola virus infection. However, the ecology of and mechanisms behind Ebola virus emergence are as little understood as they are for all other filoviruses. Consequently, the possibility of the future occurrence of a large disease outbreak caused by other less characterized filoviruses (i.e. Bundibugyo virus, Lloviu virus, Ravn virus, Reston virus and Taï Forest virus) is impossible to rule out. Yet, for many of these viruses, not even rudimentary research tools are available, let alone medical countermeasures. This review summarizes the current knowledge on these less well-characterized filoviruses.

Spatial methods for infectious disease outbreak investigations: systematic literature review

Investigations of infectious disease outbreaks are conventionally framed in terms of person, time and place. Although geographic information systems have increased the range of tools available, spatial analyses are used relatively infrequently. We conducted a systematic review of published reports of outbreak investigations worldwide to estimate the prevalence of spatial methods, describe the techniques applied and explore their utility. We identified 80 reports using spatial methods published between 1979 and 2013, ca 0.4% of the total number of published outbreaks. Environmental or waterborne infections were the most commonly investigated, and most reports were from the United Kingdom. A range of techniques were used, including simple dot maps, cluster analyses and modelling approaches. Spatial tools were usefully applied throughout investigations, from initial confirmation of the outbreak to describing and analysing cases and communicating findings. They provided valuable insights that led to public health actions, but there is scope for much wider implementation and development of new methods.

Experimental Inoculation of Egyptian Rousette Bats (Rousettus aegyptiacus) with Viruses of the Ebolavirus and Marburgvirus Genera

The Egyptian rousette bat (Rousettus aegyptiacus) is a natural reservoir for marburgviruses and a consistent source of virus spillover to humans. Cumulative evidence suggests various bat species may also transmit ebolaviruses. We investigated the susceptibility of Egyptian rousettes to each of the five known ebolaviruses (Sudan, Ebola, Bundibugyo, Taï Forest, and Reston), and compared findings with Marburg virus. In a pilot study, groups of four juvenile bats were inoculated with one of the ebolaviruses or Marburg virus. In ebolavirus groups, viral RNA tissue distribution was limited, and no bat became viremic. Sudan viral RNA was slightly more widespread, spurring a second, 15-day Sudan virus serial euthanasia study. Low levels of Sudan viral RNA disseminated to multiple tissues at early time points, but there was no viremia or shedding. In contrast, Marburg virus RNA was widely disseminated, with viremia, oral and rectal shedding, and antigen in spleen and liver. This is the first experimental infection study comparing tissue tropism, viral shedding, and clinical and pathologic effects of six different filoviruses in the Egyptian rousette, a known marburgvirus reservoir. Our results suggest Egyptian rousettes are unlikely sources for ebolaviruses in nature, and support a possible single filovirus-single reservoir host relationship.

Cell entry by a novel European filovirus requires host endosomal cysteine proteases and Niemann-Pick C1

Lloviu virus (LLOV), a phylogenetically divergent filovirus, is the proposed etiologic agent of die-offs of Schreibers's long-fingered bats (Miniopterus schreibersii) in western Europe. Studies of LLOV remain limited because the infectious agent has not yet been isolated. Here, we generated a recombinant vesicular stomatitis virus expressing the LLOV spike glycoprotein (GP) and used it to show that LLOV GP resembles other filovirus GP proteins in structure and function. LLOV GP must be cleaved by endosomal cysteine proteases during entry, but is much more protease-sensitive than EBOV GP. The EBOV/MARV receptor, Niemann-Pick C1 (NPC1), is also required for LLOV entry, and its second luminal domain is recognized with high affinity by a cleaved form of LLOV GP, suggesting that receptor binding would not impose a barrier to LLOV infection of humans and non-human primates. The use of NPC1 as an intracellular entry receptor may be a universal property of filoviruses.

Filoviruses in bats: current knowledge and future directions

Filoviruses, including Ebolavirus and Marburgvirus, pose significant threats to public health and species conservation by causing hemorrhagic fever outbreaks with high mortality rates. Since the first outbreak in 1967, their origins, natural history, and ecology remained elusive until recent studies linked them through molecular, serological, and virological studies to bats. We review the ecology, epidemiology, and natural history of these systems, drawing on examples from other bat-borne zoonoses, and highlight key areas for future research. We compare and contrast results from ecological and virological studies of bats and filoviruses with those of other systems. We also highlight how advanced methods, such as more recent serological assays, can be interlinked with flexible statistical methods and experimental studies to inform the field studies necessary to understand filovirus persistence in wildlife populations and cross-species transmission leading to outbreaks. We highlight the need for a more unified, global surveillance strategy for filoviruses in wildlife, and advocate for more integrated, multi-disciplinary approaches to understand dynamics in bat populations to ultimately mitigate or prevent potentially devastating disease outbreaks.

Animal models for Ebola and Marburg virus infections

Ebola and Marburg hemorrhagic fevers (EHF and MHF) are caused by the Filoviridae family, Ebolavirus and Marburgvirus (ebolavirus and marburgvirus), respectively. These severe diseases have high mortality rates in humans. Although EHF and MHF are endemic to sub-Saharan Africa. A novel filovirus, Lloviu virus, which is genetically distinct from ebolavirus and marburgvirus, was recently discovered in Spain where filoviral hemorrhagic fever had never been reported. The virulence of this virus has not been determined. Ebolavirus and marburgvirus are classified as biosafety level-4 (BSL-4) pathogens and Category A agents, for which the US government requires preparedness in case of bioterrorism. Therefore, preventive measures against these viral hemorrhagic fevers should be prepared, not only in disease-endemic regions, but also in disease-free countries. Diagnostics, vaccines, and therapeutics need to be developed, and therefore the establishment of animal models for EHF and MHF is invaluable. Several animal models have been developed for EHF and MHF using non-human primates (NHPs) and rodents, which are crucial to understand pathophysiology and to develop diagnostics, vaccines, and therapeutics. Rhesus and cynomolgus macaques are representative models of filovirus infection as they exhibit remarkably similar symptoms to those observed in humans. However, the NHP models have practical and ethical problems that limit their experimental use. Furthermore, there are no inbred and genetically manipulated strains of NHP. Rodent models such as mouse, guinea pig, and hamster, have also been developed. However, these rodent models require adaptation of the virus to produce lethal disease and do not mirror all symptoms of human filovirus infection. This review article provides an outline of the clinical features of EHF and MHF in animals, including humans, and discusses how the animal models have been developed to study pathophysiology, vaccines, and therapeutics.

Reston ebolavirus in humans and animals in the Philippines: a review

The 2008 Reston ebolavirus infection event in domestic pigs has triggered continuing epidemiologic investigations among Philippine health and veterinary agencies in collaboration with international filovirus experts. Prior to this, there were only 3 known and documented Reston ebolavirus outbreaks in nonhuman primates in the world, all traced back to a single geographic source in the Philippines in a monkey breeding/export facility. The first one in 1989 was the first-ever Ebola virus that emerged outside of Africa and was also the first known natural infection of Ebola virus in nonhuman primates. When it was first discovered among laboratory monkeys in the United States, the source was immediately traced back to the farm located in the Philippines. The second outbreak was in 1992-93. The third episode in 1996 was the last known outbreak before Reston ebolavirus reemerged in pigs in 2008. The isolated outbreaks involving 2 animal species bring forth issues requiring further investigations, and highlight the significance of intersectoral collaboration to effectively address zoonoses prevention and control/response in the interest of minimizing public health risk.

Serologic cross-reactivity of human IgM and IgG antibodies to five species of Ebola virus

Five species of Ebola virus (EBOV) have been identified, with nucleotide differences of 30–45% between species. Four of these species have been shown to cause Ebola hemorrhagic fever (EHF) in humans and a fifth species (Reston ebolavirus) is capable of causing a similar disease in non-human primates. While examining potential serologic cross-reactivity between EBOV species is important for diagnostic assays as well as putative vaccines, the nature of cross-reactive antibodies following EBOV infection has not been thoroughly characterized. In order to examine cross-reactivity of human serologic responses to EBOV, we developed antigen preparations for all five EBOV species, and compared serologic responses by IgM capture and IgG enzyme-linked immunosorbent assay (ELISA) in groups of convalescent diagnostic sera from outbreaks in Kikwit, Democratic Republic of Congo (n = 24), Gulu, Uganda (n = 20), Bundibugyo, Uganda (n = 33), and the Philippines (n = 18), which represent outbreaks due to four different EBOV species. For groups of samples from Kikwit, Gulu, and Bundibugyo, some limited IgM cross-reactivity was noted between heterologous sera-antigen pairs, however, IgM responses were largely stronger against autologous antigen. In some instances IgG responses were higher to autologous antigen than heterologous antigen, however, in contrast to IgM responses, we observed strong cross-reactive IgG antibody responses to heterologous antigens among all sets of samples. Finally, we examined autologous IgM and IgG antibody levels, relative to time following EHF onset, and observed early peaking and declining IgM antibody levels (by 80 days) and early development and persistence of IgG antibodies among all samples, implying a consistent pattern of antibody kinetics, regardless of EBOV species. Our findings demonstrate limited cross-reactivity of IgM antibodies to EBOV, however, the stronger tendency for cross-reactive IgG antibody responses can largely circumvent limitations in the utility of heterologous antigen for diagnostic assays and may assist in the development of antibody-mediated vaccines to EBOV.

Ebola virus (EBOV) is a highly pathogenic virus, capable of causing Ebola hemorrhagic fever in humans and non-human primates. Five species of EBOV have been identified. To examine whether infection with one EBOV species results in antibodies that cross-react with other EBOV species, we selected groups of human diagnostic samples from four outbreaks, which were each due to a different EBOV species, and compared IgM and IgG responses by ELISA to each of the five EBOV species. For samples from an individual outbreak, we found limited IgM reactivity to species of EBOV other than the virus species the individual was infected with. In contrast, for all groups of outbreak samples we observed strong cross-reactive IgG antibodies to all EBOV species. Our study demonstrates that IgG antibody responses tend to be more cross-reactive than IgM antibody responses in people infected with EBOV, a finding that has implications for the development of diagnostic assays and vaccines to EBOV.

Aerosol Exposure to the Angola Strain of Marburg Virus Causes Lethal Viral Hemorrhagic Fever in Cynomolgus Macaques

Cynomolgus macaques were exposed to the Angola strain of Lake Victoria Marburg virus (MARV) by aerosol to examine disease course and lethality. Macaques became febrile 4 to 7 days postexposure; the peak febrile response was delayed 1 to 2 days in animals that received a lower dose; viremia coincided with the onset of fever. All 6 macaques succumbed to the infection, with the 3 macaques in the low-dose group becoming moribund on day 9, a day later than the macaques in the high-dose group. Gross pathologic lesions included maculopapular cutaneous rash; pulmonary congestion and edema; pericardial effusion; enlarged, congested, and/or hemorrhagic lymphoid tissues; enlarged friable fatty liver; and pyloric and duodenal congestion and/or hemorrhage. Fibrinous interstitial pneumonia was the most consistent pulmonary change. Lymphocytolysis and lymphoid depletion, as confirmed by TUNEL (terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling), were observed in the mediastinal lymph nodes and spleen. MARV antigen was detected in the lungs, mediastinal lymph nodes, spleen, and liver of all animals examined. In infected macaques, nuclear expression of interleukin-33 was lost in pulmonary arteriolar and mediastinal lymph node high endothelial venule endothelial cells; interleukin-33-positive fibroblastic reticular cells in the mediastinal lymph node were consistently negative for MARV antigen. These macaques exhibited a number of features similar to those of human filovirus infections; as such, this model of aerosolized MARV-Angola might be useful in developing medical countermeasures under the Animal Rule.

Current knowledge on lower virulence of Reston Ebola virus (in French: Connaissances actuelles sur la moindre virulence du virus Ebola Reston)

Ebola viruses (EBOV) and Marburg virus belong to the family Filoviridae, order Mononegavirales. The genus Ebolavirus consists of four species: Zaire ebolavirus (ZEBOV), Sudan ebolavirus (SEBOV), Ivory Coast ebolavirus (ICEBOV) and Reston ebolavirus (REBOV). Three species of ebolaviruses, ZEBOV, SEBOV, ICEBOV, and Marburg virus are known to be extremely pathogenic in primates and humans and cause severe hemorrhagic fever leading up to case fatality rate of some 90%, while REBOV is thought to be pathogenic in Asian monkeys but not in African monkeys and humans. Recent studies indicated several factors involved in different virulence between African EBOV and REBOV. This article reviews the history, epidemiology, and virulence of REBOV.

Potential for Ebola transmission between gorilla and chimpanzee social groups

Over the past decade Ebola hemorrhagic fever has emerged repeatedly in Gabon and Congo, causing numerous human outbreaks and massive die-offs of gorillas and chimpanzees. Why Ebola has emerged so explosively remains poorly understood. Previous studies have tended to focus on exogenous factors such as habitat disturbance and climate change as drivers of Ebola emergence while downplaying the contribution of transmission between gorilla or chimpanzee social groups. Here we report recent observations on behaviors that pose a risk of transmission among gorilla groups and between gorillas and chimpanzees. These observations support a reassessment of ape-to-ape transmission as an amplifier of Ebola outbreaks.

GEOGRAPHIC POTENTIAL FOR OUTBREAKS OF MARBURG HEMORRHAGIC FEVER

Marburg virus represents one of the least well-known of the hemorrhagic fever-causing viruses worldwide; in particular, its geographic potential in Africa remains quite mysterious. Ecologic niche modeling was used to explore the geographic and ecologic potential of Marburg virus in Africa. Model results permitted a reinterpretation of the geographic point of infection in the initiation of the 1975 cases in Zimbabwe, and also anticipated the potential for cases in Angola, where a large outbreak recently (2004-2005) occurred. The geographic potential for additional outbreaks is outlined, including in several countries in which the virus is not known. Overall, results demonstrate that ecologic niche modeling can be a powerful tool in understanding geographic distributions of species and other biologic phenomena such as zoonotic disease transmission from natural reservoir populations.

Marburg and Ebola Viruses as Aerosol Threats

Ebola and Marburg viruses are the sole members of the genus Filovirus in the family Filoviridae. There has been considerable media attention and fear generated by outbreaks of filoviruses because they can cause a severe viral hemorrhagic fever (VHF) syndrome that has a rapid onset and high mortality. Although they are not naturally transmitted by aerosol, they are highly infectious as respirable particles under laboratory conditions. For these and other reasons, filoviruses are classified as category A biological weapons. However, there is very little data from animal studies with aerosolized filoviruses. Animal models of filovirus exposure are not well characterized, and there are discrepancies between these models and what has been observed in human outbreaks. Building on published results from aerosol studies, as well as a review of the history, epidemiology, and disease course of naturally occurring outbreaks, we offer an aerobiologist's perspective on the threat posed by aerosolized filoviruses.