Development of a murine model for aerosolized ebolavirus infection using a panel of recombinant inbred mice

Interferon- and infectious diseases: Lessons and prospects

Infectious diseases continue to claim many lives. Prevention of morbidity and mortality from these diseases would benefit not just from new medicines and vaccines but also from a better understanding of what constitutes protective immunity. Among the major immune signals that mobilize host defense against infection is interferon-γ (IFN-γ), a protein secreted by lymphocytes. Forty years ago, IFN-γ was identified as a macrophage-activating factor, and, in recent years, there has been a resurgent interest in IFN-γ biology and its role in human defense. Here we assess the current understanding of IFN-γ, revisit its designation as an "interferon," and weigh its prospects as a therapeutic against globally pervasive microbial pathogens.

Filoviruses: Scientific Gaps and Prototype Pathogen Recommendation

Viruses in the family Filoviridae, including the commonly known Ebola (EBOV) and Marburg (MARV) viruses, can cause severe hemorrhagic fever in humans and nonhuman primates. Sporadic outbreaks of filovirus disease occur in sub-Saharan Africa with reported case fatality rates ranging from 25% to 90%. The high mortality and increasing frequency and magnitude of recent outbreaks along with the increased potential for spread from rural to urban areas highlight the importance of pandemic preparedness for these viruses. Despite their designation as high-priority pathogens, numerous scientific gaps exist in critical areas. In this review, these gaps and an assessment of potential prototype pathogen candidates are presented for this important virus family.

History and impact of the mouse-adapted Ebola virus model

Ebola virus (EBOV) is a member of the filoviridae family, which are comprised of negative sense, enveloped RNA hemorrhagic fever viruses that can cause severe disease and high lethality rates. These viruses require BSL-4 containment laboratories for study. Early studies of EBOV pathogenesis relied heavily on the use of nonhuman primates, which are expensive and cumbersome to handle in large numbers. Guinea pig models were also developed, but even to this day limited reagents are available in this model. In 1998, Mike Bray and colleagues developed a mouse-adapted EBOV (maEBOV) that caused lethality in adult immunocompetent mice. This model had significant advantages, including being inexpensive, allowing for higher animal numbers for statistical analysis, availability of reagents for studying pathogenesis, and availability of a vast array of genetically modified strains. The model has been used to test vaccines, therapeutic drugs, EBOV mutants, and pathogenesis, and its importance is demonstrated by the hundreds of citations referencing the original publication. This review will cover the history of the maEBOV model and its use in filovirus research.

STAT-1 Knockout Mice as a Model for Wild-Type Sudan Virus (SUDV)

Currently there is no FDA-licensed vaccine or therapeutic against Sudan ebolavirus (SUDV) infections. The largest ever reported 2014–2016 West Africa outbreak, as well as the 2021 outbreak in the Democratic Republic of Congo, highlight the critical need for countermeasures against filovirus infections. A well-characterized small animal model that is susceptible to wild-type filoviruses would greatly add to the screening of antivirals and vaccines. Here, we infected signal transducer and activator of transcription-1 knock out (STAT-1 KO) mice with five different wildtype filoviruses to determine susceptibility. SUDV and Marburg virus (MARV) were the most virulent, and caused 100% or 80% lethality, respectively. Zaire ebolavirus (EBOV), Bundibugyo ebolavirus (BDBV), and Taï Forest ebolavirus (TAFV) caused 40%, 20%, and no mortality, respectively. Further characterization of SUDV in STAT-1 KO mice demonstrated lethality down to 3.1 × 10¹ pfu. Viral genomic material was detectable in serum as early as 1 to 2 days post-challenge. The onset of viremia was closely followed by significant changes in total white blood cells and proportion of neutrophils and lymphocytes, as well as by an influx of neutrophils in the liver and spleen. Concomitant significant fluctuations in blood glucose, albumin, globulin, and alanine aminotransferase were also noted, altogether consistent with other models of filovirus infection. Finally, favipiravir treatment fully protected STAT-1 KO mice from lethal SUDV challenge, suggesting that this may be an appropriate small animal model to screen anti-SUDV countermeasures.

The Impact of Age and Sex on Mouse Models of Melioidosis

Mouse models have been used to generate critical data for many infectious diseases. In the case of Burkholderia pseudomallei, mouse models have been invaluable for bacterial pathogenesis studies as well as for testing novel medical countermeasures including both vaccines and therapeutics. Mouse models of melioidosis have also provided a possible way forward to better understand the chronicity associated with this infection, as it appears that BALB/c mice develop an acute infection with B. pseudomallei, whereas the C57BL/6 model is potentially more suggestive of a chronic infection. Several unanswered questions, however, persist around this model. In particular, little attention has been paid to the effect of age or sex on the disease outcome in these animal models. In this report, we determined the LD₅₀ of the B. pseudomallei K96243 strain in both female and male BALB/c and C57BL/6 mice in three distinct age groups. Our data demonstrated a modest increase in susceptibility associated with sex in this model, and we documented important histopathological differences associated with the reproductive systems of each sex. There was a statistically significant inverse correlation between age and susceptibility. The older mice, in most cases, were more susceptible to the infection. Additionally, our retrospective analyses suggested that the impact of animal supplier on disease outcome in mice may be minimal. These observations were consistent regardless of whether the mice were injected with bacteria intraperitoneally or if they were exposed to aerosolized bacteria. All of these factors should be considered when designing experiments using mouse models of melioidosis.

The use of mice lacking type I or both type I and type II interferon responses in research on hemorrhagic fever viruses. Part 2: Vaccine efficacy studies

For more than 20 years, researchers have used laboratory mice lacking type I or both type I and II interferon (IFN) responses to study high-containment viruses that cause hemorrhagic fevers (HF) in humans. With the exception of Rift Valley fever virus, agents that cause viral HF in humans, such as Ebola and Lassa virus, do not cause disease in mature immunocompetent mice. In contrast, IFN-deficient mice typically develop severe or fatal disease when inoculated with these agents. The sensitivity of IFN-deficient mice to disease has led to their widespread use in biocontainment laboratories to assess the efficacy of novel vaccines against HF viruses, often without considering whether adaptive immune responses in IFN-deficient mice accurately mirror those in immunocompetent humans. Failure to recognize these questions may lead to inappropriate expectations of the predictive value of mouse experiments. In two invited articles, we investigate these questions. The present article reviews the use of IFN-deficient mice for assessing novel vaccines against HF viruses, including Ebola, Lassa, Crimean-Congo hemorrhagic fever and Rift Valley fever viruses. A companion paper examines the general question of how the lack of IFN signaling may affect adaptive immune responses and the outcome of vaccine studies in mice.

Tuning Subunit Vaccines with Novel TLR Triagonist Adjuvants to Generate Protective Immune Responses against Coxiella burnetii

Coxiella burnetii is an obligate intracellular bacterium and the causative agent of Q fever. C. burnetii is considered a potential bioterrorism agent because of its low infectious dose; resistance to heat, drying, and common disinfectants; and lack of prophylactic therapies. Q-Vax, a formalin-inactivated whole-bacteria vaccine, is currently the only prophylactic measure that is protective against C. burnetii infections but is not U.S. Food and Drug Administration approved. To overcome the safety concerns associated with the whole-bacteria vaccine, we sought to generate and evaluate recombinant protein subunit vaccines against C. burnetii To accomplish this, we formulated C. burnetii Ags with a novel TLR triagonist adjuvant platform, which used combinatorial chemistry to link three different TLR agonists together to form one adjuvanting complex. We evaluated the immunomodulatory activity of a panel of TLR triagonist adjuvants and found that they elicited unique Ag-specific immune responses both in vitro and in vivo. We evaluated our top candidates in a live C. burnetii aerosol challenge model in C56BL/6 mice and found that several of our novel vaccine formulations conferred varying levels of protection to the challenged animals compared with sham immunized mice, although none of our candidates were as protective as the commercial vaccine across all protection criteria that were analyzed. Our findings characterize a novel adjuvant platform and offer an alternative approach to generating protective and effective vaccines against C. burnetii.

Filovirus Strategies to Escape Antiviral Responses

This chapter describes the various strategies filoviruses use to escape host immune responses with a focus on innate immune and cell death pathways. Since filovirus replication can be efficiently blocked by interferon (IFN), filoviruses have evolved mechanisms to counteract both type I IFN induction and IFN response signaling pathways. Intriguingly, marburg- and ebolaviruses use different strategies to inhibit IFN signaling. This chapter also summarizes what is known about the role of IFN-stimulated genes (ISGs) in filovirus infection. These fall into three categories: those that restrict filovirus replication, those whose activation is inhibited by filoviruses, and those that have no measurable effect on viral replication. In addition to innate immunity, mammalian cells have evolved strategies to counter viral infections, including the induction of cell death and stress response pathways, and we summarize our current knowledge of how filoviruses interact with these pathways. Finally, this chapter delves into the interaction of EBOV with myeloid dendritic cells and macrophages and the associated inflammatory response, which differs dramatically between these cell types when they are infected with EBOV. In summary, we highlight the multifaceted nature of the host-viral interactions during filoviral infections.

Small Animal Models for Studying Filovirus Pathogenesis

Filovirus small animal disease models have so far been developed in laboratory mice, guinea pigs, and hamsters. Since immunocompetent rodents do not exhibit overt signs of disease following infection with wild-type filoviruses isolated from humans, rodent models have been established using adapted viruses produced through sequential passage in rodents. Rodent-adapted viruses target the same cells/tissues as the wild-type viruses, making rodents invaluable basic research tools for studying filovirus pathogenesis. Moreover, comparative analyses using wild-type and rodent-adapted viruses have provided beneficial insights into the molecular mechanisms of pathogenicity and acquisition of species-specific virulence. Additionally, wild-type filovirus infections in immunodeficient rodents have provided a better understanding of the host factors required for resistance to filovirus infection and of the immune response against the infection. This chapter provides comprehensive information on the filovirus rodent models and rodent-adapted filoviruses. Specifically, we summarize the clinical and pathological features of filovirus infections in all rodent models described to date, including the recently developed humanized and collaborative cross (CC) resource recombinant inbred (RI) intercrossed (CC-RIX) mouse models. We also cover the molecular determinants responsible for adaptation and virulence acquisition in a number of rodent-adapted filoviruses. This chapter clearly defines the characteristic and advantages/disadvantages of rodent models, helping to evaluate the practical use of rodent models in future filovirus studies.

Filovirus Virulence in Interferon α/β and γ Double Knockout Mice, and Treatment with Favipiravir

The 2014 Ebolavirus outbreak in West Africa highlighted the need for vaccines and therapeutics to prevent and treat filovirus infections. A well-characterized small animal model that is susceptible to wild-type filoviruses would facilitate the screening of anti-filovirus agents. To that end, we characterized knockout mice lacking α/β and γ interferon receptors (IFNAGR KO) as a model for wild-type filovirus infection. Intraperitoneal challenge of IFNAGR KO mice with several known human pathogenic species from the genus Ebolavirus and Marburgvirus, except Bundibugyo ebolavirus and Taï Forest ebolavirus, caused variable mortality rate. Further characterization of the prototype Ebola virus Kikwit isolate infection in this KO mouse model showed 100% lethality down to a dilution equivalent to 1.0 × 10⁻¹ pfu with all deaths occurring between 7 and 9 days post-challenge. Viral RNA was detectable in serum after challenge with 1.0 × 10² pfu as early as one day after infection. Changes in hematology and serum chemistry became pronounced as the disease progressed and mirrored the histological changes in the spleen and liver that were also consistent with those described for patients with Ebola virus disease. In a proof-of-principle study, treatment of Ebola virus infected IFNAGR KO mice with favipiravir resulted in 83% protection. Taken together, the data suggest that IFNAGR KO mice may be a useful model for early screening of anti-filovirus medical countermeasures.

Disease progression in mice exposed to low-doses of aerosolized clinical isolates of Burkholderia pseudomallei

Mouse models have been essential to generate supporting data for the research of infectious diseases. Burkholderia pseudomallei, the etiological agent of melioidosis, has been studied using mouse models to investigate pathogenesis and efficacy of novel medical countermeasures to include both vaccines and therapeutics. Previous characterization of mouse models of melioidosis have demonstrated that BALB/c mice present with an acute infection, whereas C57BL/6 mice have shown a tendency to be more resistant to infection and may model chronic disease. In this study, either BALB/c or C57BL/6 mice were exposed to aerosolized human clinical isolates of B. pseudomallei. The bacterial strains included HBPUB10134a (virulent isolate from Thailand), MSHR5855 (virulent isolate from Australia), and 1106a (relatively attenuated isolate from Thailand). The LD₅₀ values were calculated and serial sample collections were performed in order to examine the bacterial burdens in tissues, histopathological features of disease, and the immune response mounted by the mice after exposure to aerosolized B. pseudomallei. These data will be important when utilizing these models for testing novel medical countermeasures. Additionally, by comparing highly virulent strains with attenuated isolates, we hope to better understand the complex disease pathogenesis associated with this bacterium.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): Ebola virus disease

Ebola virus disease has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7 on disease profile and impacts, Article 5 on the eligibility of Ebola virus disease to be listed, Article 9 for the categorisation of Ebola virus disease according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to Ebola virus disease. The assessment has been performed following a methodology composed of information collection and compilation, expert judgement on each criterion at individual and, if no consensus was reached before, also at collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. Details on the methodology used for this assessment are explained in a separate opinion. According to the assessment performed, Ebola virus disease can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL. The disease would comply with the criteria as in Sections 4 and 5 of Annex IV of the AHL, for the application of the disease prevention and control rules referred to in points (d) and (e) of Article 9(1). The animal species to be listed for Ebola virus disease according to Article 8(3) criteria are some species of non-human primates, pigs and rodents as susceptible species and some species of fruit bats as reservoir, as indicated in the present opinion.

Characterization of pathogenesis of and immune response to Burkholderia pseudomallei K96243 using both inhalational and intraperitoneal infection models in BALB/c and C57BL/6 mice

Burkholderia pseudomallei, the etiologic agent of melioidosis, is a Gram negative bacterium designated as a Tier 1 threat. This bacterium is known to be endemic in Southeast Asia and Northern Australia and can infect humans and animals by several routes. Inhalational melioidosis has been associated with monsoonal rains in endemic areas and is also a significant concern in the biodefense community. There are currently no effective vaccines for B. pseudomallei and antibiotic treatment can be hampered by non-specific symptomology and also the high rate of naturally occurring antibiotic resistant strains. Well-characterized animal models will be essential when selecting novel medical countermeasures for evaluation prior to human clinical trials. Here, we further characterize differences between the responses of BALB/c and C57BL/6 mice when challenged with low doses of a low-passage and well-defined stock of B. pseudomallei K96243 via either intraperitoneal or aerosol routes of exposure. Before challenge, mice were implanted with a transponder to collect body temperature readings, and daily body weights were also recorded. Mice were euthanized on select days for pathological analyses and determination of the bacterial burden in selected tissues (blood, lungs, liver, and spleen). Additionally, spleen homogenate and sera samples were analyzed to better characterize the host immune response after infection with aerosolized bacteria. These clinical, pathological, and immunological data highlighted and confirmed important similarities and differences between these murine models and exposure routes.

Assessing Antiviral Countermeasures Using Mouse Models of Ebolavirus Infection

Mouse models of Ebola virus (EBOV) have demonstrated their utility as important tools for screening the efficacy of candidate therapeutics and vaccines. In this chapter we explain the various mouse models that utilize either wild-type or mouse-adapted EBOV variants.

An update on the 2014 Ebola outbreak in Western Africa

The recent Ebola outbreak in Western Africa was the most devastating outbreak witnessed in recent times. There have been remarkable local and international efforts to control the crisis. Ebola Virus Disease is the focus of immense research activity. The progression of events in the region has been evolving swiftly and it is of paramount importance to the medical community to be acquainted with the situation. Over 28000 people were inflicted with the condition, over 11000 have died. Novel data has emerged regarding modes of transmission, providing rationale for recent flare-ups. Similarly, studies on survivors are elucidating the later stages of the disease recovery process. Novel techniques for diagnosis are also discussed. Finally, the current research regarding treatment and vaccine development is reviewed, particularly the implementation of rVSV-ZEBOV vaccination programs.

Ebola Virus Makona Shows Reduced Lethality in an Immune-deficient Mouse Model

Ebola virus Makona (EBOV-Makona; from the 2013-2016 West Africa outbreak) shows decreased virulence in an immune-deficient mouse model, compared with a strain from 1976. Unlike other filoviruses tested, EBOV-Makona may be slightly more virulent by the aerosol route than by the injected route, as 2 mice died following aerosol exposure, compared with no mortality among mice that received intraperitoneal injection of equivalent or higher doses. Although most mice did not succumb to infection, the detection of an immunoglobulin G antibody response along with observed clinical signs suggest that the mice were infected but able to clear the infection and recover. We hypothesize that this may be due to the growth rates and kinetics of the virus, which appear slower than that for other filoviruses and consequently give more time for an immune response that results in clearance of the virus. In this instance, the immune-deficient mouse model is unlikely to be appropriate for testing medical countermeasures against this EBOV-Makona stock but may provide insight into pathogenesis and the immune response to virus.

Genetic Architecture of Group A Streptococcal Necrotizing Soft Tissue Infections in the Mouse

Host genetic variations play an important role in several pathogenic diseases, and we have previously provided strong evidences that these genetic variations contribute significantly to differences in susceptibility and clinical outcomes of invasive Group A Streptococcus (GAS) infections, including sepsis and necrotizing soft tissue infections (NSTIs). Our initial studies with conventional mouse strains revealed that host genetic variations and sex differences play an important role in orchestrating the severity, susceptibility and outcomes of NSTIs. To understand the complex genetic architecture of NSTIs, we utilized an unbiased, forward systems genetics approach in an advanced recombinant inbred (ARI) panel of mouse strains (BXD). Through this approach, we uncovered interactions between host genetics, and other non-genetic cofactors including sex, age and body weight in determining susceptibility to NSTIs. We mapped three NSTIs-associated phenotypic traits (i.e., survival, percent weight change, and lesion size) to underlying host genetic variations by using the WebQTL tool, and identified four NSTIs-associated quantitative genetic loci (QTL) for survival on mouse chromosome (Chr) 2, for weight change on Chr 7, and for lesion size on Chr 6 and 18 respectively. These QTL harbor several polymorphic genes. Identification of multiple QTL highlighted the complexity of the host-pathogen interactions involved in NSTI pathogenesis. We then analyzed and rank-ordered host candidate genes in these QTL by using the QTLminer tool and then developed a list of 375 candidate genes on the basis of annotation data and biological relevance to NSTIs. Further differential expression analyses revealed 125 genes to be significantly differentially regulated in susceptible strains compared to their uninfected controls. Several of these genes are involved in innate immunity, inflammatory response, cell growth, development and proliferation, and apoptosis. Additional network analyses using ingenuity pathway analysis (IPA) of these 125 genes revealed interleukin-1 beta network as key network involved in modulating the differential susceptibility to GAS NSTIs.

GAS bacteria are major human pathogens that are responsible for millions of infections worldwide, including severe and deadly NSTIs. Several studies have identified numerous GAS secreted virulence factors including proteases, DNases, and superantigens, which mediate several pathologic features of GAS NSTIs. However, the exact role of host genetic and/or nongenetic factors in GAS NSTIs has not been studied so far. To understand these contributions, we undertook the present study utilizing the ARI panel of BXD strains. We found that host genetic context and sex differences can modulate host-pathogen interplay and accordingly potentiate disease severity, manifestations, and outcomes. We also mapped the genetic susceptibility loci of GAS NSTIs to four mouse chromosomes, namely 2, 6, 7 and 18, harboring several polymorphic genes. We believe that these findings will be helpful in uncovering further regulatory events of host-mediated GAS pathogenesis that may occur once the pathogen becomes invasive.

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.

Mapping of genetic loci that modulate differential colonization by Escherichia coli O157:H7 TUV86-2 in advanced recombinant inbred BXD mice

BACKGROUND

Shiga toxin (Stx)-producing E. coli (STEC) are responsible for foodborne outbreaks that can result in severe human disease. During an outbreak, differential disease outcomes are observed after infection with the same STEC strain. One question of particular interest is why some infected people resolve infection after hemorrhagic colitis whereas others progress to the hemolytic uremic syndrome (HUS). Host age and infection dose have been implicated; however, these parameters do not appear to fully account for all of the observed variation in disease severity. Therefore, we hypothesized that additional host genetic factors may play a role in progression to HUS.

METHODS AND RESULTS

To mimic the genetic diversity in the human response to infection by STEC, we measured the capacity of an O157:H7 outbreak isolate to colonize mouse strains from the advanced recombinant inbred (ARI) BXD panel. We first infected the BXD parental strains C57BL/6 J (B6) and DBA/2 J (D2) with either 86-24 (Stx2a+) or TUV86-2, an Stx2a-negative isogenic mutant. Colonization levels were determined in an intact commensal flora (ICF) infection model. We found a significant difference in colonization levels between the parental B6 and D2 strains after infection with TUV86-2 but not with 86-24. This observation suggested that a host factor that may be masked by Stx2a affects O157:H7 colonization in some genetic backgrounds. We then determined the TUV86-2 colonization levels of 24 BXD strains in the ICF model. We identified several quantitative trait loci (QTL) associated with variation in colonization by correlation analyses. We found a highly significant QTL on proximal chromosome 9 (12.5-26.7 Mb) that strongly predicts variation in colonization levels and accounts for 15-20 % of variance. Linkage, polymorphism and co-citation analyses of the mapped region revealed 36 candidate genes within the QTL, and we identified five genes that are most likely responsible for the differential colonization.

CONCLUSIONS

The identification of the QTL on chromosome 9 supports our hypothesis that individual genetic makeup affects the level of colonization after infection with STEC O157:H7.

Host Genetic Variations and Sex Differences Potentiate Predisposition, Severity, and Outcomes of Group A Streptococcus-Mediated Necrotizing Soft Tissue Infections

Host genetic variations play an important role in several pathogenic diseases, and we previously provided strong evidence that these genetic variations contribute significantly to differences in susceptibility and clinical outcomes of invasive group A Streptococcus (GAS) patients, including sepsis and necrotizing soft tissue infections (NSTIs). The goal of the present study was to investigate how genetic variations and sex differences among four commonly used mouse strains contribute to variation in severity, manifestations, and outcomes of NSTIs. DBA/2J mice were more susceptible to NSTIs than C57BL/6J, BALB/c, and CD-1 mice, as exhibited by significantly greater bacteremia, excessive dissemination to the spleen, and significantly higher mortality. Differences in the sex of the mice also contributed to differences in disease severity and outcomes: DBA/2J female mice were relatively resistant compared to their male counterparts. However, DBA/2J mice exhibited minimal weight loss and developed smaller lesions than did the aforementioned strains. Moreover, at 48 h after infection, compared with C57BL/6J mice, DBA/2J mice had increased bacteremia, excessive dissemination to the spleen, and excessive concentrations of inflammatory cytokines and chemokines. These results indicate that variations in the host genetic context as well as sex play a dominant role in determining the severity of and susceptibility to GAS NSTIs.

Is there a way out for the 2014 Ebola outbreak in Western Africa?

The 2014 Ebola outbreak in West Africa, primarily affecting Guinea, Sierra Leone, and Liberia, has exceeded all previous Ebola outbreaks in the number of cases and in international response. Although infections only occur frequently in Western Africa, the virus has the potential to spread globally and is classified as a category A pathogen that could be misused as a bioterrorism agent. This review aims (i) to discuss the latest data to aid our current recommendations for the prevention and control of the Ebola virus infection, (ii) to review its pathophysiology as well as offering insights on the most current data available about Ebola vaccine progress and potential use.

Aerosol-Transmitted Infections-a New Consideration for Public Health and Infection Control Teams

Since the emergence of the 2003 severe acute respiratory syndrome (SARS), the 2003 reemergence of avian A/H5N1, the emergence of the 2009 pandemic influenza A/H1N1, the 2012 emergence of Middle East respiratory syndrome (MERS), the 2013 emergence of avian A/H7N9 and the 2014 Ebola virus outbreaks, the potential for the aerosol transmission of infectious agents is now routinely considered in the investigation of any outbreak. Although many organisms have traditionally been considered to be transmitted by only one route (e.g. direct/indirect contact and/or faecal-orally), it is now apparent that the aerosol transmission route is also possible and opportunistic, depending on any potentially aerosol-generating procedures, the severity of illness and the degree and duration of pathogen-shedding in the infected patient, as well as the environment in which these activities are conducted.This article reviews the evidence and characteristics of some of the accepted (tuberculosis, measles, chickenpox, whooping cough) and some of the more opportunistic (influenza, Clostridium difficile, norovirus) aerosol-transmitted infectious agents and outlines methods of detecting and quantifying transmission.

Animal models for ebolavirus countermeasures discovery: what defines a useful model?

INTRODUCTION

Ebolaviruses are highly pathogenic filoviruses, which cause disease in humans and nonhuman primates (NHP) in Africa. The Zaire ebolavirus outbreak in 2014, which continues to greatly affect Western Africa and other countries to which the hemorrhagic fever was exported due to travel of unsymptomatic yet infected individuals, was complicated by the lack of available licensed vaccines or therapeutics to combat infection. After almost a year of research at an increased pace to find and test vaccines and therapeutics, there is now a deeper understanding of the available disease models for ebolavirus infection. Demonstration of vaccine or therapeutic efficacy in NHP models of ebolavirus infection is crucial to the development and eventual licensure of ebolavirus medical countermeasures, so that safe and effective countermeasures can be accelerated into human clinical trials.

AREAS COVERED

The authors describe ebolavirus hemorrhagic fever (EHF) disease in various animal species: mice, guinea pigs, hamsters, pigs and NHP, to include baboons, marmosets, rhesus and cynomolgus macaques, as well as African green monkeys. Because the NHP models are supremely useful for therapeutics and vaccine testing, emphasis is placed on comparison of these models, and their use as gold-standard models of EHF.

EXPERT OPINION

Animal models of EHF varying from rodents to NHP species are currently under evaluation for their reproducibility and utility for modeling infection in humans. Complete development and licensure of therapeutic agents and vaccines will require demonstration that mechanisms conferring protection in NHP models of infection are predictive of protective responses in humans, for a given countermeasure.

Viral Respiratory Infections of Adults in the Intensive Care Unit

Viral lower respiratory tract infections (LRTIs) are an underappreciated cause of critical illness in adults. Recent advances in viral detection techniques over the past decade have demonstrated viral LRTIs are associated with rates of morbidity, mortality, and health care utilization comparable to those of seen with bacterial community acquired and nosocomial pneumonias. In this review, we describe the relationship between viral LRTIs and critical illness, as well as discuss relevant clinical features and management strategies for the more prevalent respiratory viral pathogens.

Interferon α/β Receptor-Deficient Mice as a Model for Ebola Virus Disease

A major obstacle in ebolavirus research is the lack of a small-animal model for Sudan virus (SUDV), as well as other wild-type (WT) ebolaviruses. Here, we expand on research by Bray and by Lever et al suggesting that WT ebolaviruses are pathogenic in mice deficient for the type 1 interferon (IFN) α/β receptor (IFNα/βR-/-). We examined the disease course of several WT ebolaviruses: Boneface (SUDV/Bon) and Gulu variants of SUDV, Ebola virus (EBOV), Bundibugyo virus (BDBV), Taï Forest virus, and Reston virus (RESTV). We determined that exposure to WT SUDV or EBOV results in reproducible signs of disease in IFNα/βR-/- mice, as measured by weight loss and partial lethality. Vaccination with the SUDV or EBOV glycoprotein (GP)-expressing Venezuelan equine encephalitis viral replicon particle vaccine protected these mice from SUDV/Bon and EBOV challenge, respectively. Treatment with SUDV- or EBOV-specific anti-GP antibodies protected mice from challenge when delivered 1-3 days after infection. Serial sampling experiments revealed evidence of disseminated intravascular coagulation in the livers of mice infected with the Boneface variant of SUDV, EBOV, and BDBV. Taken together, these data solidify the IFNα/βR-/- mouse as an important and useful model for the study of WT EBOV disease.

Quantitative genetics in the study of virus-induced disease

While the role of viral variants has long been known to play a key role in causing variation in disease severity, it is also clear that host genetic variation plays a critical role in determining virus-induced disease responses. However, a variety of factors, including confounding environmental variables, rare genetic variants requiring extremely large cohorts, the temporal dynamics of infections, and ethical limitation on human studies, have made the identification and dissection of variant host genes and pathways difficult within human populations. This difficulty has led to the development of a variety of experimental approaches used to identify host genetic contributions to disease responses. In this chapter, we describe the history of genetic associations within the human population, the development of experimentally tractable systems, and the insights these specific approaches provide. We conclude with a discussion of recent advances that allow for the investigation of the role of complex genetic networks that underlie host responses to infection, with the goal of drawing connections to human infections. In particular, we highlight the need for robust animal models with which to directly control and assess the role of host genetics on viral infection outcomes.

Experimental Aerosolized Guinea Pig–Adapted Zaire Ebolavirus (Variant

Eight guinea pigs were aerosolized with guinea pig–adapted Zaire ebolavirus (variant: Mayinga) and developed lethal interstitial pneumonia that was distinct from lesions described in guinea pigs challenged subcutaneously, nonhuman primates challenged by the aerosol route, and natural infection in humans. Guinea pigs succumbed with significant pathologic changes primarily restricted to the lungs. Intracytoplasmic inclusion bodies were observed in many alveolar macrophages. Perivasculitis was noted within the lungs. These changes are unlike those of documented subcutaneously challenged guinea pigs and aerosolized filoviral infections in nonhuman primates and human cases. Similar to findings in subcutaneously challenged guinea pigs, there were only mild lesions in the liver and spleen. To our knowledge, this is the first report of aerosol challenge of guinea pigs with guinea pig–adapted Zaire ebolavirus (variant: Mayinga). Before choosing this model for use in aerosolized ebolavirus studies, scientists and pathologists should be aware that aerosolized guinea pig–adapted Zaire ebolavirus (variant: Mayinga) causes lethal pneumonia in guinea pigs.

A forward phenotypically driven unbiased genetic analysis of host genes that moderate herpes simplex virus virulence and stromal keratitis in mice

Both viral and host genetics affect the outcome of herpes simplex virus type 1 (HSV-1) infection in humans and experimental models. Little is known about specific host gene variants and molecular networks that influence herpetic disease progression, severity, and episodic reactivation. To identify such host gene variants we have initiated a forward genetic analysis using the expanded family of BXD strains, all derived from crosses between C57BL/6J and DBA/2J strains of mice. One parent is highly resistant and one highly susceptible to HSV-1. Both strains have also been fully sequenced, greatly facilitating the search for genetic modifiers that contribute to differences in HSV-1 infection. We monitored diverse disease phenotypes following infection with HSV-1 strain 17syn+ including percent mortality (herpes simplex encephalitis, HSE), body weight loss, severity of herpetic stromal keratitis (HSK), spleen weight, serum neutralizing antibody titers, and viral titers in tear films in BXD strains. A significant quantitative trait locus (QTL) on chromosome (Chr) 16 was found to associate with both percent mortality and HSK severity. Importantly, this QTL maps close to a human QTL and the gene proposed to be associated with the frequency of recurrent herpetic labialis (cold sores). This suggests that a single host locus may influence these seemingly diverse HSV-1 pathogenic phenotypes by as yet unknown mechanisms. Additional suggestive QTLs for percent mortality were identified—one on Chr X that is epistatically associated with that on Chr 16. As would be anticipated the Chr 16 QTL also modulated weight loss, reaching significance in females. A second significant QTL for maximum weight loss in male and female mice was mapped to Chr 12. To our knowledge this is the first report of a host genetic locus that modulates the severity of both herpetic disease in the nervous system and herpetic stromal keratitis.

Animal Models of Human Viral Diseases

As the threat of exposure to emerging and reemerging viruses within a naive population increases, it is vital that the basic mechanisms of pathogenesis and immune response be thoroughly investigated. By using animal models in this endeavor, the response to viruses can be studied in a more natural context to identify novel drug targets, and assess the efficacy and safety of new products. This is especially true in the advent of the Food and Drug Administration's animal rule. Although no one animal model is able to recapitulate all the aspects of human disease, understanding the current limitations allows for a more targeted experimental design. Important facets to be considered before an animal study are the route of challenge, species of animals, biomarkers of disease, and a humane endpoint. This chapter covers the current animal models for medically important human viruses, and demonstrates where the gaps in knowledge exist.