Lrp1 is a host entry factor for Rift Valley fever virus.

Rift Valley fever virus (RVFV) is a zoonotic pathogen with pandemic potential. RVFV entry is mediated by the viral glycoprotein (Gn), but host entry factors remain poorly defined. Our genome-wide CRISPR screen identified low-density lipoprotein receptor-related protein 1 (mouse Lrp1/human LRP1), heat shock protein (Grp94), and receptor-associated protein (RAP) as critical host factors for RVFV infection. RVFV Gn directly binds to specific Lrp1 clusters and is glycosylation independent. Exogenous addition of murine RAP domain 3 (mRAPD3) and anti-Lrp1 antibodies neutralizes RVFV infection in taxonomically diverse cell lines. Mice treated with mRAPD3 and infected with pathogenic RVFV are protected from disease and death. A mutant mRAPD3 that binds Lrp1 weakly failed to protect from RVFV infection. Together, these data support Lrp1 as a host entry factor for RVFV infection and define a new target to limit RVFV infections.

The lipopeptide Pam3CSK4 inhibits Rift Valley fever virus infection and protects from encephalitis.

Rift Valley fever virus (RVFV) is an encephalitic bunyavirus that can infect neurons in the brain. There are no approved therapeutics that can protect from RVFV encephalitis. Innate immunity, the first line of defense against infection, canonically antagonizes viruses through interferon signaling. We found that interferons did not efficiently protect primary cortical neurons from RVFV, unlike other cell types. To identify alternative neuronal antiviral pathways, we screened innate immune ligands and discovered that the TLR2 ligand Pam3CSK4 inhibited RVFV infection, and other bunyaviruses. Mechanistically, we found that Pam3CSK4 blocks viral fusion, independent of TLR2. In a mouse model of RVFV encephalitis, Pam3CSK4 treatment protected animals from infection and mortality. Overall, Pam3CSK4 is a bunyavirus fusion inhibitor active in primary neurons and the brain, representing a new approach toward the development of treatments for encephalitic bunyavirus infections.

Long-term persistence of viral RNA and inflammation in the CNS of macaques exposed to aerosolized Venezuelan equine encephalitis virus.

Venezuelan equine encephalitis virus (VEEV) is a positively-stranded RNA arbovirus of the genus Alphavirus that causes encephalitis in humans. Cynomolgus macaques are a relevant model of the human disease caused by VEEV and are useful in exploring pathogenic mechanisms and the host response to VEEV infection. Macaques were exposed to small-particle aerosols containing virus derived from an infectious clone of VEEV strain INH-9813, a subtype IC strain isolated from a human infection. VEEV-exposed macaques developed a biphasic fever after infection similar to that seen in humans. Maximum temperature deviation correlated with the inhaled dose, but fever duration did not. Neurological signs, suggestive of virus penetration into the central nervous system (CNS), were predominantly seen in the second febrile period. Electroencephalography data indicated a statistically significant decrease in all power bands and circadian index during the second febrile period that returned to normal after fever resolved. Intracranial pressure increased late in the second febrile period. On day 6 post-infection macaques had high levels of MCP-1 and IP-10 chemokines in the CNS, as well as a marked increase of T lymphocytes and activated microglia. More than four weeks after infection, VEEV genomic RNA was found in the brain, cerebrospinal fluid and cervical lymph nodes. Pro-inflammatory cytokines & chemokines, infiltrating leukocytes and pathological changes were seen in the CNS tissues of macaques euthanized at these times. These data are consistent with persistence of virus replication and/or genomic RNA and potentially, inflammatory sequelae in the central nervous system after resolution of acute VEEV disease.

Rift Valley Fever Virus Infects the Posterior Segment of the Eye and Induces Inflammation in a Rat Model of Ocular Disease.

People infected with the mosquito-borne Rift Valley fever virus (RVFV) can suffer from eye-related problems resulting in ongoing vision issues or even permanent blindness. Despite ocular disease being the most frequently reported severe outcome, it is vastly understudied compared to other disease outcomes caused by RVFV. Ocular manifestations of RVFV include blurred vision, uveitis, and retinitis. When an infected individual develops macular or paramacular lesions, there is a 50% chance of permanent vision loss in one or both eyes. The cause of blinding ocular pathology remains unknown in part due to the lack of a tractable animal model. Using 3 relevant exposure routes, both subcutaneous (SC) and aerosol inoculation of Sprague Dawley rats led to RVFV infection of the eye. Surprisingly, direct inoculation of the conjunctiva did not result in successful ocular infection. The posterior segment of the eye, including the optic nerve, choroid, ciliary body, and retina, were all positive for RVFV antigen in SC-infected rats, and live virus was isolated from the eyes. Proinflammatory cytokines and increased leukocyte counts were also found in the eyes of infected rats. Additionally, human ocular cell lines were permissive for Lrp1-dependent RVFV infection. This study experimentally defines viral tropism of RVFV in the posterior segment of the rat eye and characterizes virally-mediated ocular inflammation, providing a foundation for evaluation of vaccines and therapeutics to protect against adverse ocular outcomes. IMPORTANCE Rift Valley fever virus (RVFV) infection leads to eye damage in humans in up to 10% of reported cases. Permanent blindness occurs in 50% of individuals with significant retinal scarring. Despite the prevalence and severity of this outcome, very little is known about the mechanisms of pathogenesis. We addressed this gap by developing a rodent model of ocular disease. Subcutaneous infection of Sprague Dawley rats resulted in infection of the uvea, retina, and optic nerve along with the induction of inflammation within the posterior eye. Infection of human ocular cells induced inflammatory responses and required host entry factors for RVFV infection similar to rodents. This work provides evidence of how RVFV infects the eye, and this information can be applied to help mitigate the devastating outcomes of RVF ocular disease through vaccines or treatments.

Physiological and immunological changes in the brain associated with lethal eastern equine encephalitis virus in macaques.

Aerosol exposure to eastern equine encephalitis virus (EEEV) can trigger a lethal viral encephalitis in cynomolgus macaques which resembles severe human disease. Biomarkers indicative of central nervous system (CNS) infection by the virus and lethal outcome of disease would be useful in evaluating potential medical countermeasures, especially for therapeutic compounds. To meet requirements of the Animal Rule, a better understanding of the pathophysiology of EEEV-mediated disease in cynomolgus macaques is needed. In this study, macaques given a lethal dose of clone-derived EEEV strain V105 developed a fever between 2-3 days post infection (dpi) and succumbed to the disease by 6 dpi. At the peak of the febrile phase, there was a significant increase in the delta electroencephalography (EEG) power band associated with deep sleep as well as a sharp rise in intracranial pressure (ICP). Viremia peaked early after infection and was largely absent by the onset of fever. Granulocytosis and elevated plasma levels of IP-10 were found early after infection. At necropsy, there was a one hundred- to one thousand-fold increase in expression of traumatic brain injury genes (LIF, MMP-9) as well as inflammatory cytokines and chemokines (IFN-γ, IP-10, MCP-1, IL-8, IL-6) in the brain tissues. Phenotypic analysis of leukocytes entering the brain identified cells as primarily lymphoid (T, B, NK cells) with lower levels of infiltrating macrophages and activated microglia. Massive amounts of infectious virus were found in the brains of lethally-infected macaques. While no infectious virus was found in surviving macaques, quantitative PCR did find evidence of viral genomes in the brains of several survivors. These data are consistent with an overwhelming viral infection in the CNS coupled with a tremendous inflammatory response to the infection that may contribute to the disease outcome. Physiological monitoring of EEG and ICP represent novel methods for assessing efficacy of vaccines or therapeutics in the cynomolgus macaque model of EEEV encephalitis.

Rift Valley Fever: a Threat to Pregnant Women Hiding in Plain Sight?

The potential for emerging mosquito-borne viruses to cause fetal infection in pregnant women was overlooked until the Zika fever outbreak several years ago. Rift Valley fever virus (RVFV) is an emerging arbovirus with a long history of fetal infection and death in pregnant livestock. The effect of RVFV infection on pregnant women is not well understood. This Gem examines the effects that this important emerging pathogen has during pregnancy, its potential impact on pregnant women, and the current research efforts designed to understand and mitigate adverse effects of RVFV infection during pregnancy.

SARS-CoV-2 infection of African green monkeys results in mild respiratory disease discernible by PET/CT imaging and shedding of infectious virus from both respiratory and gastrointestinal tracts.

Vaccines are urgently needed to combat the global coronavirus disease 2019 (COVID-19) pandemic, and testing of candidate vaccines in an appropriate non-human primate (NHP) model is a critical step in the process. Infection of African green monkeys (AGM) with a low passage human isolate of SARS-CoV-2 by aerosol or mucosal exposure resulted in mild clinical infection with a transient decrease in lung tidal volume. Imaging with human clinical-grade 18F-fluoro-2-deoxy-D-glucose positron emission tomography (18F-FDG PET) co-registered with computed tomography (CT) revealed pulmonary lesions at 4 days post-infection (dpi) that resolved over time. Infectious virus was shed from both respiratory and gastrointestinal (GI) tracts in all animals in a biphasic manner, first between 2-7 dpi followed by a recrudescence at 14-21 dpi. Viral RNA (vRNA) was found throughout both respiratory and gastrointestinal systems at necropsy with higher levels of vRNA found within the GI tract tissues. All animals seroconverted simultaneously for IgM and IgG, which has also been documented in human COVID-19 cases. Young AGM represent an species to study mild/subclinical COVID-19 disease and with possible insights into live virus shedding. Future vaccine evaluation can be performed in AGM with correlates of efficacy being lung lesions by PET/CT, virus shedding, and tissue viral load.

Development of Rift valley fever encephalitis in rats is mediated by early infection of olfactory epithelium and neuroinvasion across the cribriform plate.

The zoonotic emerging Rift Valley fever virus (RVFV) causes sporadic disease in livestock and humans throughout Africa and the Saudi Arabian peninsula. Infection of people with RVFV can occur through mosquito bite or mucosal exposure during butchering or milking of infected livestock. Disease typically presents as a self-limiting fever; however, in rare cases, hepatitis, encephalitis and ocular disease may occur. Recent studies have illuminated the neuropathogenic mechanisms of RVFV in a rat aerosol infection model. Neurological disease in rats is characterized by breakdown of the blood-brain barrier late in infection, infiltration of leukocytes to the central nervous system (CNS) and massive viral replication in the brain. However, the route of RVFV entry into the CNS after inhalational exposure remains unknown. Here, we visualized the entire nasal olfactory route from snout to brain after RVFV infection using RNA in situ hybridization and immunofluorescence microscopy. We found widespread RVFV-infected cells within the olfactory epithelium, across the cribriform plate, and in the glomerular region of the olfactory bulb within 2 days of infection. These results indicate that the olfactory tract is a major route of infection of the brain after inhalational exposure. A better understanding of potential neuroinvasion pathways can support the design of more effective therapeutic regiments for the treatment of neurological disease caused by RVFV.

Neutrophil and macrophage influx into the central nervous system are inflammatory components of lethal Rift Valley fever encephalitis in rats.

Rift Valley fever virus (RVFV) causes severe disease in livestock concurrent with zoonotic transmission to humans. A subset of people infected with RVFV develop encephalitis, and significant gaps remain in our knowledge of how RVFV causes pathology in the brain. We previously found that, in Lewis rats, subcutaneous inoculation with RVFV resulted in subclinical disease while inhalation of RVFV in a small particle aerosol caused fatal encephalitis. Here, we compared the disease course of RVFV in Lewis rats after each different route of inoculation in order to understand more about pathogenic mechanisms of fatal RVFV encephalitis. In aerosol-infected rats with lethal encephalitis, neutrophils and macrophages were the major cell types infiltrating the CNS, and this was concomitant with microglia activation and extensive cytokine inflammation. Despite this, prevention of neutrophil infiltration into the brain did not ameliorate disease. Unexpectedly, in subcutaneously-inoculated rats with subclinical disease, detectable viral RNA was found in the brain along with T-cell infiltration. This study sheds new light on the pathogenic mechanisms of RVFV encephalitis.

SARS-CoV-2 growth, furin-cleavage-site adaptation and neutralization using serum from acutely infected hospitalized COVID-19 patients.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), emerged at the end of 2019 and by mid-June 2020 the virus had spread to at least 215 countries, caused more than 8 000 000 confirmed infections and over 450 000 deaths, and overwhelmed healthcare systems worldwide. Like severe acute respiratory syndrome coronavirus (SARS-CoV), which emerged in 2002 and caused a similar disease, SARS-CoV-2 is a betacoronavirus. Both viruses use human angiotensin-converting enzyme 2 (hACE2) as a receptor to enter cells. However, the SARS-CoV-2 spike (S) glycoprotein has a novel insertion that generates a putative furin cleavage signal and this has been postulated to expand the host range. Two low-passage (P) strains of SARS-CoV-2 (Wash1 : P4 and Munich : P1) were cultured twice in Vero E6 cells and characterized virologically. Sanger and MinION sequencing demonstrated significant deletions in the furin cleavage signal of Wash1 : P6 and minor variants in the Munich : P3 strain. Cleavage of the S glycoprotein in SARS-CoV-2-infected Vero E6 cell lysates was inefficient even when an intact furin cleavage signal was present. Indirect immunofluorescence demonstrated that the S glycoprotein reached the cell surface. Since the S protein is a major antigenic target for the development of neutralizing antibodies, we investigated the development of neutralizing antibody titres in serial serum samples obtained from COVID-19 human patients. These were comparable regardless of the presence of an intact or deleted furin cleavage signal. These studies illustrate the need to characterize virus stocks meticulously prior to performing either in vitro or in vivo pathogenesis studies.

Peripheral Blood Biomarkers of Disease Outcome in a Monkey Model of Rift Valley Fever Encephalitis.

Rift Valley Fever (RVF) is an emerging arboviral disease of livestock and humans. Although the disease is caused by a mosquito-borne virus, humans are infected through contact with, or inhalation of, virus-laden particles from contaminated animal carcasses. Some individuals infected with RVF virus (RVFV) develop meningoencephalitis, resulting in morbidity and mortality. Little is known about the pathogenic mechanisms that lead to neurologic sequelae, and thus, animal models that represent human disease are needed. African green monkeys (AGM) exposed to aerosols containing RVFV develop a reproducibly lethal neurological disease that resembles human illness. To understand the disease process and identify biomarkers of lethality, two groups of 5 AGM were infected by inhalation with either a lethal or a sublethal dose of RVFV. Divergence between lethal and sublethal infections occurred as early as 2 days postinfection (dpi), at which point CD8+ T cells from lethally infected AGM expressed activated caspase-3 and simultaneously failed to increase levels of major histocompatibility complex (MHC) class II molecules, in contrast to surviving animals. At 4 dpi, lethally infected animals failed to demonstrate proliferation of total CD4+ and CD8+ T cells, in contrast to survivors. These marked changes in peripheral blood cells occur much earlier than more-established indicators of severe RVF disease, such as granulocytosis and fever. In addition, an early proinflammatory (gamma interferon [IFN-γ], interleukin 6 [IL-6], IL-8, monocyte chemoattractant protein 1 [MCP-1]) and antiviral (IFN-α) response was seen in survivors, while very late cytokine expression was found in animals with lethal infections. By characterizing immunological markers of lethal disease, this study furthers our understanding of RVF pathogenesis and will allow the testing of therapeutics and vaccines in the AGM model.IMPORTANCE Rift Valley Fever (RVF) is an important emerging viral disease for which we lack both an effective human vaccine and treatment. Encephalitis and neurological disease resulting from RVF lead to death or significant long-term disability for infected people. African green monkeys (AGM) develop lethal neurological disease when infected with RVF virus by inhalation. Here we report the similarities in disease course between infected AGM and humans. For the first time, we examine the peripheral immune response during the course of infection in AGM and show that there are very early differences in the immune response between animals that survive infection and those that succumb. We conclude that AGM are a novel and suitable monkey model for studying the neuropathogenesis of RVF and for testing vaccines and therapeutics against this emerging viral pathogen.

Vaccine strains of Rift Valley fever virus exhibit attenuation at the maternal-fetal placental interface.

Rift Valley fever virus (RVFV) infection causes abortions in ruminant livestock and is associated with an increased likelihood of miscarriages in women. Using sheep and human placenta explant cultures, we sought to identify tissues at the maternal-fetal interface targeted by RVFV. Sheep villi and fetal membranes were highly permissive to RVFV infection resulting in markedly higher virus titers than human cultures. Sheep cultures were most permissive to wild-type RVFV and ΔNSm infection, while live attenuated RVFV vaccines (LAVs; MP-12, ΔNSs, and ΔNSs/ΔNSm) exhibited reduced replication. The human fetal membrane restricted wild-type and LAV replication, and when infection occurred, it was prominent in the maternal-facing side. Type-I and type-III interferons were induced in human villi exposed to LAVs lacking the NSs protein. This study supports the use of sheep and human placenta explants to understand vertical transmission of RVFV in mammals and whether LAVs are attenuated at the maternal-fetal interface.

No evidence for enhanced disease with human polyclonal SARS-CoV-2 antibody in the ferret model.

Since SARS-CoV-2 emerged in late 2019, it spread from China to the rest of the world. An initial concern was the potential for vaccine- or antibody-dependent enhancement (ADE) of disease as had been reported with other coronaviruses. To evaluate this, we first developed a ferret model by exposing ferrets to SARS-CoV-2 by either mucosal inoculation (intranasal/oral/ocular) or inhalation using a small particle aerosol. Mucosal inoculation caused a mild fever and weight loss that resolved quickly; inoculation via either route resulted in virus shedding detected in the nares, throat, and rectum for 7-10 days post-infection. To evaluate the potential for ADE, we then inoculated groups of ferrets intravenously with 0.1, 0.5, or 1 mg/kg doses of a human polyclonal anti-SARS-CoV-2 IgG from hyper-immunized transchromosomic bovines (SAB-185). Twelve hours later, ferrets were challenged by mucosal inoculation with SARS-CoV-2. We found no significant differences in fever, weight loss, or viral shedding after infection between the three antibody groups or the controls. Signs of pathology in the lungs were noted in infected ferrets but no differences were found between control and antibody groups. The results of this study indicate that healthy, young adult ferrets of both sexes are a suitable model of mild COVID-19 and that low doses of specific IgG in SAB-185 are unlikely to enhance the disease caused by SARS-CoV-2.

A highly potent human neutralizing antibody prevents vertical transmission of Rift Valley fever virus in a rat model.

Rift Valley fever virus (RVFV) is an emerging mosquito-transmitted virus that circulates in livestock and humans in Africa and the Middle East. Outbreaks lead to high rates of miscarriages in domesticated livestock. Women are also at risk of vertical virus transmission and late-term miscarriages. MAb RVFV-268 is a highly potent recombinant neutralizing human monoclonal antibody that targets RVFV. Here we show that mAb RVFV-268 reduces viral replication in rat placenta explant cultures and prevents vertical transmission in a rat model of congenital RVF. Passive transfer of mAb RVFV-268 from mother to fetus occurs as early as 6 h after administration and persists through 24 h. Administering mAb RVFV-268 2 h prior to RVFV challenge or 24 h post-challenge protects the dams and offspring from RVFV infection. These findings support mAb RVFV-268 as a pre- and post-infection treatment to subvert RVFV infection and vertical transmission, thus protecting the mother and offspring.

Lrp1 is essential for lethal Rift Valley fever hepatic disease in mice.

Rift Valley fever virus (RVFV) is an emerging arbovirus found in Africa. While RVFV is pantropic and infects many cells and tissues, viral replication and necrosis within the liver play a critical role in mediating severe disease. The low-density lipoprotein receptor-related protein 1 (Lrp1) is a recently identified host factor for cellular entry and infection by RVFV. The biological significance of Lrp1, including its role in hepatic disease in vivo, however, remains to be determined. Because Lrp1 has a high expression level in hepatocytes, we developed a mouse model in which Lrp1 is specifically deleted in hepatocytes to test how the absence of liver Lrp1 expression affects RVF pathogenesis. Mice lacking Lrp1 expression in hepatocytes showed minimal RVFV replication in the liver, longer time to death, and altered clinical signs toward neurological disease. In contrast, RVFV infection levels in other tissues showed no difference between the two genotypes. Therefore, Lrp1 is essential for RVF hepatic disease in mice.

Detection of infectious influenza virus in cough aerosols generated in a simulated patient examination room.

BACKGROUND: The potential for aerosol transmission of infectious influenza virus (ie, in healthcare facilities) is controversial. We constructed a simulated patient examination room that contained coughing and breathing manikins to determine whether coughed influenza was infectious and assessed the effectiveness of an N95 respirator and surgical mask in blocking transmission. METHODS: National Institute for Occupational Safety and Health aerosol samplers collected size-fractionated aerosols for 60 minutes at the mouth of the breathing manikin, beside the mouth, and at 3 other locations in the room. Total recovered virus was quantitated by quantitative polymerase chain reaction and infectivity was determined by the viral plaque assay and an enhanced infectivity assay. RESULTS: Infectious influenza was recovered in all aerosol fractions (5.0% in >4 µm aerodynamic diameter, 75.5% in 1-4 µm, and 19.5% in <1 µm; n = 5). Tightly sealing a mask to the face blocked entry of 94.5% of total virus and 94.8% of infectious virus (n = 3). A tightly sealed respirator blocked 99.8% of total virus and 99.6% of infectious virus (n = 3). A poorly fitted respirator blocked 64.5% of total virus and 66.5% of infectious virus (n = 3). A mask documented to be loosely fitting by a PortaCount fit tester, to simulate how masks are worn by healthcare workers, blocked entry of 68.5% of total virus and 56.6% of infectious virus (n = 2). CONCLUSIONS: These results support a role for aerosol transmission and represent the first reported laboratory study of the efficacy of masks and respirators in blocking inhalation of influenza in aerosols. The results indicate that a poorly fitted respirator performs no better than a loosely fitting mask.

Congenital Rift Valley fever in Sprague Dawley rats is associated with diffuse infection and pathology of the placenta.

Rift Valley fever (RVF) is a disease of animals and humans associated with abortions in ruminants and late-gestation miscarriages in women. Here, we use a rat model of congenital RVF to identify tropisms, pathologies, and immune responses in the placenta during vertical transmission. Infection of late-gestation pregnant rats resulted in vertical transmission to the placenta and widespread infection throughout the decidua, basal zone, and labyrinth zone. Some pups from infected dams appeared normal while others had gross signs of teratogenicity including death. Histopathological lesions were detected in placenta from pups regardless of teratogenicity, while teratogenic pups had widespread hemorrhage throughout multiple placenta layers. Teratogenic events were associated with significant increases in placental pro-inflammatory cytokines, type I interferons, and chemokines. RVFV displays a high degree of tropism for all placental tissue layers and the degree of hemorrhage and inflammatory mediator production is highest in placenta from pups with adverse outcomes. Given the potential for RVFV to emerge in new locations and the recent evidence of emerging viruses, like Zika and SARS-CoV-2, to undergo vertical transmission, this study provides essential understanding regarding the mechanisms by which RVFV crosses the placenta barrier.

SARS-CoV-2 growth, furin-cleavage-site adaptation and neutralization using serum from acutely infected, hospitalized COVID-19 patients.

SARS-CoV-2, the causative agent of COVID-19, emerged at the end of 2019 and by mid-June 2020, the virus has spread to at least 215 countries, caused more than 8,000,000 confirmed infections and over 450,000 deaths, and overwhelmed healthcare systems worldwide. Like SARS-CoV, which emerged in 2002 and caused a similar disease, SARS-CoV-2 is a betacoronavirus. Both viruses use human angiotensin-converting enzyme 2 (hACE2) as a receptor to enter cells. However, the SARS-CoV-2 spike (S) glycoprotein has a novel insertion that generates a putative furin cleavage signal and this has been postulated to expand the host range. Two low passage (P) strains of SARS-CoV-2 (Wash1: P4 and Munich: P1) were cultured twice in Vero-E6 cells and characterized virologically. Sanger and MinION sequencing demonstrated significant deletions in the furin cleavage signal of Wash1: P6 and minor variants in the Munich: P3 strain. Cleavage of the S glycoprotein in SARS-CoV-2-infected Vero-E6 cell lysates was inefficient even when an intact furin cleavage signal was present. Indirect immunofluorescence demonstrated the S glycoprotein reached the cell surface. Since the S protein is a major antigenic target for the development of neutralizing antibodies we investigated the development of neutralizing antibody titers in serial serum samples obtained from COVID-19 human patients. These were comparable regardless of the presence of an intact or deleted furin cleavage signal. These studies illustrate the need to characterize virus stocks meticulously prior to performing either in vitro or in vivo pathogenesis studies.

Rift Valley fever in animals and humans: Current perspectives.

Rift Valley fever (RVF) is an ecologically complex emerging arboviral disease that causes significant illness in both livestock and people. This review article is designed to assist the reader in understanding the varied aspects of RVF disease in animals and humans. The historical facets of RVF disease, including the evolution of human outbreaks, are presented and discussed. The different clinical presentations of human RVF disease and the underlying causes are then addressed. We explore the exposure and transmission potential of RVF in animals and people. In the concluding section, we discuss the historical role of RVF as a biological weapon. We conclude with an outline of the important unanswered questions for ongoing research into this important zoonotic disease.

Rift Valley fever virus induces fetal demise in Sprague-Dawley rats through direct placental infection.

Rift Valley fever virus (RVFV) infections in pregnant livestock cause high rates of fetal demise; miscarriage in pregnant women has also been associated with RVFV infection. To address how RVFV infection during pregnancy causes detrimental effects on the fetus, we developed a pregnant rodent model of RVFV infection. We found that pregnant rats were more susceptible to RVFV-induced death than their nonpregnant counterparts and that RVFV infection resulted in intrauterine fetal death and severe congenital abnormalities, even in pups from infected asymptomatic pregnant rats. Virus distribution in infected dams was widespread, with a previously unrecognized preference for infection, replication, and tissue damage in the placenta. In human mid-gestation placental tissue, RVFV directly infected placental chorionic villi, with replication detected in the outermost syncytial layer. Our work identifies direct placental infection by RVFV as a mechanism for vertical transmission. This is the first study to show vertical transmission of RVFV with a lethal outcome in a species other than livestock. This study highlights the potential impact of a future epidemic of this emerging mosquito-borne virus.