Advances in Rift Valley fever research: insights for disease prevention

Heterogeneity of Rift Valley fever virus transmission potential across livestock hosts, quantified through a model-based analysis of host viral load and vector infection

Quantifying the variation of pathogens’ life history traits in multiple host systems is crucial to understand their transmission dynamics. It is particularly important for arthropod-borne viruses (arboviruses), which are prone to infecting several species of vertebrate hosts. Here, we focus on how host-pathogen interactions determine the ability of host species to transmit a virus to susceptible vectors upon a potentially infectious contact. Rift Valley fever (RVF) is a viral, vector-borne, zoonotic disease, chosen as a case study. The relative contributions of livestock species to RVFV transmission has not been previously quantified. To estimate their potential to transmit the virus over the course of their infection, we 1) fitted a within-host model to viral RNA and infectious virus measures, obtained daily from infected lambs, calves, and young goats, 2) estimated the relationship between vertebrate host infectious titers and probability to infect mosquitoes, and 3) estimated the net infectiousness of each host species over the duration of their infectious periods, taking into account different survival outcomes for lambs. Our results indicate that the efficiency of viral replication, along with the lifespan of infectious particles, could be sources of heterogeneity between hosts. Given available data on RVFV competent vectors, we found that, for similar infectious titers, infection rates in the Aedes genus were on average higher than in the Culex genus. Consequently, for Aedes-mediated infections, we estimated the net infectiousness of lambs to be 2.93 (median) and 3.65 times higher than that of calves and goats, respectively. In lambs, we estimated the overall infectiousness to be 1.93 times higher in individuals which eventually died from the infection than in those recovering. Beyond infectiousness, the relative contributions of host species to transmission depend on local ecological factors, including relative abundances and vector host-feeding preferences. Quantifying these contributions will ultimately help design efficient, targeted, surveillance and vaccination strategies.

Viruses spread by mosquitoes present a major threat to animal and public health worldwide. When these pathogenic viruses can infect multiple species, controlling their spread becomes difficult. Rift Valley fever virus (RVFV) is such a virus. It spreads predominantly among ruminant livestock but can also spill over and cause severe disease in humans. Understanding which of these ruminant species are most important for the transmission of RVFV can help for effective control. One piece of this puzzle is to assess how effective infected animals are at transmitting RVFV to mosquitoes. To answer this question, we combine mathematical models with observations from experimental infections in cattle, sheep, and goats, and model changes in viremia over time within individuals. We then quantify the relationship between hosts’ viremia and the probability to infect mosquitoes. In combining these two analyses, we estimate the overall transmission potential of sheep, when in contact with mosquitoes, to be 3 to 5 times higher than that of goats and cattle. Further, sheep that experience a lethal infection have an even larger overall transmission potential. Once applied at the level of populations, with setting-specific herd composition and exposure to mosquitoes, these results will help unravel species’ role in RVF outbreaks.

Zoonoses and global epidemics

PURPOSE OF REVIEW

The purpose of the review is to summarize recent advances in understanding the origins, drivers and clinical context of zoonotic disease epidemics and pandemics. In addition, we aimed to highlight the role of clinicians in identifying sentinel cases of zoonotic disease outbreaks.

RECENT FINDINGS

The majority of emerging infectious disease events over recent decades, including the COVID-19 pandemic, have been caused by zoonotic viruses and bacteria. In particular, coronaviruses, haemorrhagic fever viruses, arboviruses and influenza A viruses have caused significant epidemics globally. There have been recent advances in understanding the origins and drivers of zoonotic epidemics, yet there are gaps in diagnostic capacity and clinical training about zoonoses.

SUMMARY

Identifying the origins of zoonotic pathogens, understanding factors influencing disease transmission and improving the diagnostic capacity of clinicians will be crucial to early detection and prevention of further epidemics of zoonoses.

Seroprevalence and associated risk factors of selected zoonotic viral hemorrhagic fevers in Tanzania

OBJECTIVE

To determine the seroprevalence of selected zoonotic viral hemorrhagic fevers (VHFs) and their associated risk factors in Tanzania.

METHODS

Blood samples were collected from consenting outpatients and community members in eight districts selected from five ecological zones of Tanzania. Serum was harvested and tested for the presence of immunoglobulin G (IgG) and M (IgM) antibodies against Crimean-Congo hemorrhagic fever (CCHF), Ebola virus disease (EVD), Marburg virus disease (MVD), Rift Valley fever (RVF), and yellow fever (YF).

RESULTS

The presence of IgM and IgG antibodies against CCHF, EVD, MVD, RVF, and YF was detected in 64 of 500 samples (12.8%). The prevalences of IgM and IgG antibodies to CCHF, EVD, MVD, RFV, and YF were 2.0%, 3.4%, 1.2%, 4.8%, and 1.4%, respectively. Contact with wild animals (OR = 1.2, CI = 1.3-1.6) and keeping goats (OR = 1.3, CI = 1.5-1.9) were significantly associated with RVF, while contact with bats (OR = 1.2, CI = 1.1-1.5) was associated with MVD.

CONCLUSION

The findings of this study provide evidence of exposure to CCHF, EVD, MVD, RVF, and YF in Tanzania. Since most of these VHFs occurred without apparent clinical forms of the disease, these findings call for the need to strengthen the surveillance system and management of febrile illnesses in Tanzania.

Rift Valley Fever: Important Considerations for Risk Mitigation and Future Outbreaks

Rift Valley fever virus (RVFV) is a zoonotic phlebovirus of the Phenuiviridae family with great opportunity for emergence in previously unaffected regions, despite its current geographical limits. Outbreaks of RVFV often infect humans or domesticated animals, such as livestock, concurrently and occur sporadically, ranging from localized outbreaks in villages to multi-country events that spread rapidly. The true burden of Rift Valley fever (RVF) is not well defined due to underreporting, misdiagnosis caused by the broad spectrum of disease presentation, and minimal access for rapid and accurate laboratory confirmation. Severe symptoms may include hemorrhagic fever, loss of vision, psychological impairment or disturbances, and organ failure. Those living in endemic areas and travelers should be aware of the potential for exposure to ongoing outbreaks or interepidemic transmission, and engage in behaviors to minimize exposure risks, as vaccinations in humans are currently unavailable and animal vaccinations are not used routinely or ubiquitously. The lack of vaccines approved for use in humans is concerning, as RVFV has proven to be highly pathogenic in naïve populations, causing severe disease in a large percent of confirmed cases, which could have considerable impact on human health.

Evaluation of an Indirect Enzyme-Linked Immunosorbent Assay Based on Recombinant Baculovirus-Expressed Rift Valley Fever Virus Nucleoprotein as the Diagnostic Antigen

The increasing risk of Rift Valley fever virus (RVFV) infection as a global veterinary and public health threat demands the development of safe and accurate diagnostic tests. The aim of this study was to assess the suitability of a baculovirus expression system to produce recombinant RVFV nucleoprotein (N) for use as serodiagnostic antigen in an indirect enzyme-linked immunosorbent assay (ELISA). The ability of the recombinant N antigen to detect RVFV antibody responses was evaluated in ELISA format using antisera from sheep and cattle experimentally infected with two genetically distinct wild-type RVFV strains and sera from indigenous sheep and goat populations exposed to natural RVFV field infection in The Gambia. The recombinant N exhibited specific reactivity with the N-specific monoclonal antibody and various hyperimmune serum samples from ruminants. The indirect ELISA detected N-specific antibody responses in animals with 100% sensitivity compared to the plaque reduction neutralization test (6 to 21 days postinfection) and with 97% and 100% specificity in sheep and cattle, respectively. There was a high level of correlation between the indirect N ELISA and the virus neutralization test for sheep sera (R ² = 0.75; 95% confidence interval [CI] = 0.73 to 0.92) and cattle sera (R ² = 0.80; 95% CI = 0.67 to 0.97); in addition, the N-specific ELISA detected RVFV seroprevalence levels of 26.1% and 54.3% in indigenous sheep and goats, respectively, in The Gambia. The high specificity and correlation with the virus neutralization test support the idea of the feasibility of using the recombinant baculovirus-expressed RVFV N-based indirect ELISA to assess RVFV seroprevalence in livestock in areas of endemicity and nonendemicity.

The influence of raw milk exposures on Rift Valley fever virus transmission

Rift Valley fever virus (RVFV) is a zoonotic phlebovirus that can be transmitted to humans or livestock by mosquitoes or through direct contact with contaminated bodily fluids and tissues. Exposure to bodily fluids and tissues varies by types of behaviors engaged for occupational tasks, homestead responsibilities, or use in dietary or therapeutic capacities. While previous studies have included milk exposures in their analyses, their primary focus on livestock exposures has been on animal handling, breeding, and slaughter. We analyzed data from multiple field surveys in Kenya with the aim of associating RVFV infection to raw milk exposures from common animal species. Of those with evidence of prior RVFV infection by serology (n = 267), 77.2% engaged in milking livestock compared to 32.0% for 3,956 co-local seronegative individuals (p < 0.001), and 86.5% of seropositive individuals consumed raw milk compared to 33.4% seronegative individuals (p < 0.001). Individuals who milked and also consumed raw milk had greater odds of RVFV exposure than individuals whose only contact to raw milk was through milking. Increased risks were associated with exposure to milk sourced from cows (p < 0.001), sheep (p < 0.001), and goats (p < 0.001), but not camels (p = 0.98 for consuming, p = 0.21 for milking). Our data suggest that exposure to raw milk may contribute to a significant number of cases of RVFV, especially during outbreaks and in endemic areas, and that some animal species may be associated with a higher risk for RVFV exposure. Livestock trade is regulated to limit RVFV spread from endemic areas, yet further interventions designed to fully understand the risk of RVFV exposure from raw milk are imperative.

Safety and immunogenicity of Rift Valley fever MP-12 and arMP-12ΔNSm21/384 vaccine candidates in goats (Capra aegagrus hircus) from Tanzania

Vaccination of domestic ruminants is considered to be an effective strategy for protecting these animals against Rift Valley fever (RVF), but available vaccines have limitations. Therefore, the aim of this study was to determine the safety and immunogenicity of RVF virus (RVFV) mutagenesis passage 12 (MP-12) and arMP-12ΔNSm21/384 vaccine candidates in goats (Capra aegagrus hircus) in Tanzania. Goats were vaccinated intramuscularly with RVFV MP-12 or arMP-12ΔNSm21/384, and then on Day 87 post-vaccination (PV) all animals were revaccinated using the RVFV MP-12 vaccine candidate. Serum samples were collected from the animals before and after vaccination at various intervals to test for RVFV using a Vero cell culture assay and reverse transcription polymerase chain reaction and for RVFV-neutralising antibody using a plaque reduction neutralisation assay. Serum samples collected before vaccination on Days -14 and 0, and on Days 3, 4 and 5 PV were negative for RVFV and neutralising antibody. All animals remained healthy, and viremia was not detected in any of the animals. Rift Valley fever virus antibody was first detected on Day 5 PV at a 1:10 dilution in five of five animals vaccinated with the MP-12 vaccine and in five of eight animals vaccinated with arMP-12ΔNSm21/384. Titres then increased and were sustained at 1:40 to 1:640 through to Day 87 PV. All animals that were revaccinated on Day 87 PV with MP-12 developed antibody titres ranging from 1:160 to as high as 1:10 240 on Days 14 and 21 PV. Although the antibody titres for goats vaccinated with RVF MP-12 were slightly higher than titres elicited by the arMP-12ΔNSm21/384 vaccine, these findings demonstrated that both vaccines are promising candidates for the prevention of RVF among Tansanian goats.

Participatory survey of Rift Valley fever in nomadic pastoral communities of North-central Nigeria: The associated risk pathways and factors

BACKGROUND

Rift Valley fever (RVF) is an emerging neglected mosquito-borne viral zoonotic disease of domestic animals and humans, with potential for global expansion. The objectives of this study were: to assess perceived relative burden and seasonality of RVF in nomadic cattle herds and validate the burden with sero-prevalence impact; and assess perceived risk factors associated with the disease and risk pathways for RVF virus in nomadic pastoral herds of North-central Nigeria using pastoralists' existing veterinary knowledge.

METHODS

Participatory Epidemiology (PE) survey was conducted in Fulani nomadic pastoral communities domiciled in Niger State between January and December 2015. A cross-sectional sero-prevalence investigation was also carried out in nomadic pastoral cattle herds to validate outcomes of PE. A total of nine nomadic pastoral communities were purposively selected for qualitative impact assessment using Participatory Rural Appraisal tools, while 97 cattle randomly sampled from 15 purposively selected nomadic herds and had their sera analyzed using c-ELISA. Kendall's Coefficient of Concordance W statistics and OpenEpi 2.3.1 were used for statistical analyses.

RESULTS

Mean proportional piles (relative burden) of RVF (Gabi-gabiF) was 8.3%, and nomads agreement on the burden was strong (W = 0.6855) and statistically significant (P<0.001). This was validated by 11.3% (11/97; 95% CI: 6.1-18.9) sero-positivity (quantitative impact). Mean matrix scores of prominent clinical signs associated with RVF were fever (3.1), anorexia (2.1), abortion (4.1), nasal discharge (3.3), neurological disorder (8.4), diarrhoea (3.2), and sudden death (4.4), with strong agreement (W = 0.6687) and statistically significant (p<0.001). Mean proportional piles of pastoralists' perceived risk factors identified to influenced RVF occurrence were: availability of mosquitoes (18 piles, 17.6%), high cattle density (16 piles, 15.9%) and high rainfall (12 piles, 12.2%). Agreement on the risk factors was strong (W = 0.8372) and statistically significant (p<0.01). Mean matrix scores for the Entry pathway of RVF virus into the nomadic pastoral herds were: presence of RVFV infected mosquitoes (tiny biting flies) (7.9), presence of infected cattle in herds (8.4), and contacts of herd with infected wild animals at grazing (10.1). Mean matrix scores for the Spread pathway of RVF virus in herds were bites of infected mosquitoes (5.1), contacts with infected aborted fetuses/fluids (7.8), and contaminated pasture with aborted fetuses/fluids (9.7). Agreement on risk pathways was strong (W = 0.6922) and statistically significant (p<0.03). Key informants scored RVF to occurred more in Damina or late rainy season (5.3), followed by Kaka or early dry season (3.3), with strong agreement (W = 0.8719) and statistically significant (P<0.01). This study highlighted the significant existing knowledge level about RVF contained in nomadic pastoralists.

CONCLUSIONS

The use of PE approach is needful in active surveillance of livestock diseases in pastoral communities domiciled in highly remote areas. RVF surveillance system, control and prevention programmes that take the identified risk factors and pathways into consideration will be beneficial to the livestock industry in Nigeria, and indeed Africa. An 'OneHealth' approach is needed to improve efficiency of RVF research, surveillance, prevention and control systems, so as to assure food security and public health in developing countries.

Environmental limits of Rift Valley fever revealed using ecoepidemiological mechanistic models

Vector-borne diseases (VBDs) of humans and domestic animals are a significant component of the global burden of disease and a key driver of poverty. The transmission cycles of VBDs are often strongly mediated by the ecological requirements of the vectors, resulting in complex transmission dynamics, including intermittent epidemics and an unclear link between environmental conditions and disease persistence. An important broader concern is the extent to which theoretical models are reliable at forecasting VBDs; infection dynamics can be complex, and the resulting systems are highly unstable. Here, we examine these problems in detail using a case study of Rift Valley fever (RVF), a high-burden disease endemic to Africa. We develop an ecoepidemiological, compartmental, mathematical model coupled to the dynamics of ambient temperature and water availability and apply it to a realistic setting using empirical environmental data from Kenya. Importantly, we identify the range of seasonally varying ambient temperatures and water-body availability that leads to either the extinction of mosquito populations and/or RVF (nonpersistent regimens) or the establishment of long-term mosquito populations and consequently, the endemicity of the RVF infection (persistent regimens). Instabilities arise when the range of the environmental variables overlaps with the threshold of persistence. The model captures the intermittent nature of RVF occurrence, which is explained as low-level circulation under the threshold of detection, with intermittent emergence sometimes after long periods. Using the approach developed here opens up the ability to improve predictions of the emergence and behaviors of epidemics of many other important VBDs.

Rift Valley Fever in Egypt and other African countries: Historical review, recent outbreaks and possibility of disease occurrence in Egypt

This article reviews and discusses the historical and recent status of Rift Valley Fever (RVF) in Egypt and the other African countries based on the available and scattered reports. The recent outbreaks in African countries were reviewed and mapped out. Four major epidemics have been recorded in Egypt (1977, 1978, 1993 and 2003). The outbreak resulted in unpredicted human disease with severe clinical manifestations and heavy mortality as well as many abortions and deaths in sheep, goats, cattle, water buffalo and camels. Of the 18 culicine mosquito species that occur in Egypt, Culex pipiens and Cx. antennatus were implicated as vectors of RVF in Egypt based on their natural infection with RVF virus. Aedes caspius was also suspected of disseminating the virus among livestock based on host feeding and vector competence studies. The epidemiological factors related to the introduction and spread of RVF in Egypt are discussed. The study concluded that due to the availability and abundance of the potential vectors, suitability of environmental conditions, continuous importation of livestock's from Sudan, and the close association of susceptible domestic animals with humans, the RVF virus could possibly occur and circulate in Egypt.

Translating Predictions of Zoonotic Viruses for Policymakers

Recent outbreaks of Ebola virus disease and Zika virus disease highlight the need for disseminating accurate predictions of emerging zoonotic viruses to national governments for disease surveillance and response. Although there are published maps for many emerging zoonotic viruses, it is unknown if there is agreement among different models or if they are concordant with national expert opinion. Therefore, we reviewed existing predictions for five high priority emerging zoonotic viruses with national experts in Cameroon to investigate these issues and determine how to make predictions more useful for national policymakers. Predictive maps relied primarily on environmental parameters and species distribution models. Rift Valley fever virus and Crimean-Congo hemorrhagic fever virus predictions differed from national expert opinion, potentially because of local livestock movements. Our findings reveal that involving national experts could elicit additional data to improve predictions of emerging pathogens as well as help repackage predictions for policymakers.

Current Status of Rift Valley Fever Vaccine Development

Rift Valley Fever (RVF) is a mosquito-borne zoonotic disease that presents a substantial threat to human and public health. It is caused by Rift Valley fever phlebovirus (RVFV), which belongs to the genus Phlebovirus and the family Phenuiviridae within the order Bunyavirales. The wide distribution of competent vectors in non-endemic areas coupled with global climate change poses a significant threat of the transboundary spread of RVFV. In the last decade, an improved understanding of the molecular biology of RVFV has facilitated significant progress in the development of novel vaccines, including DIVA (differentiating infected from vaccinated animals) vaccines. Despite these advances, there is no fully licensed vaccine for veterinary or human use available in non-endemic countries, whereas in endemic countries, there is no clear policy or practice of routine/strategic livestock vaccinations as a preventive or mitigating strategy against potential RVF disease outbreaks. The purpose of this review was to provide an update on the status of RVF vaccine development and provide perspectives on the best strategies for disease control. Herein, we argue that the routine or strategic vaccination of livestock could be the best control approach for preventing the outbreak and spread of future disease.

Rift Valley fever trasmission dynamics described by compartmental models

Rift Valley fever (RVF) is one of the most important zoonotic Transboundary Animal Diseases able to cross international borders and cause devastating effect on animal health and food security. Climate changes and the presence of competent vectors in the most of the current RVF-free temperate countries strongly support the inclusion of RVF virus (RVFV) among the most significant emerging viral threats for public and animal health. The transmission of RVFV is driven by complex eco-climatic factors making the epidemiology of RVF infection difficult to study and to understand. Mathematical, statistical and spatial models are often used to explain the mechanisms underlying these biological processes, providing new and effective tools to plan measures for public health protection. In this paper we performed a systematic literature review on RVF published papers with the aim of identifying and describing the most recent papers developing compartmental models for the study of RVFV transmission dynamics.

Rift Valley Fever Virus MP-12 Vaccine Is Fully Attenuated by a Combination of Partial Attenuations in the S, M, and L Segments

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to Africa and characterized by a high rate of abortion in ruminants and hemorrhagic fever, encephalitis, or blindness in humans. RVF is caused by Rift Valley fever virus (RVFV; family Bunyaviridae, genus Phlebovirus), which has a tripartite negative-stranded RNA genome (consisting of the S, M, and L segments). Further spread of RVF into countries where the disease is not endemic may affect the economy and public health, and vaccination is an effective approach to prevent the spread of RVFV. A live-attenuated MP-12 vaccine is one of the best-characterized RVF vaccines for safety and efficacy and is currently conditionally licensed for use for veterinary purposes in the United States. Meanwhile, as of 2015, no other RVF vaccine has been conditionally or fully licensed for use in the United States. The MP-12 strain is derived from wild-type pathogenic strain ZH548, and its genome encodes 23 mutations in the three genome segments. However, the mechanism of MP-12 attenuation remains unknown. We characterized the attenuation of wild-type pathogenic strain ZH501 carrying a mutation(s) of the MP-12 S, M, or L segment in a mouse model. Our results indicated that MP-12 is attenuated by the mutations in the S, M, and L segments, while the mutations in the M and L segments confer stronger attenuation than those in the S segment. We identified a combination of 3 amino acid changes, Y259H (Gn), R1182G (Gc), and R1029K (L), that was sufficient to attenuate ZH501. However, strain MP-12 with reversion mutations at those 3 sites was still highly attenuated. Our results indicate that MP-12 attenuation is supported by a combination of multiple partial attenuation mutations and a single reversion mutation is less likely to cause a reversion to virulence of the MP-12 vaccine.

IMPORTANCE

Rift Valley fever (RVF) is a mosquito-transmitted viral disease that is endemic to Africa and that has the potential to spread into other countries. Vaccination is considered an effective way to prevent the disease, and the only available veterinary RVF vaccine in the United States is a live-attenuated MP-12 vaccine, which is conditionally licensed. Strain MP-12 is different from its parental pathogenic RVFV strain, strain ZH548, because of the presence of 23 mutations. This study determined the role of individual mutations in the attenuation of the MP-12 strain. We found that full attenuation of MP-12 occurs by a combination of multiple mutations. Our findings indicate that a single reversion mutation will less likely cause a major reversion to virulence of the MP-12 vaccine.

The role of the IKK complex in viral infections

The NF‐κB signal transduction pathway is a critical regulator of multiple cellular functions that ultimately shift the balance between cell survival and death. The cascade is activated by many intrinsic and extrinsic stimuli, which is transduced via adaptor proteins to phosphorylate the IκB kinase (IKK) complex, which in turn phosphorylates the inhibitory IκBα protein to undergo proteasomal degradation and sets in motion nuclear events in response to the initial stimulus. Viruses are important modulators of the NF‐κB cascade and have evolved multiple mechanisms to activate or inhibit this pathway in a manner conducive to viral multiplication and establishment of a productive infectious cycle. This is a subject of extensive research by multiple laboratories whereby unraveling the interactions between specific viral components and members of the NF‐κB signal transduction cascade can shed unique perspectives on infection associated pathogenesis and novel therapeutic targets. In this review, we highlight the interactions between components of the IKK complex and multiple RNA and DNA viruses with the emphasis on mechanisms by which the interaction feeds the infection. Understanding these interactions will shed light on the exploitative capabilities of viruses to maintain an environment favorable for a productive infection.

Epidemiologic Investigations into Outbreaks of Rift Valley Fever in Humans, South Africa, 2008-2011

Rift Valley fever continues to pose a notable public health threat to humans.

Rift Valley fever (RVF) is an emerging zoonosis posing a public health threat to humans in Africa. During sporadic RVF outbreaks in 2008–2009 and widespread epidemics in 2010–2011, 302 laboratory-confirmed human infections, including 25 deaths (case-fatality rate, 8%) were identified. Incidence peaked in late summer to early autumn each year, which coincided with incidence rate patterns in livestock. Most case-patients were adults (median age 43 years), men (262; 87%), who worked in farming, animal health or meat-related industries (83%). Most case-patients reported direct contact with animal tissues, blood, or other body fluids before onset of illness (89%); mosquitoes likely played a limited role in transmission of disease to humans. Close partnership with animal health and agriculture sectors allowed early recognition of human cases and appropriate preventive health messaging.

Towards a better understanding of Rift Valley fever epidemiology in the south-west of the Indian Ocean

Rift Valley fever virus (Phlebovirus, Bunyaviridae) is an arbovirus causing intermittent epizootics and sporadic epidemics primarily in East Africa. Infection causes severe and often fatal illness in young sheep, goats and cattle. Domestic animals and humans can be contaminated by close contact with infectious tissues or through mosquito infectious bites. Rift Valley fever virus was historically restricted to sub-Saharan countries. The probability of Rift Valley fever emerging in virgin areas is likely to be increasing. Its geographical range has extended over the past years. As a recent example, autochthonous cases of Rift Valley fever were recorded in 2007-2008 in Mayotte in the Indian Ocean. It has been proposed that a single infected animal that enters a naive country is sufficient to initiate a major outbreak before Rift Valley fever virus would ever be detected. Unless vaccines are available and widely used to limit its expansion, Rift Valley fever will continue to be a critical issue for human and animal health in the region of the Indian Ocean.

Rift Valley fever risk map model and seroprevalence in selected wild ungulates and camels from Kenya

Since the first isolation of Rift Valley fever virus (RVFV) in the 1930s, there have been multiple epizootics and epidemics in animals and humans in sub-Saharan Africa. Prospective climate-based models have recently been developed that flag areas at risk of RVFV transmission in endemic regions based on key environmental indicators that precede Rift Valley fever (RVF) epizootics and epidemics. Although the timing and locations of human case data from the 2006–2007 RVF outbreak in Kenya have been compared to risk zones flagged by the model, seroprevalence of RVF antibodies in wildlife has not yet been analyzed in light of temporal and spatial predictions of RVF activity. Primarily wild ungulate serum samples from periods before, during, and after the 2006–2007 RVF epizootic were analyzed for the presence of RVFV IgM and/or IgG antibody. Results show an increase in RVF seropositivity from samples collected in 2007 (31.8%), compared to antibody prevalence observed from 2000–2006 (3.3%). After the epizootic, average RVF seropositivity diminished to 5% in samples collected from 2008–2009. Overlaying maps of modeled RVF risk assessments with sampling locations indicated positive RVF serology in several species of wild ungulate in or near areas flagged as being at risk for RVF. Our results establish the need to continue and expand sero-surveillance of wildlife species Kenya and elsewhere in the Horn of Africa to further calibrate and improve the RVF risk model, and better understand the dynamics of RVFV transmission.

Legal, technical, and interpretational considerations in the forensic analysis of viruses

The forensic evaluation of viruses presents new challenges to the forensic science community. Although many criminal cases have been adjudicated involving the deliberate transmission of viruses, especially HIV, this review provides a general approach to viral forensics, especially in light of significant biodefense challenges. Newly emerging techniques of nucleic acid sequencing are discussed in a forensic context. Human mitochondrial DNA analysis, wherein mixed profiles are routinely assessed in a forensic context, provides the groundwork for an interpretational approach to the issue of mixed DNA sequences. The importance of phylogenetic classification is discussed as both providing an integrated graphical depiction of the structure of viral nucleic acid variation as well as offering a tool that can be used to assess the relatedness of complex populations of nucleic acids.

Curcumin inhibits Rift Valley fever virus replication in human cells

Rift Valley fever virus (RVFV) is an arbovirus that is classified as a select agent, an emerging infectious virus, and an agricultural pathogen. Understanding RVFV-host interactions is imperative to the design of novel therapeutics. Here, we report that an infection by the MP-12 strain of RVFV induces phosphorylation of the p65 component of the NFκB cascade. We demonstrate that phosphorylation of p65 (serine 536) involves phosphorylation of IκBα and occurs through the classical NFκB cascade. A unique, low molecular weight complex of the IKK-β subunit can be observed in MP-12-infected cells, which we have labeled IKK-β2. The IKK-β2 complex retains kinase activity and phosphorylates an IκBα substrate. Inhibition of the IKK complex using inhibitors impairs viral replication, thus alluding to the requirement of an active IKK complex to the viral life cycle. Curcumin strongly down-regulates levels of extracellular infectious virus. Our data demonstrated that curcumin binds to and inhibits kinase activity of the IKK-β2 complex in infected cells. Curcumin partially exerts its inhibitory influence on RVFV replication by interfering with IKK-β2-mediated phosphorylation of the viral protein NSs and by altering the cell cycle of treated cells. Curcumin also demonstrated efficacy against ZH501, the fully virulent version of RVFV. Curcumin treatment down-regulated viral replication in the liver of infected animals. Our data point to the possibility that RVFV infection may result in the generation of novel versions of host components (such as IKK-β2) that, by virtue of altered protein interaction and function, qualify as unique therapeutic targets.

Pathogenesis

Virus pathogenesis is a complex, variable, and relatively rare state. Like the course of a virus infection, pathogenesis is determined by the balance between host and virus factors. Not all the pathogenic symptoms seen in virus infections are caused directly by the virus but are side effects of the immune response. In the past few decades, molecular genetic analysis has contributed enormously in understanding the virus pathogenesis. In the chapter, three major aspects of virus pathogenesis are considered: direct cell damage resulting from virus replication, damage resulting from immune activation or suppression, and cell transformation caused by viruses. The chapter also discusses the mechanisms of cellular injury, virus-induced immunodeficiency, virus-related diseases, bacteriophages, and cell transformation by viruses, retroviruses, and DNA viruses. The chapter ends with a discussion of new pathogenic viruses that are being discovered all the time, and changes in human activities that have resulted in the emergence of new or previously unrecognized diseases.

The 2007 Rift Valley fever outbreak in Sudan

Rift Valley fever (RVF) is a neglected, emerging, mosquito-borne disease with severe negative impact on human and animal health and economy. RVF is caused by RVF virus (RVFV) affecting humans and a wide range of animals. The virus is transmitted through bites from mosquitoes and exposure to viremic blood, body fluids, or tissues of infected animals. During 2007 a large RVF outbreak occurred in Sudan with a total of 747 confirmed human cases including 230 deaths (case fatality 30.8%); although it has been estimated 75,000 were infected. It was most severe in White Nile, El Gezira, and Sennar states near to the White Nile and the Blue Nile Rivers. Notably, RVF was not demonstrated in livestock until after the human cases appeared and unfortunately, there are no records or reports of the number of affected animals or deaths. Ideally, animals should serve as sentinels to prevent loss of human life, but the situation here was reversed. Animal contact seemed to be the most dominant risk factor followed by animal products and mosquito bites. The Sudan outbreak followed an unusually heavy rainfall in the country with severe flooding and previous studies on RVF in Sudan suggest that RVFV is endemic in parts of Sudan. An RVF outbreak results in human disease, but also large economic loss with an impact beyond the immediate influence on the directly affected agricultural producers. The outbreak emphasizes the need for collaboration between veterinary and health authorities, entomologists, environmental specialists, and biologists, as the best strategy towards the prevention and control of RVF.

Characterization of wild-type and alternate transcription termination signals in the Rift Valley fever virus genome

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease caused by a phlebovirus of the family Bunyaviridae, which affects humans and ruminants in Africa and the Middle East. RFV virus (RVFV) possesses a single-stranded tripartite RNA genome of negative/ambisense polarity. The S segment utilizes the ambisense strategy and codes for two proteins, the N nucleoprotein and the nonstructural NSs protein, in opposite orientations. The two open reading frames (ORFs) are separated by an intergenic region (IGR) highly conserved among strains and containing a motif, 5'-GCUGC-3', present on the genome and antigenome, which was shown previously to play a role in transcription termination (C. G. Albarino, B. H. Bird, and S. T. Nichol, J. Virol. 81:5246-5256, 2007; T. Ikegami, S. Won, C. J. Peters, and S. Makino, J. Virol. 81:8421-8438, 2007). Here, we created recombinant RVFVs with mutations or deletions in the IGR and showed that the substitution of the motif sequence by a series of five A's inactivated transcription termination at the wild-type site but allowed the transcriptase to recognize another site with the consensus sequence present in the opposite ORF. Similar situations were observed for mutants in which the motif was still present in the IGR but located close to the stop codon of the translated ORF, supporting a model in which transcription is coupled to translation and translocating ribosomes abrogate transcription termination. Our data also showed that the signal tolerated some sequence variations, since mutation into 5'-GCAGC-3' was functional, and 5'-GUAGC-3' is likely the signal for the termination of the 3' end of the L mRNA.

Alteration in superoxide dismutase 1 causes oxidative stress and p38 MAPK activation following RVFV infection

Rift Valley fever (RVF) is a zoonotic disease caused by Rift Valley fever virus (RVFV). RVFV is a category A pathogen that belongs to the genus Phlebovirus, family Bunyaviridae. Understanding early host events to an infectious exposure to RVFV will be of significant use in the development of effective therapeutics that not only control pathogen multiplication, but also contribute to cell survival. In this study, we have carried out infections of human cells with a vaccine strain (MP12) and virulent strain (ZH501) of RVFV and determined host responses to viral infection. We demonstrate that the cellular antioxidant enzyme superoxide dismutase 1 (SOD1) displays altered abundances at early time points following exposure to the virus. We show that the enzyme is down regulated in cases of both a virulent (ZH501) and a vaccine strain (MP12) exposure. Our data demonstrates that the down regulation of SOD1 is likely to be due to post transcriptional processes and may be related to up regulation of TNFα following infection. We also provide evidence for extensive oxidative stress in the MP12 infected cells. Concomitantly, there is an increase in the activation of the p38 MAPK stress response, which our earlier published study demonstrated to be an essential cell survival strategy. Our data suggests that the viral anti-apoptotic protein NSm may play a role in the regulation of the cellular p38 MAPK response. Alterations in the host protein SOD1 following RVFV infection appears to be an early event that occurs in multiple cell types. Activation of the cellular stress response p38 MAPK pathway can be observed in all cell types tested. Our data implies that maintaining oxidative homeostasis in the infected cells may play an important role in improving survival of infected cells.

Rift Valley fever epidemiology, surveillance, and control: what have models contributed?

BACKGROUND

Rift Valley fever (RVF) is an emerging vector-borne zoonotic disease that represents a threat to human health, animal health, and livestock production, particularly in Africa. The epidemiology of RVF is not well understood, so that forecasting RVF outbreaks and carrying out efficient and timely control measures remains a challenge. Various epidemiological modeling tools have been used to increase knowledge on RVF epidemiology and to inform disease management policies.

AIM

This narrative review gives an overview of modeling tools used to date to measure or model RVF risk in animals, and presents how they have contributed to increasing our understanding of RVF occurrence or informed RVF surveillance and control strategies.

METHODOLOGY

Systematic literature searches were performed in PubMed and ISI Web of Knowledge. Additional research work was identified from other sources.

RESULTS

Literature was scarce. Research work was highly heterogeneous in methodology, level of complexity, geographic scale of approach, and geographical area of study. Gaps in knowledge and data were frequent, and uncertainty was not always explored. Spatial approaches were the most commonly utilized techniques and have been used at both local and continental scales, the latter leading to the implementation of an early warning system. Three articles using dynamic transmission models explored the potential of RVF endemicity. Risk factor studies identified water-related environmental risk factors associated with RVF occurrence in domestic livestock. Risk assessments identified importation of infected animals, contaminated products, or infected vectors as key risk pathways for the introduction of RVF virus into disease-free areas.

CONCLUSIONS

Enhanced outbreak prediction and control and increased knowledge on RVF epidemiology would benefit from additional field data, continued development, and refinement of modeling techniques for exploring plausible disease transmission mechanisms and the impact of intervention strategies.