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Research Project: Rift Valley Fever Pathogenesis, Epidemiology, and Control Measures

Location: Foreign Arthropod Borne Animal Disease Research

2018 Annual Report

1. Identify factors associated with Bunyaviridae (Rift Valley Fever virus) infections, pathogenesis, and maintenance in arthropod vector and vertebrate animal hosts, including identifying viral molecular determinants of virulence and mechanisms of viral pathogenesis in relevant animal hosts associated with arthropod-transmitted virus, and characterizing host, vector and bunyavirus interactions (molecular and cellular) associated with virus infection. Sub-objective 1A: Create a network based stochastic model that accounts for mosquitoes, cattle and humans to determine the best mitigation strategies in the event of an outbreak. Sub-objective 1B: Develop tools for rapid detection and characterization of emergent viruses. 2. Identify epidemiological and ecological factors affecting the inter-epidemic cycle and disease emergence caused by Bunyaviridae (Rift Valley Fever virus), including developing means to detect and characterize emergent arboviral diseases and use these data to generate models that predict future outbreaks, and developing epidemiological models to identify biotic and abiotic factors that contribute to virus establishment, evolution, inter-epidemic maintenance, transmission and disease emergence. Sub-objective 2A: Develop RVFV “vector-transmitted” infectious models in target ruminant species to facilitate studies of disease pathogenesis, disease transmission and vaccine efficacy. Sub-objective 2B: Identify mammalian host innate and adaptive responses to insect transmitted RVFV.

The potential introduction of Rift Valley fever (RVF) virus (RVFV) is the most significant arthropod-borne animal disease threat to U.S. livestock according to the USDA-APHIS National Veterinary Stockpile (NVS) Steering Committee. A number of challenges exist for the control and prevention of RVF in the areas of disease surveillance, diagnostics, vaccines and vector control. RVFV is the third biological threat agent on the NVS Steering Committee’s priority list for generation and stockpiling of countermeasures for diagnosis, vaccination, and insect control. Understanding the epidemiological factors affecting disease outbreak and the interepizootic maintenance of RVFV is necessary for the development of appropriate countermeasures strategies. This includes the ability to detect and characterize emergent viruses since RVFV is an RNA virus and could evolve to adapt to a new environment. Also, the proposed research will identify determinants of RVFV infection, pathogenesis and maintenance in mammalian and insect vector hosts. Information derived from these studies will also provide a better vaccine evaluation challenge model. Vaccine formulations will be developed to improve immunogenicity, onset of immunity and stability to provide better response to outbreaks and prevent RVFV epizootics. The overall goals of this project are to utilize the unit’s unique multidisciplinary expertise to fill knowledge gaps about the interepidemic cycle of RVFV and provide the tools necessary for detecting, controlling and eradicating RVFV should it be introduced into the U.S.

Progress Report
The Rift Valley fever (RVF) virus reverse genetic system has been established in the laboratory and synthetic viruses representing two wildtype RVF virus strains have been rescued (i.e. viable virus recovered). The pathogenicity in sheep of one of these synthetic strains has been compared to the wild-type strain has been performed. The rate which Rift Valley fever virus exchanges gene segments or reassorts has been investigated in cell-culture and sheep, a target host species. A reverse transcriptase-polymerase chain reaction (RT-PCR) with melt curve analysis assay was developed to distinguish between distinct viral lineages. Plaque purified reassortants that are identified are confirmed by sequence analysis. This study is ongoing. The multiplex pathogen detection and characterization continue to be evaluated including the Fluorescence Microsphere Immunoassay (FMIA) and MassTag multiple pathogen detection systems. A patent application for the MassTag system has been submitted. A manuscript describing the FMIA laboratory evaluation was published. A manuscript describing a field validation stud of the FMIA is in preparation. Large scale production of experimental lots of recombinant RVF viral Np and Gn proteins were produced for use in further development of immunological assays. The assay development has been extended to lateral flow assays for both laboratory confirmatory testing as well as rapid, presumptive field diagnostics. One of the enzyme linked immunosorbent assay (ELISA) systems is in commercial development and the evaluation locally and through international collaborations was compiled and a manuscript submitted. Animal models to better evaluate vaccine control approaches have been developed that demonstrate virus produced in mosquito cells provided more consistent host response to infection. Experimental infections of young lambs and calves using two virulent strains of RVF virus have been conducted. A preliminary vaccine challenge study with and Arthropod-Borne Animal Diseases Research Unit (ABADRU)/Kansas State University subunit vaccine candidate has been conducted with a promising outcome. This vaccine is now in commercial development and the verification of the commercial product’s efficacy was conducted in mice. The first animal studies were conducted in previous project demonstrated strain differences in clinical pathology in an experimental model. Sequence analysis has indicated that quasi-species that is genetic variation amongst the population of viruses within an inoculum contributes to the pathological outcome of infection. Further analysis revealed that the virus population is dependent on the host species. A manuscript describing this analysis is in preparation. Additional studies looking at the quasi-species isolated from experimentally infected mosquitoes is ongoing. To further evaluate insect vector-host-virus interactions and to improve RVF virus challenge models, a Culex tarsalis colony was established and is being used to understand mosquito saliva enhancement of infection by evaluating the host responses to virus and/or mosquito saliva in primary bovine macrophage cells. This has been expanded to include ovine macrophage cells. The virus growth characteristics in the primary cell-lines has been established. This work is ongoing but immunological markers effected by presence of virus and/or mosquito saliva have been identified. The Invasive Mosquito Project, a citizen science based crowd sourced method for mosquito collections, continues to partner community members with local experts. The project is expanding to California and is working with the Centers for Disease Control Centers of Excellence to expand the outreach. Mosquitoes from the project continue to be shipped from around the country.

1. Development of a subunit vaccine for Rift Valley fever (RVF) virus. RVF virus is an important animal and human mosquito-transmitted pathogen in Africa that could be introduced into the United States. ARS researchers and collaborators at Kansas State University in Manhattan, Kansas developed an efficacious, safe to produce and use sub-unit RVF vaccine for livestock. This candidate vaccine has been patented and was licensed for commercial company for development (APHIS license pending). The RVF subunit vaccine is compatible with a differentiating infected from vaccinated animals approach to prevention and/or control of RVF disease outbreaks using new diagnostic tools also developed by this team.