Location: Foreign Arthropod Borne Animal Disease Research
2019 Annual Report
Objectives
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.
Approach
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
Rift Valley fever (RVF) virus (RVFV) is an exotic zoonotic pathogen the poses a significant arthropod-borne animal disease threat to U.S. livestock if introduced. The previous project began to address a number of challenges for the detection (surveillance), control and prevention of RVF. The current project focuses on identifying factor associated with infections, pathogenesis, and epidemiological maintenance in the arthropod vector and vertebrate animal hosts (Objective 1). An important aspect to RVFV evolution is that the genome consists of three segments of single stranded RNA. These RNA segments can be exchanged between different genetic types of RVFV and potential very closely related viruses. The rate this can occur is especially important to know especially when an attenuated live vaccine is being deployed. Currently only conditionally approved vaccine in the U.S. is an attenuated live vaccine. Tools to evaluate RVF reassortment have been developed. The rate of reassortment from two distinct virulent strains and the attenuate live vaccine and virulent strain have been accessed in sheep.
To further evaluate insect vector-host-virus interactions and to improve RVFV challenge models, a Culex tarsalis colony 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. The virus growth characteristics of both the attenuated vaccine strain and a virulent strain of RVFV in this primary cell-lines has been established. Work to quantitate effects of presence of virus and/or mosquito saliva on specific immunological markers is nearly complete.
Another way to access factors affecting infection and pathogenesis is development and understanding the mechanism of antiviral treatments for RVFV infection. There are currently no antiviral treatments for RVFV; however, we have identified two potential candidates. Work is ongoing to understand the mechanisms of the candidate antiviral effects and to identify other potential small molecules RVFV antivirals.
The second objective of this project is to identify epidemiological and ecological factors affecting the inter-epidemic cycle and disease emergence. This includes developing the means to detect and characterize emergent arboviruses. Two assays to detect antibodies to RVFV developed in the previous project were further validated in the laboratory and the field. Additionally, new pathological tools were developed to assess presence of RVF viral genome or proteins in tissues.
This objective also includes development of epidemiological models. This is being done by an 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.
Accomplishments
1. Rift Valley fever virus genetic reassortment. ARS researchers and collaborators at Kansas State University in Manhattan, Kansas, developed methods to evaluate Rift Valley fever virus genetic reassortment. The rate which Rift Valley fever virus (RVFV) 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. An additional method was developed to detect low copy genetic targets. A methods development paper and complete analysis of this work is near completion. Genetic reassortment can have significant impact on viral evolution affecting virulence and the ability of the virus to spread. A review paper describing the importance of understanding genetic reassortment in RVFV epidemiology has been published.
2. Improved Rift Valley fever diagnostics. ARS researchers and collaborators at Kansas State University in Manhattan, Kansas, improved Rift Valley fever diagnostics. Development and evaluation of improved diagnostic tools for Rift Valley fever has continued. The multiplex pathogen detection using the Fluorescence Microsphere Immunoassay (FMIA) was evaluated in the RVFV endemic country, Kenya. A manuscript demonstrating that the assays was useful for diagnostic or surveillance was published. The RVF competitive Enzyme Linked Immunosorbent Assay (cELSIA) developed through a three-way collaboration (ARS-Manhattan, Kansas, Kansas State University, and Texas A&M) was packaged commercially and manuscripts demonstrating that the assay reliable sensitivity and specificity were published. These assays allow safe generation of serological assays for antibodies to RVFV that can be lethal to humans in an non-endemic country such as the U.S. and tests that could provide early warning of an introduction.
3. Rift Valley fever pathological assessment tools. Rift Valley fever pathological assessment tools were developed by ARS researchers and collaborators at Kansas State University in Manhattan, Kansas. Rift Valley fever virus (RVFV) is a pathogen endemic to sub-Saharan Africa and the Arabian Peninsula transmitted by mosquitoes. There are concerns this virus could be introduced to the U.S. or other non-endemic countries. Although there are international recommendations for diagnosis pathological tools have been limited. Improvements in pathological tools for RVFV have been made that include the ability to distinguish between infected and vaccinated animals. This is important due to concerns by farmers of potential ill effects of vaccination with attenuated vaccines. A manuscript describing a test to detect RVF viral RNA in fixed tissues was published and a one describing detect of RVF viral proteins has been submitted. Also, a comprehensive review was prepared to assist establishment of procedures to propagate, quantitate and store the virus.
4. Discovery of potential antivirals for Rift Valley fever. Discovery of potential antivirals for Rift Valley fever was accomplished by ARS researchers and collaborators at Kansas State University, in Manhattan, Kansas. There currently are no antiviral treatments for Rift Valley fever virus (RVFV) infection. Potential antiviral candidates that were effective against RVFV were identified by screening a National Institutes of Health panel of potential drugs. This research is paving the way to providing more than just palliative care for RVFV infected animals and humans.
Review Publications
Gaudreault, N., Kim, I., Indran, S., Wilson, W.C., Richt, J. 2018. Molecular aspects of Rift Valley fever virus and the emergence of reassortants. Virus Genes. https://doi.org/10.1007/s11262-018-1611-y.
Lindahl, J., Ragan, I., Rowland, R., Wainaina, M., Mbotha, D., Wilson, W.C. 2019. A multiplex fluorescent microsphere immunoassay for increased understanding of Rift Valley fever epidemiology in ruminants in Kenya. PLOS Neglected Tropical Diseases. https://doi.org/10.1016/j.jviromet.2019.04.011.
Lang, Y., Li, Y., Jasperson, D.C., Henningson, J., Lee, J., Ma, J., Li, Y., Duff, M., Liu, H., Bai, D., McVey, D.S., Richt, J., Ikegami, T., Wilson, W.C., Ma, W. 2019. Identification and evaluation of antivirals for Rift Valley fever virus. Veterinary Microbiology. 230:110-116. https://doi.org/10.1016/j.vetmic.2019.01.027.
