Page Banner

United States Department of Agriculture

Agricultural Research Service

Research Project: Countermeasures to Prevent, Mitigate, and Control Rift Valley Fever (Rvf)

Location: Arthropod-Borne Animal Diseases Research

2013 Annual Report


1a.Objectives (from AD-416):
1. Identify epidemiological factors affecting disease outbreak and inter-epizootic maintenance of RVFV.

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 factors associated with RVFV infection, pathogenesis and maintenance.

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.

3. Develop and validate vaccine strategies for preventing RVFV epizootics.

Sub-Objective 3.A. Develop needle-free vaccine platforms that reduce the time to effective onset of immunity.

Sub-Objective 3.B. Develop vaccine platforms that do not require refrigeration for extended periods of time.


1b.Approach (from AD-416):
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 inter-epizootic 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.


3.Progress Report:
In Objective 1A, an improved RVF network based stochastic model for the United States was developed that contains the migration rates, estimated abundance and a mortality data. The model accounts for two species of mosquitoes (Aedes vexans and Culex tarsalis), cattle, humans, and pathogen transmission along a contact network model providing insight into the potential epidemiology should the virus be introduced.

To provide additional information related to RVF models, the potential susceptibility of North American wildlife has been accessed using RVF virus MP-12 vaccine strain infection of wildlife cell cultures as a model have suggested that North American wildlife including native deer species could be epidemiologically important. This finding is being further verified using virulent RVFV infection studies of representative cell-lines.

The tools necessary to collect epidemiological data are being further developed in Objective 1B, including development of expressed and purified RVF viral antigens and serological diagnostic reagents. An external RNA control was added to the multiple RVFV gene targets genetic assay and field-tested in South Africa. New technologies for multiple pathogen detection and characterization have been explored and funding was acquired for this modern and expensive technology. Significant progress has been made in the development of a fluorescent microsphere based assay for RVF.

To develop better animal models under Objective 2A, a commercially available intradermal injection system was evaluated. The system is not acceptable for a zoonotic pathogen delivery. An alternative method has been identified but not yet tested. A Culex tarsalis colony was established and we continue to optimize rearing conditions to facilitate the research in objective 2B directed toward understanding mosquito saliva enhancement of infection. In addition, procedures have been adapted to assess the host responses to virus and/or mosquito saliva in primary bovine macrophage cells.

The information gained in Objective 2 will be used to enhance objective 3 related to developing better countermeasures. Under Objective 3A, ABADRU has demonstrated the ability to perform large animal research at the high biosecurity conditions required. Under Objective 3 B, preliminary evaluations of the stability of MP-12 in formulations of the vaccine with various adjuvants and stabilizers have been conducted and further work is in progress. ABADRU continues to address concerns of CDC inspectors concerning large animal research with virulent RVFV but has received authorization for laboratory and small animal research. Laboratory research with virulent RVFV has been initiated. Only two of the ABADRU staff members have received the RVF investigational vaccination thus are authorized by ARS to work in the RVF laboratory. Up to four new staff members will submit applications to be enrolled in the recently reinitiated US Army Medical Research Institute for Infectious Diseases special immunization program.

These accomplishments align with the Bio-defense research and the control of zoonotic diseases components of the NP-103 Animal Health Action Plan.


4.Accomplishments
1. A Rift Valley fever risk map model was compared to antibody prevalence in wildlife and camels from Kenya. Rift Valley Fever (RVF) is an acute viral disease of animal and humans that is endemic in Africa and Arabian Peninsula. The mosquito-transmitted Rift Valley fever virus (RVFV) causes the disease. Previous studies have demonstrated that wild animals may play central roles in RVF outbreaks. ARS researchers in Manhattan, KS and collaborators examined temporal and spatial change patterns in RVF serological prevalence in Kenyan wildlife. This data was used to determine if there is a relationship between predicted RVF risk and the prevalence. This information enhances the understanding of RVF epidemiology and will be useful in future development of spatial models predicting high risk of exposure to RVFV in sub-Sahara Africa.

2. Development of a Rift Valley Fever (RVF) network based stochastic model for the United States. The previously published RVF model for Africa was further adapted to model introduction into the United States by ARS researchers in Manhattan, KS. The new model contains the migration rates, estimated abundance and a normalized mortality curve. It accounts for two species of mosquitoes, cattle, humans, and pathogen transmission along a contact network model. The combination of susceptible-infected-recovered (SIR) modeling and network modeling allows for fine scale tuning of the parameters to accurately model the interactions between the various components. The model provides insights into how the virus might move through the U.S. livestock population should it be introduced.

3. Development and evaluation of one-step robust Rift Valley fever genetic assay for detection of virulent viruses and capable of differentiation RNA from some vaccines. Rift Valley fever virus (RVFV) is an insect transmitted virus endemic to Africa and the Arabian Peninsula that affect animals and humans. Current assays for RVFV genomic RNA detect a single target gene thus could result in a false negative result should a mutation occur in the target region. A multiplex assay developed by ARS researchers in Manhattan, KS includes quick virus inactivation for use on samples from infected animals, detects all three genome segments and includes an external RNA control. The assay is specifically designed to differentiate RNA from vaccines containing a specific genetic deletion from that of virulent viral RNA. Once validated and approved by national regulatory agencies, these assays can be safely produced and distributed to regional diagnostic laboratories, providing capacity for operator safe early detection of RVFV in suspected ruminant samples.

4. Evaluation of sheep immune response to Rift Valley fever virus (RVFV) proteins. Development of differentiates infected from vaccinated animal (DIVA) control strategies and companion diagnostic tests is hindered by lack of information on vaccine target species immune responses to specific viral proteins. ARS researchers in Manhattan, KS and collaborators characterized RVFV MP-12 vaccinated sheep immune profiles to expressed recombinant RVFV proteins demonstrating their potential for DIVA control and companion diagnostic strategies. These serological reagents and assays will be useful for immunological characterization of responses to wild type virus infection, in experimental challenge of immunity studies and in the field (in Africa).


Review Publications
Xue, L., Cohnstaedt, L.W., Scott, M.H., Scoglio, C. 2013. A hierarchical network approach for modeling Rift Valley fever epidemics with applications in North America. PLOS Neglected Tropical Diseases. 8(5): e62049.

Last Modified: 9/2/2014
Footer Content Back to Top of Page