2012 Annual Report 1a.Objectives (from AD-416): To develop a challenge model for young vaccine age animal and evaluate North American vaccine candidates for the ability to block transmission to mosquitoes following challenge. 1b.Approach (from AD-416): ARS and CFIA have been working together to develop modern diagnostic tests for detecting RVFV, viral antigen, nucleic acids, and antibodies. During this development, very young sheep and cattle were experimentally infected with RVFV to generate positive diagnostic material. This new study will expand the research to develop a vaccine age challenge model for sheep. The ability of known competent mosquito species to feed on challenge animal to become infected and transmit the virus to susceptible hamsters will be demonstrated. Once this model is confirmed, young sheep will be vaccinated with RVFV MP-12 strain vaccine candidate, held for 28 days then challenged with a virulent strain of RVFV. The ability of the vaccine induced immune response to block the same competent mosquito species fed on challenged animals to become infected and transmit the virus to susceptible hamsters will be evaluated. 3.Progress Report: The goal of this project was to develop a challenge model for young vaccine age animal and evaluate North American vaccine candidates for the ability to block transmission to mosquitoes following challenge. Two young sheep RVFV challenge studies were performed that resulted in detectable viremias and fever. In addition, CFI’s Culex pipiens colony has been demonstrated to be competent for RVFV. Based on these results an experimental design for vaccine challenge study has been developed that include insect feeding post challenge. This will not only determine the effectiveness of the trial attenuated vaccine to protect the vertebrate host, but also the potential to block virus transmission to the insect vector. The young sheep RVFV infection studies and vaccine challenge studies were conducted. These studies demonstrated that this model could be used to evaluate vaccine candidates.