Animals were housed in the Institut Pasteur animal facilities accredited by the French Ministry of Agriculture to perform experiments on live mice, in appliance of the French and European regulations on care and protection of the Laboratory Animals (accreditation number B 75 15-01 and B 75 15-07). The veterinary staff of the Institut Pasteur animal facility approved protocols. Protocols were performed in compliance with the NIH Animal Welfare Insurance #A5476-01 issued on 02/07/2007. Inbred 129S2/SvPas mice with knockout at the interferon a and b receptor 1 locus (Ifnar1 2/2 ) and control mice (Ifnar1 +/+ ) were bred at the Institut Pasteur [34] . Vero E6 cells were grown in DMEM supplemented with 10% FCS. BHK21/T7 cells [35] were grown in MEM supplemented with 5% FCS and tryptose phosphate broth powder (Sigma-Aldrich, Gillingham, UK). The cell culture media were supplemented with 10 IU/ml of penicillin and 10 mg/ ml of streptomycin. Stocks of the virulent RVFV Egyptian ZH548 strain were produced under biosafety level 3 (BSL3) conditions.