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The safety and immunogenicity of two authentic recombinant (ar) Rift Valley fever (RVF) viruses, one with a deletion in the NSs region of the S RNA segment (arMP-12ΔNSs16/198) and the other with a large deletion of the NSm gene in the pre Gn region of the M RNA segment (arMP-12ΔNSm21/384) of the RVF MP-12 vaccine virus were tested in crossbred ewes at 30-50 days of gestation. First, we evaluated the neutralizing antibody response, measured by plaque reduction neutralization (PRNT(80)), and clinical response of the two viruses in groups of four ewes each. The virus dose was 1×10(5)plaque forming units (PFU). Control groups of four ewes each were also inoculated with a similar dose of RVF MP-12 or the parent recombinant virus (arMP-12). Neutralizing antibody was first detected in 3 of 4 animals inoculated with arMP-12ΔNSm21/384 on Day 5 post inoculation and all four animals had PRNT(80) titers of ≥1:20 on Day 6. Neutralizing antibody was first detected in 2 of 4 ewes inoculated with arMP-12ΔNSs16/198 on Day 7 and all had PRNT(80) titers of ≥1:20 on Day 10. We found the mean PRNT(80) response to arMP-12ΔNSs16/198 to be 16- to 25-fold lower than that of ewes inoculated with arMP-12ΔNSm21/384, arMP-12 or RVF MP-12. No abortions occurred though a single fetal death in each of the arMP-12 and RVF MP-12 groups was found at necropsy. The poor PRNT(80) response to arMP-12ΔNSs16/198 caused us to discontinue further testing of this candidate and focus on arMP-12ΔNSm21/384. A dose escalation study of arMP-12ΔNSm21/384 showed that 1×10(3)plaque forming units (PFU) stimulate a PRNT(80) response comparable to doses of up to 1×10(5)PFU of this virus. With further study, the arMP-12ΔNSm21/384 virus may prove to be a safe and efficacious candidate for a livestock vaccine. The large deletion in the NSm gene may also provide a negative marker that will allow serologic differentiation of naturally infected animals from vaccinated animals., (Copyright © 2012 Elsevier Ltd. All rights reserved.)