Rift Valley fever virus lacking the NSs and NSm genes is highly attenuated, confers protective immunity from virulent virus challenge and allows for differential identification of infected and vaccinated animals

Special Pathogens Branch, Division of Viral and Rickettsial Diseases, Coordinating Center for Infectious Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road MS G-14 Atlanta, GA 30329, USA; University of California, Davis, School of Veterinary Medicine, Davis, CA 95616

^* To whom correspondence should be addressed. Email: stn1{at}cdc.gov.

	Abstract

Rift Valley fever (RVF) virus is a mosquito-borne human and veterinary pathogen associated with large outbreaks of severe disease throughout Africa and more recently the Arabian peninsula. Infection of livestock can result in sweeping "abortion storms" and high mortality among young animals. Human infection results in self-limiting febrile disease that in 1-2% of patients progresses to more serious complications including hepatitis, encephalitis, retinitis or a hemorrhagic syndrome with high fatality. The virus S segment-encoded NSs protein and the M segment-encoded NSm proteins are important virulence factors. The development of safe effective vaccines and tools to screen and evaluate antiviral compounds is critical for future control strategies. Here we report the successful reverse genetics generation of multiple recombinant eGFP tagged RVF viruses containing either full length complete virus genome or precise deletions of the NSs alone or NSs/NSm genes in combination, thus creating attenuating deletions on multiple virus genome segments. These viruses were highly attenuated with no detectable viremia or clinical illness observed at high challenge dosages (1.0x10⁴ PFU) in a rat lethal disease model. A single dose immunization regimen induced robust anti-RVF virus IgG antibodies (titer 1:6400) by day 26 post-vaccination. All vaccinated animals subsequently challenged with a high dose of virulent RVF virus survived infection and could be serologically differentiated from naïve experimentally infected animals by the lack of NSs antibodies. These rationally designed marker RVF virus vaccine viruses will be useful tools for in vitro screening of therapeutic compounds and provide a basis for further development of RVF virus marker vaccines for use in endemic regions or following natural or intentional introduction into previously unaffected areas.