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Evaluation of Competitive ELISA For Detection of Antibodies to Rift Valley Fever Virus in Cattle and Sheep Sera

Rift Valley fever virus (RVFV) is a mosquito‐borne, zoonotic pathogen that causes mass abortions and deaths in ruminants. In humans, it can cause illnesses ranging from fever to occasionally fatal hemorrhagic fever, encephalitis and liver failure. RVFV is an enveloped, single‐stranded, negative‐sense tripartite RNA virus of the Phlebovirus genus (family: Phenuiviridae, order: Bunyavirales). It is categorized as a Select Agent by the CDC and USDA. Originally endemic to sub‐Saharan Africa only, it has recently spread beyond the continent to the Arabian Peninsula. Thus, the high likelihood of RVFV's spread to non‐endemic countries spurs the need for rapid diagnostics and surveillance tests. In this study, we assessed the efficacy of the recombinant RVFV nucleoprotein based competitive ELISA (cELISA) assay to detect RVFV antibodies in cattle and sheep sera. This cELISA is a new prototype packaged by Veterinary Medical Research & Development (VMRD). We used heat inactivated serum samples from ruminants (cattle=66, sheep=99) that were experimentally infected with either the MP‐12 RVFV vaccine or a virulent strain as well as known RVFV negative sera (cattle=330, sheep=179). We compared the cELISA assay with a plaque reduction neutralization test (PRNT80), the gold standard method for the detection of anti‐RVFV neutralizing antibodies. The recommended cut‐off value for the cELISA was 60%. Additionally, using experimental sera determined positive or negative by PRNT80 (at cut‐offs <1:10 and 1:40) and ROC analysis, we determined the optimal cut‐offs to be 46% and 68% respectively. With the cut‐off of 60% for cELISA and 1:40 titer for PRNT80, the sensitivity and specificity of the cELISA assay was determined to be 95.1% and 91.8% respectively. Antibodies to RVFV were first detected at 5 days post inoculation (dpi) in both sheep and cattle. Interestingly, at 5 dpi, the cELISA detected antibodies in 2/4 samples from RVFV inoculated cattle in comparison to 0/4 by PRNT80. We found the prototype cELISA to be an easy, sensitive, specific, and safe test for the detection of antibodies to RVFV in cattle and sheep. The prototype cELISA thus, could be used for early diagnosis and surveillance that will help in diminish the disease burden.