The open reading frame of the nucleocapsid protein (NP) of Rift Valley fever virus (RVFV) strain MP12 was cloned and expressed in Vero E6 cells. The recombinant NP (rNP)-expressing cells were used as antigens for an indirect immunofluorescent antibody assay (IFA). The rNP-based IFA and RVFV-infected Vero E6 cell (authentic antigen)-based IFA showed similar IFA profiles with immune rabbit serum, which was prepared by immunization with rNP expressed using a baculovirus vector. A total of 942 traditional cattle sera obtained in five districts in Central, Southern, and Western provinces of Zambia were screened for anti-RVFV antibodies by the authentic antigen-based and rNP-based IFAs. Significant agreement was obtained between the two IFAs. The findings show that the rNP-based IFA is a safe and useful diagnostic tool as an alternative to the authentic antigen-based IFA. The antibody titers given by the rNP-based IFA were higher than those by the authentic antigen-based IFA. Therefore, the rNP-based IFA might be useful for serosurveillance of RVFV infection among cattle. Antibody prevalence rates in the five districts were 1.3% to 13.5% in the authentic antigen-based IFA and 6.0% to 21.4% in the rNP-based IFA. The results indicated that despite no reports of active cases of RVF in these provinces of Zambia, the virus is circulating among cattle herds.
Keywords: Rift Valley fever virus; Zambia; immunofluorescence assay.