Development of an indirect ELISA detecting Swine acute diarrhea syndrome coronavirus IgG antibodies based on a recombinant spike protein

Swine acute diarrhea syndrome coronavirus (SADS-CoV) is a newly identified swine enteropathogenic coronavirus that causes watery diarrhea in neonatal piglets, leading to significant economic losses to the swine industry. Currently there are no suitable serological method to assess the infection of SADV-CoV and effectiveness of vaccines, making an urgent need to exploit effect enzyme-linked immunosorbent assay (ELISA) to compensate for this deficiency. In this study, an indirect ELISA (S-iELISA) based on recombinant spike (S) protein expressed in Baculovirus was developed and evaluated. The reaction conditions of S-iELISA were optimized and cut-off value determined as 0.3711 by analyzing OD450nm values of 40 SADS-CoV-negative sera confirmed by immunoinfluscent assay (IFA) and Western Blot. The coefficients of variation of 6 positive sera within and between runs of S-iELISA were both less than 10% and cross-reactivity assays demonstrated that S-iELISA was non-cross-reactive with other swine viruses' sera. Furthermore, the overall coincidence rate between IFA and S-iELISA was 97.3% based on testing 111 clinical serum samples. Virus neutralization test with 7 different OD450nm value sera showed that the OD450nm values tested by S-iELISA are positive correlated with virus neutralization. Finally, a total of 300 pig field serum samples were tested by S-iELISA and commercial kits of other swine enteroviruses showed that the IgG-positive for SADS-CoV, TGEV, PDCoV and PEDV were 81.7%, 54%, 65.3%, 6%, respectively. The results suggest this S-iELISA is specific, sensitive, repeatable and can be applied for vaccines evaluation and detection the SADS-CoV infection in swine industry.