Development and Application of an iELISA for the Detection of Antibody against Salmonella Abortusequi

Equine abortus salmonellosis is a bacterial disease that causes high abortion rates in susceptible equids and therefore significant economic losses. Although the tube agglutination test (TAT) is a commonly used serological test for S. Abortusequi, it is not highly specific or sensitive, and the development of more sensitive, specific and rapid assays is therefore urgently required. In this study, an indirect enzyme-linked immunosorbent assay (iELISA) was developed for the specific detection of flagellum protein (FljB) antibodies against S. Abortusequi. Negative sera from horses in China (n = 1030) were used to establish the baseline for a negative population, and reference antisera positive against other viruses or bacteria were used to test the cross reactivity of the technique. The performance of the FljB iELISA was evaluated against that of the standard TAT, and was tested using field serum samples. The FljB iELISA assay was 8-16 times more sensitive than TAT. ROC analysis showed that the FljB iELISA was accurate, with an area under the curve (AUC) = 0.9943 (95% CI, 0.9815-1.000). The diagnostic sensitivity (DSe) of the FljB iELISA was 98.9% (95% Cl, 93.84%-100.00%), which was higher than that of TAT (DSe 38.6; 95% CI, 29.14%-49.08%). The diagnostic specificity (DSp) of the iELISA was 100.0% (95% CI, 95.82%-100.0%). When the 508 clinical samples were tested, the FljB iELISA had a positive detection rate of 51.38% (261/508, 95% CI, 51.24%-51.51%), which was higher than that of TAT (44/508). We also performed a serological survey for S. Abortusequi infection, using a series of samples collected from across eighteen provinces of China in 2021. The results showed that all provinces except Jiangsu had a certain number of cases, and the positive rates ranged from 0% to 96.9%, indicating the wide spread of S. Abortusequi in China. The abovementioned results suggest that the FljB iELISA developed in this study is rapid, sensitive, specific, and repeatable and is likely to be a suitable test for large-scale serological surveys for the detection and control of S. Abortusequi infection.