Early Diagnosis of Human Leptospirosis by Detection of Antibodies to Leptospira-Secreted Virulence-Modifying Protein Exotoxins

The ambiguous clinical presentation of leptospirosis poses challenges for accurately assessing the global burden of this emerging disease. As a result, leptospirosis has not been recognized for inclusion in the WHO's neglected tropical diseases list. This underscores the urgent need for a diagnostic biomarker for early detection of illness and well-defined estimation of disease burden in urban and low-income settings. The recently discovered PF07598 gene family encoding virulence-modifying proteins (VMPs), including full-length LA3490, its truncated N-terminal ricin-binding lectin (RBL) domain, and a natural variant encoding only the C-terminal domain (LA0591), was evaluated for its potential to detect anti-VMP-specific IgG antibodies in early infection. The study was conducted on a well-characterized sample of febrile patients from Sri Lanka, with leptospirosis confirmation by microscopic agglutination test (MAT) and Quantitative PCR. The sample included 113 confirmed cases, 45 probable cases, 75 febrile patients, and 41 healthy subjects. Among confirmed cases, mean ELISA optical density (OD) values for LA0591, LA3490, and RBLs were 1.045 (standard error [SE]: 0.063), 0.835 (SE: 0.032), and 0.536 (SE: 0.019), respectively, compared with 0.261 (SE: 0.043), 0.697 (SE: 0.026), and 0.303 (SE: 0.019) in healthy subjects. Anti-VMP antibodies were detectable as early as day 2. In seroconversion cases, ELISA OD for LA0591 in acute MAT-negative samples was 1.347, comparable with high MAT titers. ROC analysis showed AUCs of 0.947 for LA0591 and 0.930 for LA3490, confirming their reliability as diagnostic markers. LA0591 demonstrated superior sensitivity, specificity, and early diagnostic capability, establishing it as a valuable tool for leptospirosis detection.