Wastewater surveillance for Salmonella Typhi and its association with seroincidence of enteric fever in Vellore, India.

Background: Blood culture-based surveillance for typhoid fever has limited sensitivity, and operational challenges are encountered in resource-limited settings. Environmental surveillance targeting Salmonella Typhi (S. Typhi) shed in wastewater (WW), coupled with cross-sectional serosurveys of S. Typhi-specific antibodies estimating exposure to infection, emerges as a promising alternative.

Methods: We assessed the feasibility and effectiveness of wastewater (WW) and sero-surveillance for S. Typhi in Vellore, India, from May 2022 to April 2023. Monthly samples were collected from 40 sites in open drainage channels and processed using standardized protocols. DNA was extracted and analyzed via quantitative PCR for S. Typhi genes (ttr, tviB, staG) and the fecal biomarker HF183. Clinical cases of enteric fever were recorded from four major hospitals, and a cross-sectional serosurvey measured hemolysin E (HlyE) IgG levels in children under 15 years of age to estimate seroincidence.

Results: 7.50% (39/520) of grab and 15.28% (79/517) Moore swabs were positive for all 3 S. Typhi genes. Moore swab positivity was significantly associated with HF183 (adjusted odds ratio (aOR): 3.08, 95% CI: 1.59-5.95) and upstream catchment population (aOR: 4.67, 1.97-11.04), and there was increased detection during monsoon season - membrane filtration (aOR: 2.99, 1.06-8.49), and Moore swab samples (aOR: 1.29, 0.60-2.79). Only 11 blood culture-confirmed typhoid cases were documented over the study period. Estimated seroincidence was 10.4/100 person-years (py) (95% CI: 9.61 - 11.5/100 py). The number of S. Typhi positive samples at a site was associated with the estimated sero-incidence in the site catchment population (incidence rate ratios: 1.14 (1.07-1.23) and 1.10 (1.02-1.20) for grab and Moore swabs respectively.

Conclusions: These findings underscore the utility and effectiveness of alternate surveillance approaches to estimating the incidence of S. Typhi infection in resource-limited settings, offering valuable insights for public health interventions and disease monitoring strategies where conventional methods are challenging to implement.