Tamil Nadu: Wastewater monitoring can offer early warning for typhoid, finds Lancet study in Vellore

CHENNAI: A new study conducted in Vellore has established that wastewater surveillance can accurately track typhoid transmission, identify disease hotspots and support targeted vaccination strategies. The study revealed a strong link between the presence of Salmonella typhi in sewage and laboratory-confirmed typhoid cases.

Published in The Lancet Microbe, the findings strengthen the case for integrating environmental surveillance into routine public health programmes in typhoid-endemic regions.

The prospective study was carried out by researchers from Christian Medical College (CMC), Vellore, Imperial College London and collaborating institutions across 33 urban wards in Vellore between December 12, 2022, and June 30, 2023, before the rollout of the TyphiBEV typhoid conjugate vaccine (TCV) trial. The objective was to establish baseline S. Typhi detection in wastewater and to determine whether it reflected the clinical burden of typhoid in the community.

Researchers collected 1,446 wastewater samples, comprising 723 grab samples and 723 Moore swabs, from 50 surveillance sites serving a population of about 2.25 lakh.

The bacterium was detected in 80 samples, accounting for 5.5 per cent of the total. Grab samples yielded a higher positivity rate, with 60 samples (8.3 per cent) testing positive compared with 20 samples (2.8 per cent) collected through Moore swabs. The highest detection rates were recorded in the northeastern and central parts of the city.

Hospital-based surveillance conducted simultaneously recorded 207 blood culture-confirmed typhoid cases. Infections peaked between March and May 2023, when Vellore witnessed a localised outbreak. Spatial mapping showed that areas reporting frequent detection of S. Typhi in wastewater closely overlapped with neighbourhoods that recorded the highest number of clinical cases, underlining the value of environmental surveillance in identifying transmission clusters.

The study estimated the overall incidence of typhoid at 180.2 cases per 100,000 person-years. Children aged 5 to 14 years bore the highest burden, with an incidence of 699.1 per 100,000 person-years, followed by children younger than five years at 685.1 per 100,000 person-years. Adults aged over 30 years recorded the lowest incidence, at 19.9 per 100,000 person-years.

The analysis found a significant association between wastewater positivity and disease incidence. Statistical modelling showed that every ten-fold increase in typhoid incidence raised the odds of detecting S. Typhi in wastewater by 2.43 times after accounting for rainfall, laboratory methods and human faecal contamination. Researchers also observed a strong correlation between wastewater positivity and typhoid incidence across non-overlapping catchment areas, indicating that environmental sampling can reliably reflect disease activity at the community level.

"Our findings show that wastewater surveillance data correlate with clinical disease incidence in an endemic setting and could be used to monitor targeted interventions, including typhoid conjugate vaccine introduction," the authors said.

The researchers said environmental surveillance can capture infections that routine hospital surveillance often misses, including asymptomatic carriers and untreated cases, offering a more comprehensive picture of transmission. They concluded that integrating wastewater monitoring with clinical surveillance could provide an early warning of outbreaks, guide vaccine deployment, strengthen water, sanitation and hygiene interventions, and help assess the long-term impact of typhoid vaccination programmes in resource-constrained urban settings.

Highlights

1,446 wastewater samples were collected from 50 locations across Vellore

The bacterium was detected in 80 samples

The highest detection rates were recorded in the northeastern and central parts of the city

Every ten-fold increase in typhoid incidence raised the odds of detecting S. Typhi in wastewater by 2.43 times

Children aged 5 to 14 years bore the highest burden, with an incidence of 699.1 per 100,000 person-years

* The researchers said environmental surveillance can capture infections that routine hospital surveillance often misses, including asymptomatic carriers and untreated cases, offering a more comprehensive picture of transmission