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

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.