Blantyre Typhoid

Case Study #2 - “Environmental surveillance for Salmonella Typhi in rivers and wastewater from an informal sewage network in Blantyre, Malawi”

The aim of the study was to assess the feasibility of utilising ES in an urban environment drained by a natural river system and investigate whether this approach might be a useful tool for confirming the presence and sustained transmission of S. Typhi within the local population.

Surveillance Strategy/System Design:

  • Sampling location: Blantyre, Malawi (metropolitan area)
  • Sampling sites: A combination of 41 river sites and 2 informal sewage channels. River sites selected based on broad geographic coverage, catchment population >1,200, sufficient river flow, and safe and consistent accessibility. Catchment sizes ranged from 4000 to 30000 individuals.
  • Sampling frequency: Monthly between May 2021 and May 2022
  • Sample collection method: One litre water grab samples, collected in autoclavable PPCO bottles; composite sampling with the Moore swab, manufactured with sterile gauze and high-tensile braided fishing line
  • Reason for sampling method(s): The Grab Sample method provides a snapshot of pathogen presence at a single time point. Moore swabs are low-cost and potentially more sensitive to pathogen concentration and therefore its detection over time, and are suitable for turbid waters and intermittent pathogen shedding. Since Blantyre lacks formal wastewater systems, these two methods were chosen to evaluate if environmental surveillance using these methods could detect sustained Salmonella Typhi transmission in such natural water systems.
  • Detection & sequencing approach: qPCR for S. Typhi-specific gene targets (ttr, tviB and staG); no sequencing
  • How data was used: Identification of local hotspots in resource-limited regions experiencing a high disease burden; comparisons with clinical incidence data from Queen Elizabeth Central Hospital

Ethical/Legal Considerations and Approvals:
Core ethical considerations:

  • Beneficience and non-maleficence: Typhoid fever is a major cause of morbidity and mortality, and this study aimed to pilot the use of ES for identification of high-burden regions for subsequent vaccination campaigns. The surveillance does not target or identify specific groups, and is purely used for identification of clinically under-monitored regions, and to guide vaccination strategy.
  • Distributive Justice: All of the Blantyre metro area was monitored, with sites selected to cover as much of the population as possible, leveraging available wastewater streams (rivers) and drainage ditches.
  • Privacy and Autonomy: Sites were selected to maximize catchment size, with a minimal catchment of 1200 individuals. Data was not provided in real time, as this represents a pilot effort to identify approach utility. Ethics approvals were obtained both from University of Malawi College of Medicine Research Ethics Committee (COMREC) and Imperial College London Joint Research Compliance Office.
  • Data Custodianship, Distribution, and Communication: Only positivity data (binary) was generated, and all data (aggegated by month) was included in the publication.
  • Transparency and accountability: No public health dashboards or dissemination tools were used for this work.

Core legal considerations:

  • Legal authorization: This project was a primarily academic study, performed by Imperial College, Malawi-Liverpool Wellcome Trust, and Kamuzu Unviersity researchers, aimed at developing new strategies for public health surveillance and vaccination for S. typhi.
  • Mandatory Reporting Requirements: No legal reporting mandate specific to S. typhi.
  • Data Protection and Privacy: All data is de-identified and only binary (positive/negative) information is available for monitored sites. Data is not publicly released, and aggregated for publication.
  • Public Health Integration: Data was used to compare with clinical case trends, and to inform future surveillance and vaccination efforts.
  • Community and Indigenous Rights: Surveillance efforts were performed with community approvals. All published data is aggregated across time, and catchment resolution is sufficient for general guidance of vaccination strategy, but prevents targeting of any specific population.

Data Analysis:

  • Associations between site and sample-level characteristics: sampling data analyzed separately for Moore swabs and grab samples by fitting univariant and multivariate mixed effects logistic regression models. Comparisons were performed to assess the impact of river vs sewage sites, HF183 copy number (indicator of fecal contamination) at different sites, sample- and site-specific factors such as river channel depth and stream velocity, and catchment population sizes on odds of S. Typhi detection.
  • Spatiotemporal trends: monthly cases plotted with monthly detection rates to identify trends in detection and correlation with hospital reported cases of typhoid fever.
  • Geographic visualization: QGIX v3.22.4 used to visualize 1) the spatial distribution of S. Typhi rates of detection, 2) fecal contamination, and 3) cumulative incidence rates of typhoid fever.

Data Sharing Strategy: Data on sampling sites included as supplementary tables, available in aggregated form (e.g., by month) as word documents, but not in easily parseable. No raw data is available.