Where do We Poop? City-Wide Simulation of Defecation Behavior for Wastewater-Based Epidemiology

Abstract

Wastewater surveillance, which regularly examines the pathogen biomarkers in wastewater samples, is a valuable tool for monitoring infectious diseases circulating in communities. Yet, most wastewater-based epidemiology methods, which use wastewater surveillance results for disease inferences, implicitly assume that individuals excrete only at their residential locations and that the population contribute to wastewater samples are static. These simplifying assumptions ignore daily mobility, social interactions, and heterogeneous toilet use behavior patterns, which can lead to biased interpretation of wastewater results, especially at upstream sampling locations such as neighborhoods, institutions, or buildings. Here, we introduce an agent-based geospatial simulation framework: Building on an established Patterns of Life model, we simulate daily human activities, mobility, and social contacts within a realistic urban environment and extend this agent-based framework with a physiologically motivated defecation cycle and toilet usage patterns. We couple this behavioral model with an infectious disease model to simulate transmissions through spatial and social interactions. When a defecation occurs for an infected agent, we use a pathogen shedding model to determine the amount of pathogen shed in the feces. Such a framework, integrating population mobility, disease transmission, toilet use behavior, and pathogen shedding models, is capable to simulate the Spatial-temporal dynamics of wastewater signals for a city. Using a case study of 10,000 simulated agents in Fulton County, Georgia, we examine how varying infection rates alter epidemic trajectories, pathogen loads in wastewater, and the spatial distribution of contamination across time.

Figures (9)

• Figure 1: Conceptual overview of wastewater surveillance. (1) In a specific geographic area covered by a sewer system, referred to as a sewershed, infected (in red) and non-infected (in black) persons from different buildings contribute fecal waste into a sewer system. (2) We can collect wastewater samples either from manholes located across the sewer system or at the downstream wastewater treatment plant. (3) We can analyze samples to detect the presence of pathogens and quantify their levels. (4 and 5) The wastewater surveillance results can help us estimate trends in pathogen prevalence over time in the sewershed, informing public health response. *Sewer manhole icon used in this diagram obtained from Flaticon.com.
• Figure 2: Defecation cycle within the simulation. The process begins with the 'Just Defecated' state and progresses through subsequent stages before returning to the start. The full cycle may repeat multiple times per day, depending on each agent’s individual defecation rate.
• Figure 3: Disease progression over time in the Fulton County 10K simulation across different infection rates. Full results for infection rates 0.1, 0.125, 0.15, 0.175, 0.2, up to 0.5 are available on GitHub.
• Figure 4: Daily number of new cases in the Fulton County 10K simulation across different infection rates.
• Figure 5: Daily total pathogen load in wastewater for the Fulton County 10K simulation across different infection rates. Full results for infection rates 0.1, 0.125, 0.15, 0.175, 0.2, up to 0.5 are available on GitHub.