Development of a computational model to inform environmental surveillance sampling plans for Salmonella enterica serovar Typhi in wastewater

Typhoid fever–an acute febrile disease caused by infection with the bacterium Salmonella enterica serotype Typhi (S. Typhi)—continues to be a leading cause of global morbidity and mortality, particularly in developing countries with limited access to safe drinking water and adequate sanitation. Environmental surveillance, the process of detecting and enumerating disease-causing agents in wastewater, is a useful tool to monitor the circulation of typhoid fever in endemic regions. The design of environmental surveillance sampling plans and the interpretation of sampling results is complicated by a high degree of uncertainty and variability in factors that affect the final measured pathogens in wastewater samples, such as pathogen travel time through a wastewater network, pathogen dilution, decay and degradation, and laboratory processing methods. Computational models can, to an extent, assist in the design of sampling plans and aid in the evaluation of how different contributing factors affect sampling results. This study presents a computational model combining dynamic and probabilistic modeling techniques to estimate–on a spatial and temporal scale–the approximate probability of detecting S. Typhi within a wastewater system. This model may be utilized to inform environmental surveillance sampling plans and may provide useful insight into selecting appropriate sampling locations and times and interpreting results. A simulated applied modeling scenario is presented to demonstrate the model’s functionality for aiding an environmental surveillance study in a typhoid-endemic community.Author summary: Millions of cases of typhoid fever occur each year, causing thousands of deaths, particularly in children. In order to distribute vaccines to communities with the highest need, it is important to understand disease prevalence in different regions. Clinical surveillance for typhoid fever requires testing of blood and fecal samples, which can be challenging, especially in communities that do not have easy access to healthcare. Environmental surveillance is another method of disease surveillance where wastewater samples are tested for the bacteria that causes typhoid fever–Salmonella Typhi. Environmental surveillance allows researchers to infer information about the number of people who may be infected in a community by measuring the number of bacteria in wastewater samples. Researchers must determine when and where to collect wastewater samples in a complex wastewater network, and the interpretation of this data is complicated. Many different variables affect the concentration of bacteria in wastewater, including how many bacteria are shed in the stool of people with infections and how much the bacteria is diluted and degraded before it is measured. This study describes a model developed to aid with the design of an environmental sampling plan for typhoid fever and to help with the interpretation of results.