Environmental Transmission of Pathogens and Implications on Infectious Disease Surveillance

Abstract

Indirect disease transmission occurs when pathogens are shed into the environment during transmission from infected to susceptible people. This environmentally-mediated transmission is characteristic of respiratory, gastrointestinal, and nosocomial pathogens, and can be conceptualized by the commonly described “F-diagram”. The “F-diagram” is a model of the role environmental compartments (food, fluids, fingers, fomites, fields, and flies) play in disease transmission, and can be used to inform design of interventions. Nevertheless, there is a substantial data gap in understanding the relative contributions of distinct environmental transmission routes on disease burden. This gap is driven by a lack of data, informed partly by the difficulties of traditional methods such as epidemiology to estimate and attribute disease burden to specific routes. Nevertheless, elucidating routes of transmission provides opportunities to develop more effective interventions. The relative importance of transmission routes is influenced by a combination of human-environment interactions, pathogen-specific characteristics (such as shedding loads and duration, infectivity), and pathogen fate and transport processes. Human-environment interactions include those of both infected and susceptible people. Infected people shed pathogens into environments within which they interact, exposing the susceptible people who interact with the same environments. Shedding loads and durations, which are pathogen-specific, also influence the transmission routes of pathogens. For example, gastrointestinal viruses such as norovirus and rotavirus are characterized by extremely high shedding loads (exceeding 1010 genome copies / ml) and high infectivity (fewer than 100 virus particles are associated with substantial risk of infection). As a consequence, transmission can occur even in scenarios where fecal contamination is heavily diluted. Pathogen fate and transport processes further influence transmission routes. For example, bacterial pathogens may multiply in environmental compartments such as soil and food. In this thesis, I discuss the occurrence of pathogens (gastrointestinal, respiratory, and antimicrobial resistant) in diverse environmental matrices including water, soil, hands, and surfaces; the human-environment interactions that influence exposure risks to these matrices; and the integration of this information to inform relative importance of transmission routes and potential interventions. The thesis culminates with a discussion of the use of environmental pathogen detection to inform infectious disease epidemiology.