"Everything comes from somewhere"

Abstract

The emergence and spread of infectious diseases, especially zoonoses, is complex and depends on various determinants or drivers, including their interrelations at the human-animal-environment interface (HAEI). This thesis provides novel insights into the potential role of environmental transmission in the epidemiology of selected exemplary zoonotic pathogens (i.e. Campylobacter, Shiga toxin-producing E. coli O157, and non-typhoid Salmonella) that are typically considered as foodborne pathogens. This was done mainly by studying some of their characteristics in the environment and the spatial associations between their incidence among human cases and the level of exposure to local livestock density, using novel methods within an eco-epidemiological framework. In addition, this thesis explored the potential human health effects linked to extreme rainfall events and the spatial association between the urban-rural gradient and the diversity and composition of the human gut microbiome. Chapter 2 showed that direct exposure to pluvial floodwater, and activities leading to such exposure, are associated with an increased risk of developing acute gastroenteritis (AGE) and acute respiratory infection (ARI). Chapter 3 showed that surface water is mainly contaminated with Campylobacter originating from meat-producing poultry (i.e. broilers and turkeys) in areas with high poultry densities and from wild birds in recreational waters and WWTP discharge points. The results of Chapter 4 and 5 revealed that human STEC O157 infections can be caused by environmental exposure to small ruminants besides food, while the route of transmission of Salmonella and Campylobacter to humans is mainly foodborne. Chapter 6 revealed that bacterial diversity, richness and composition of the adult gut microbiome are associated with the urban-rural gradient in the Netherlands. This thesis showed that we cannot ignore the role of the environment in mediating the transmission of (foodborne) zoonotic pathogens from livestock and wildlife, and more holistically in shaping the human gut microbiome. However, evidence about specific routes of transmission (e.g. air-borne, via soil or water) and their relative importance, was not provided. To study all interactions between different drivers of zoonotic pathogens, we recommend future studies to focus on pathogen transmission and their related transmission pathways to be able to look beyond the three drivers of change included in the One Health disease triangle (humans, animals, environment). This can be visualized by including a white triangle based on the Gestalt idea that the sum of visual perception is more than its parts. This triangle illustrates the interactions to include to study zoonotic pathogens and provide a complete picture of how they may influence one another. This visualization can be considered as a first step towards further unraveling the complexity of zoonotic pathogens by showing “the complete picture” or at least an attempt to do so, to, in the end, achieve optimal health outcomes.