Environmental Persistence of the World's Most Burdensome Infectious and Parasitic Diseases

Abstract

Humans live in complex socio-ecological systems where we interact with parasites and pathogens that spend time in abiotic and biotic environmental reservoirs (e.g., water, air, soil, other vertebrate hosts, vectors, intermediate hosts). Through a synthesis of published literature, we reviewed the life cycles and environmental persistence of 150 parasites and pathogens tracked by the World Health Organization's Global Burden of Disease study. We used those data to derive the time spent in each component of a pathogen's life cycle, including total time spent in humans versus all environmental stages. We found that nearly all infectious organisms were "environmentally mediated" to some degree, meaning that they spend time in reservoirs and can be transmitted from those reservoirs to human hosts. Correspondingly, many infectious diseases were primarily controlled through environmental interventions (e.g., vector control, water sanitation), whereas few (14%) were primarily controlled by integrated methods (i.e., combining medical and environmental interventions). Data on critical life history attributes for most of the 150 parasites and pathogens were difficult to find and often uncertain, potentially hampering efforts to predict disease dynamics and model interactions between life cycle time scales and infection control strategies. We hope that this synthetic review and associated database serve as a resource for understanding both common patterns among parasites and pathogens and important variability and uncertainty regarding particular infectious diseases. These insights can be used to improve systems-based approaches for controlling environmentally mediated diseases of humans in an era where the environment is rapidly changing.

Keywords: DALYs; disease dynamics; environmental control; human health; human–environment interaction.

Figure 1

An overview of the 150 most burdensome parasites and pathogens that infect humans.

Figure 2

(A) Duration of infectious stages outside primary vertebrate hosts (but including vertebrates as obligate intermediate hosts) according to primary transmission strategy, excluding normal flora/opportunistic pathogens and directly zoonotic pathogens, which were data-limited, and sapronoses, which persist indefinitely in abiotic reservoirs. (B) Duration of infectious stages outside primary vertebrate hosts according to standard strategies for disease prevention and control, excluding sapronoses. (C) The minimum estimated global cases according to obligate transmission pathways, categorized as direct human-to-human transmission (e.g., STDs), human-to-human transmission with obligate vertebrate stages (e.g., soil-transmitted helminths), and environment-to-human transmission (e.g., rabies virus). (D) The minimum estimated global cases by obligate vertebrate host ranges; mixed category includes some combination of livestock, poultry, domestic animals, wild animals (including birds), or humans.