Abstract and Introduction
Abstract
Background: Discoveries that emerging and re-emerging pathogens have their origin in environmental change has created an urgent need to understand how these environmental changes impact disease burden. In this article we present a framework that provides a context from which to examine the relationship between environmental changes and disease transmission and a structure from which to unite disparate pieces of information from a variety of disciplines.
Methods: The framework integrates three interrelated characteristics of environment-disease relationships: a) Environmental change manifests in a complex web of ecologic and social factors that may ultimately impact disease ; these factors are represented as those more distally related and those more proximally related to disease. b) Transmission dynamics of infectious pathogens mediate the effects that environmental changes have on disease. c) Disease burden is the outcome of the interplay between environmental change and the transmission cycle of a pathogen.
Results: To put this framework into operation, we present a matrix formulation as a means to define important elements of this system and to summarize what is known and unknown about the these elements and their relationships. The framework explicitly expresses the problem at a systems level that goes beyond the traditional risk factor analysis used in public health, and the matrix provides a means to explicitly express the coupling of different system components.
Conclusion: This coupling of environmental and disease transmission processes provides a much-needed construct for furthering our understanding of both specific and general relationships between environmental change and infectious disease.
Introduction
Public health scientists are increasingly discovering that the recent emergence or re-emergence of infectious diseases has an origin in environmental change (McMichael and Martens 2002; Morse 1995; Patz et al. 2000). These environmental changes encompass social processes such as urbanization and creation of transportation infrastructure, as well as ecologic processes such as land and water use, biodiversity loss, and climate change. Concern surrounding these trends has inspired much exploratory research because these phenomena are often anthropogenic, interrelated, and accelerating. Yet there remains a pressing need to more clearly define the causal relationships, leading from a distal environmental change to alterations in more proximal environmental characteristics and disease transmission cycles, which eventually lead to a shift in the prevalence, distribution, or severity of an infectious disease (Figure 1).
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Figure 1.
Environmental determinants of infectious disease (EnvID) framework.
In this article we focus on the intermediary relationships between proximal environmental characteristics and transmission cycles. Environmental sciences have traditionally focused on the links between distal environmental changes and their effects on proximal environmental characteristics, whereas public health scholarship has focused on the link between transmission cycles and disease burden. We argue and provide a framework for leveraging the wealth of prior research in both realms by highlighting the links between them. These links are conveniently defined through a matrix formulation in which system elements from one component are mapped onto system elements from another component. The matrix cells can then be used to provide information on what is known about the particular link. This matrix formulation is consistent with a dynamic systems approach that accounts for feedbacks, a central feature of complex systems.
The Environmental Change and Infectious Disease (EnvID) framework uses a systems theory structure to integrate and analyze disparate information from a variety of disciplines. Our ultimate goal is to identify knowledge gaps and define research directions as well as to develop relevant study designs and approaches for data analysis so that knowledge about environmental change can be incorporated appropriately into the study and control of infectious diseases. In the ensuing section, we survey the literature on contemporary frameworks of environmental change and infectious disease. Next, we motivate and describe the EnvID framework. We then use this framework to generate a putative matrix of plausible relationships between proximal environmental characteristics and transmission cycles. This matrix can be used to assess the strengths and weaknesses of existing knowledge and thus prioritize avenues for future research.