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Page 8 1 Introduction Whoever would study medicine aright must learn of the following subjects. First he must consider the effect of the seasons of the year and the differences between them. Secondly he must study the warm and the cold winds, both those which are in common to every country and those peculiar to a particular locality. Lastly, the effect of water on the health must not be forgotten. (Hippocrates, Airs, Waters, and Places) A change in weather can lead to the appearance of epidemic disease. This simple observation has been appreciated since the dawn of medical science when Hippocrates taught that many specific human illnesses were linked to changes of season or temperature. Indeed, the very terms we use today for infectious diseases often preserve ancient notions of disease being caused by environmental change and other external factors. Familiar etymological examples are “influenza,” which is derived from “influence”; “malaria,” contracted from “mal” and “aria” (bad air); or simply “a cold,” the quaintly preserved term for an upper respiratory tract infection. Perhaps the best reflection of these widespread beliefs is the colloquial phrase “under the weather,” which is taken to signify a temporary illness or indisposition without other explanation. The birth of modern microbiology and, with it, the systematic study of the epidemiology of specific microbes altered fundamental scientific concepts of disease transmission. New laboratory techniques for isolation and characterization of bacteria, viruses, and other classes of microbes identified specific agents to be the proximate cause of diseases, and control efforts accordingly shifted to a more focused scientific attack on these specific “germs.” This approach ap-
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Page 9 peared to be hugely successful. By the 1960s highly effective vaccines or drugs had been developed against many globally important pathogenic microbes, and many countries successfully protected their populations from disease through the use of pesticides, water treatment, and other public health measures. It appeared to be just a matter of time before the war against infectious diseases would be won, but optimism was premature. Reasons for setbacks included the rapid evolution of drug- and pesticide-resistant variants, the surprise emergence of new microbial pathogens, swift global dissemination of microbes and vectors through expanding transportation networks, and the dissipation of political will needed to sustain successful public health programs. Given the obvious and long-appreciated linkages between climate and human health, it might seem a simple task to use climate forecasts in predictive disease models. Unfortunately, the mathematical relationships between climate and disease are neither so obvious nor so simple. Clearly establishing causal relationships between climate and disease has proven very difficult, largely for the following reasons: Unlike some of the other sectors commonly studied in climate impact assessments, the health sciences do not have a tradition of using predictive mathematical models or other forecasting tools. Infectious disease transmission patterns are affected by many factors other than climate, and the relationship between climatic variations and disease outbreaks is often mediated by ecological, biological, or societal changes. Data on infectious disease incidence in many parts of the world are sparse or nonexistent, which makes it difficult to develop a solid empirical understanding of climate/disease relationships. Over the past 15 years or so, as observational data and climate modeling studies have confirmed the likelihood of a long-term global warming trend, a question that naturally arose was “what effect will this have on human health, specifically on infectious diseases?” This question has generated considerable public interest and stimulated the publication of numerous research and review papers. It has also been the focus of assessments carried out by the World Health Organization (WHO), the Intergovernmental Panel on Climate Change (IPCC), the U.S. Global Change Research Program (USGCRP), and other organizations. Some of these publications have made claims that climate change will have wide-ranging, adverse impacts on human health. For instance, it has been projected that with warmer temperatures, the mosquito vectors that transmit malaria, dengue, and yellow fever could move northward into the United States and Europe, that development of virus and parasites would accelerate, that epidemics of diseases such as cholera will intensify with increasing sea surface temperatures, and that the emergence of new disease threats will become more common.
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Page 10 Studies have claimed that recent changes in infectious disease patterns (for instance, increasing malaria incidence in high altitude regions) can be linked to global warming trends. Likewise, some have concluded that interannual climate fluctuations, in particular El Niño events, have been at least partially responsible for major outbreaks of disease such as cholera and dengue; and these associations between El Niño events and disease outbreaks have been extrapolated to infer the potential impacts of long-term climate change. At present, however, there is little solid scientific evidence to support such conclusions and few studies that take into account the full range of factors influencing pathogen transmission such as human travel and migration patterns, the collapse of public health measures in some regions, and an increase in drug resistant parasites and pesticide-resistant vectors. In addition, infectious disease experts have pointed out that mosquito vectors of malaria, dengue, and yellow fever have been in the United States for centuries, but epidemics of these diseases have vanished due to public health measures and lifestyle changes. They also make the point that humans can and do adapt to mitigate the harmful impacts of a changing climate. A lack of consensus among the scientific community about the magnitude and relative importance of the effects of climate on infectious diseases provided one of the motivations for this NRC study. A second motivation was the level of public concern generated by press reports that have sometimes been misleading or inaccurate. Yet another motivation for the study is the hope that recent advances in climate forecasting and remote-sensing technologies can be used to provide early warnings of conditions conducive to disease outbreak. Currently, public health systems rely primarily on “surveillance and response” approaches to controlling infectious disease. It is hoped that a “prediction and prevention” approach may become more feasible with a solid understanding of the climate and ecological conditions that favor disease transmission. This challenge involves developing predictive models to make reliable disease “forecasts” and creating operational early warning systems that can effectively reduce the risk to vulnerable populations. This report reviews the current knowledge of the relationships between climate, ecosystems, and infectious diseases, evaluates the potential for disease early warning systems, and offers recommendations about how a predictive science of infectious disease epidemiology might be realized. The report is organized as follows: Chapter 2 provides a historical review of how the concept of “environmental medicine” has developed over the last few centuries, and how this has shaped current perspectives on climate and infectious disease linkages. Chapter 3 reviews some basic concepts in climatology and infectious disease epidemiology, and gives an overview of the linkages among climate, ecosystems, and infectious diseases.
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Page 11 Chapter 4 reviews the current state of understanding of how climate influences some specific diseases. Chapter 5 discusses the analytical approaches that can be used to study these linkages including different types of observational studies, laboratory experiments, and modeling analyses. Chapter 6 describes how the lessons learned from ecological studies can provide insights into the challenges of extrapolating study results from one temporal/spatial scale to another. Chapter 7 examines the feasibility of using climate forecasts to predict disease outbreaks and describes the different components necessary for an effective warning system. Chapter 8 summarizes the committee's key findings about these issues and recommendations for future research needs.
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