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Changes in the natural environment through human activities will have broad impacts on global health and human habitation. The links between rapid environmental change and novel pathogens suggest we are entering a new transition in the history of emerging infectious disease. Scientists, however, have not reached consensus regarding an increase of emerging infectious diseases under the broad conditions of environmental change and climate change. This is an area of intense scientific scrutiny.
An emerging infectious disease [EID] is one that that has newly appeared in a population or that has been known for some time but is rapidly increasing in incidence or geographic range. Familiar examples include Influenza, E. coli, HIV, SARS, Tuberculosis, Dengue Fever, West Nile Virus, but there are many others. Influenza is the most common.
This bibliographic essay focuses on the emergence of novel pathogens, highlighting areas of scientific agreement as well as controversy. These selections can serve as a guide for the informed general reader as well as for health professionals. They draw from international books, articles and websites that provide the most engaging histories, authoritative sources, and current information. The themes in this essay begin with broad topics and become increasing more specific. These EID themes include: Microbes through History; Past Pandemics; Zoonoses - focus on Avian Influenza; Ecological Factors; Links to Climate Change; Public Health Preparedness Strategies; Impacts on National Security; and Strategies for the Future.
Emerging Infectious Disease - Microbes through History
The first known texts related to infectious disease were part of the Hippocratic corpus written in the fourth and third centuries B.C.E. "Airs, Waters, Places" was on environmental health and "On Epidemics" contains descriptions of contagious and other diseases of public health importance during the ancient period. These texts appear in Hippocratic Writings, edited by G.E.R. Lloyd.
The epidemiological transition model describes the changing relationship between humans and their diseases. Based on this model, the first transition occurred with the shift to agriculture about 10,000 YBP, resulting in a pattern of infectious diseases still evident today. In "The Changing Disease-Scape in the Third Epidemiological Transition", Kristin Harper and George Armelagos, medical anthropologists, propose that during the last two centuries, some populations have undergone a second transition with a decline in infectious disease and rise in degenerative disease. The authors maintain that we are now in a third epidemiological transition, in which a resurgence of familiar infections is accompanied by an array of novel diseases, all of which have the potential to spread rapidly.
Several books and articles are appropriate for historians and medical researchers. Lois Magner, a distinguished medical historian, produced a comprehensive and up-to-date account of the development of medical microbiology. In A History of Infectious Diseases and the Microbial World, the author places modern infectious diseases within their historical context, and highlights the links between disease and social, cultural, political, and economic factors. In Deadly Companions: How Microbes Shaped our History, Dorothy Crawford combines tales of epidemics with science and history. Crawford is Professor of Medical Microbiology at the University of Edinburgh. She reveals how microbes have evolved with humans over the millennia, shaping civilizations through infection, disease, and pandemic. In Conquest of Epidemic Disease: a Chapter in the History of Ideas, Charles-Edward Amory Winslow traces the human understanding of the causes and control of epidemic communicable diseases. Winslow had a rich knowledge of public health, history, and philosophy.
According to Joshua Lederberg, molecular biologist and Nobel Prize winner, the success of the "wonder drugs" of the 1950s led many to believe that the war on microbes had been fought and won, but the emergence of new infectious agents shattered that illusion. "Infectious History" is a very engaging global history of the cohabitation of humanity and microbes. Ultimate survival, Lederberg suggests, may require humans to embrace a more microbial point of view, in which microbes and their human hosts constitute a superorganism. In a related argument, the Mirage of Health: Utopias, Progress, and Biological Change by René J. Dubos emphasizes that the process of living cannot be separated from the disease process. The book is primarily concerned with the limitations of medicine in the search for the solution of health problems. Dubos, a microbiologist and environmentalist, argues that humankind should not ignore the dynamic process of adaptation to a constantly changing environment that every living organism must face. A. J. McMichael, a notable expert on the environment and emerging infectious disease, reports similar conclusions in "Environmental and Social Influences on Emerging Infectious Diseases: Past, Present and Future". McMichael urges that humankind come to terms with the fact that microbial species help to make up the interdependent system of life on Earth. He claims that humans and microbes are engaged in amoral, self-interested, co-evolutionary struggle.
Pandemics from the Past
There are several books on pandemic disease that would appeal to historians and medical professionals. Historians Tamara Giles-Vernick and Susan Craddock edited Influenza and Public Health, an investigation of past influenza pandemics with insights into possible transmission patterns, experiences, mistakes, and interventions. It explores several pandemics over the past century including the infamous 1918 Spanish Influenza, the avian flu epidemic of 2003, and the novel H1N1 pandemic of 2009. In Dread: How Fear and Fantasy have Fueled Epidemics from the Black Death to Avian Flu by Philip Alcabes a writer and professor of Urban Public Health, writes in an engaging style with comprehensive literary, historical and medical references.
For readers interested in the history of great plagues and pandemics, there are a number of recommended books. John H. Powell's Bring out Your Dead: The Great Plague of Yellow Fever in Philadelphia in 1793 is a historical account of the impact of a yellow fever epidemic that claimed the lives of over 10 percent of the population of Philadelphia and caused its virtual evacuation. Plagues and Peoples by historian William H. McNeill provides an account of the impact of epidemic diseases on the rise and fall of civilizations. Laurie Garrett's The Coming Plague: Newly Emerging Diseases in a World out of Balance is an engaging survey of new and emerging infections - the chapter on "future history" is especially good.
There is a bit of good news among the gloom. Scientists from the National Institute of Health (David Morens, Jeffery Taubenberger, and Anthony Fauci) suggest that successive pandemics generally appear to be decreasing in severity over time. This reflects viral evolution that favors optimal transmissibility with minimal pathogenicity. However, the scientists suggest it is important to understand in greater depth the determinants and dynamics of the pandemic era in which we live. In related research, NIH researchers reported in the New England Journal of Medicine (Miller et al.) that the signature features of past pandemics can help health authorities prioritize national strategies and provide aid to international collaborators.
Zoonoses - Case Study in Avian Influenza
The incidence and frequency of epidemic transmission of zoonotic diseases, both known and newly recognized, has increased dramatically in the past 30 years. It is thought that this dramatic disease emergence is primarily the result of the social, demographic, and environmental transformation that has occurred globally. Most human pathogens also circulate in animals or originate in nonhuman hosts. Influenza is a classic example of a zoonotic infection that transmits from animals to humans. Smith et al., scientists from the Georgia Institute of Ecology, report that infectious agents specific to humans are broadly and uniformly distributed, whereas zoonotic infectious agents are far more localized in their geographical distribution. These results have critical implications for public-health policy and future research pathways of infectious disease ecology. As reported in "Epidemic Dynamics at the Human-Animal Interface." Lloyd-Smith et al., maintain that understanding zoonoses requires a new generation of models that addresses a broader set of pathogen life histories and integrates across scientific disciplines.
Outbreaks of pathogenic avian influenza have been relatively uncommon around the world in the last 50 years with limited spread within a country or region. There is one major exception, Asian lineage H5N1 that was first identified in 1996. According to David Suarez, a researcher with the U.S.D.A., this lineage of virus has spread to over 60 countries and has become endemic in poultry in at least four countries. As reported in "Avian influenza: our current Understanding", this virus represents a public health threat, with some infected humans having severe disease and a high case fatality rate. Suarez maintains that it is a difficult disease to control because of its highly infectious nature and the interface of domestic and wild animals.
Influenza surveillance in wild birds has established that the aquatic birds of the world are the source of influenza A viruses, which occasionally spread to domestic avian species and to mammals, including humans. In "Avian Influenza Virus Surveillance and Wild Birds: Past and Present," Scott Krauss and Robert Webster report that much more attention has been given to understanding the ecology of influenza in wild aquatic birds. Robert Webster has been cited in over 400 articles on the influenza virus, particularly its link to wild birds.
At Qinghai Lake in China, migrating birds are being tracked by satellite, part of a series of investigations that began after highly pathogenic avian influenza (H5N1 subtype) first swept the region in 2005. The studies are attempting to pinpoint the viral reservoir and the role that wild birds play in transmission. L. Jiao reports results in "In China's Backcountry, Tracking Lethal Bird Flu" that no reservoir for the virus has yet been found, but transmission routes have come into clearer focus. However, in another scientific report, "New Avian Influenza Virus (H5N1) in Wild Birds, Qinghai, China," scientist Yanbing Li reports that in 2010 the virus mutated into yet another viral subtype. This occurred in a wetland region very close to Qinghai Lake. This increases concerns about a potential pandemic and the likelihood that avian influenza virus will again spread and continue to increase its genetic diversity. Scientists stress that determining movements of wild birds from Qinghai Lake is essential to track H5N1.
Wallenstein et al. investigated the transmission of H5N1 among people who had unprotected contact with infected wild mute swans in Dorset, England. Results in "No evidence of transmission of H5N1 highly pathogenic avian influenza to humans after unprotected contact with infected wild swans" reveal that no evidence of transmission of H5N1 to humans was found. The incident provided a rare opportunity to study the direct transmissibility of the virus from wild birds to humans.
Emerging Infectious Disease - Ecological Phenomena
Ecological approaches to the understanding of health and disease have a long history. The links between the environment and health are complex and can occur over long time scales that obscure those connections. Emerging zoonotic diseases have assumed increasing importance in both public and animal health, as the last few years have seen a steady increase of new cases, each emerging from an unsuspected geographic area and causing serious problems, often leading to mortalities among animals and humans. Investigating these pathogens as ecological phenomena can provide insights into why these pathogens have jumped species.
In a highly recommended article by Samuel Myers and Jonathan Patz, "Emerging Threats to Humans from Global Environmental Change", the scientists assert that changes to the natural environment are all accelerating with increased exposure to infectious disease. The authors maintain that these threats represent the greatest public health challenge humanity has faced.
In "Environmental Determinants of Infectious Disease", Joseph Eisenburg concludes that emerging and re-emerging pathogens have their origin in environmental change.
However, in "Causal Inference in Disease Ecology: Investigating Ecological Drivers of Disease Emergence", Raina Plowright maintains that few studies have rigorously analyzed the underlying environmental drivers of disease emergence. As an expert on ecological drivers of disease, Plowright reports that ecological change and disease emerge from complex, large-scale processes that are not amenable to traditional approaches to causal inference.
Emerging Infectious Disease - Links to Climate Change
Paul Epstein of Harvard School of Public Health and been sounding the warning bell on climate change and infectious disease for over 10 years. In "Climate Change and Emerging Infectious Disease" he presented a theory that deforestation and climatic volatility are a potent combination creating conditions conducive to disease emergence and spread. In "Primary, Secondary and Tertiary Effects of Eco-Climate Change: The Medical Response", Colin Butler and David Harley propose that the primary effects of climate change to global health include the acute and chronic stress of heat waves, and trauma from increased fires and flooding. Secondary signs are indirect, such as an altered distribution of vectors, intermediate hosts and pathogens. More severe future health consequences of climate change are classified here as tertiary effects: famine, war and significant population displacement.
There is near unanimous scientific consensus that greenhouse gas emissions generated by human activity will change Earth's climate. Yet, there is little hard evidence that recent global warming has already affected some health outcomes. Researchers, A.J. McMichael, Rosalie Woodruff, and Simon Hales, suggest in "Climate Change and human health: present and future risks", that anticipation of adverse health effects will strengthen the case for pre-emptive policies and guide priorities for planned adaptive strategies. In a related argument in "Climate Change and Infectious Disease in North America: the Road Ahead", researchers Amy Greer, Victoria Ng and David Fisman recommend that the best defense against increases in infectious disease related to climate change lies in strengthening existing public health infrastructure. In "Biodiversity Loss Affects Global Disease Ecology", Montira J. Pongsiri et al. propose that climate change is a phenomenon that will require a biogeographic perspective to predict its affect on global disease. Currently, biogeography and disease ecology are disciplinary communities with little overlap.
Not all scientists agree on climate change and emerging infectious disease. In "The Ecology of Climate Change and Infectious Disease", Kevin Lafferty claims that although the globe is significantly warmer than it was a century ago, there is little evidence that climate change has favored infectious diseases. Lafferty reports that recent models predict range shifts in disease distributions, with little net increase in area. Researchers Bruce Wilcox and Duane Gubler link demographic and societal factors to land use, land cover change, and ecological factors to disease emergence in "Disease Ecology and the Global Emergence of Zoonotic Pathogens." They report that the scale and magnitude of these changes are more significant than those associated with climate change, the effects of which are largely not yet understood. Joshua Rosenthal of the National Institutes of Health reports that the ability to predict the effects of climate change on the spread of infectious diseases is in its infancy. In "Climate Change and the geographic distribution of Infectious Disease", the authors report that there are conflicting predictions.
Emerging Infectious Disease and Public Health Preparedness
In a 2010 report, Infectious Disease Movement in a Borderless World, David Relman and the Institute of Medicine report that infectious diseases (pre-pandemic stage) now emerge more frequently, spread greater distances, pass more easily between humans and animals, and evolve into new and more virulent strains. Literature from the U.S. Institute of Medicine intended for clinicians, researchers, and public policy makers includes The Impact of globalization on infectious disease emergence and control exploring the consequences and opportunities by Stacey Knobler. Keith Fukuda and The World Health Organization released Ethical considerations developing a public health response to pandemic influenza. A product of expert international opinion, this publication is intended for public health and pandemic influenza preparedness activities at the national level. In "Communicating about Emerging Infectious Disease: the Importance of Research", Bev Holmes reviews risk communication strategies for emerging infectious diseases.
In "The Generic Biothreat, Or, How we became Unprepared", sociologist Andrew Lakoff argues that over the course of the past three decades, a new way of thinking about the threat of infectious disease has coalesced. The problem of infectious disease is no longer only one of prevention, but one of preparedness. Jonathan D. Moreno, professor of Medical Ethics, wrote In the Wake of Terror: Medicine and the Morality in a Time of Crisis, a book intended for public health policymakers. He maintains that responses to bioterrorism and emerging infectious diseases of the twenty-first century are destined to overwhelm the public health system.
Emerging Infectious Disease and National Security
In Contagion and Chaos, Andrew Price-Smith offers an examination of disease through the lens of national security. This book will be of interest to political scientists and those in public health and medicine as it highlights the interdependence between political science and public health. The author stresses that the association between the health of a population and perception of national security is ancient but largely forgotten. He argues that epidemic disease represents a direct threat to the power of a state, eroding prosperity and destabilizing both its internal politics and its relationships with other states. The danger of an infectious pathogen to national security depends on lethality, transmissability, fear, and economic damage. The author maintains that warfare and ecological change contribute to the spread of disease and act as "disease amplifiers."
Strategies for the Future
There are several valuable data collection tools and collaboration networks in existence. To be useful, data must assist in the effort to identify population-based strategies for pandemics. Advance knowledge of which subpopulations are most likely at increased risk can lead to quicker public health response and disease control. Lessons from the past would suggest that public health preparedness include human health surveillance that is integrated with monitoring of climate and other environmental conditions that favor emerging infectious disease.
In an important work by the Centers for Disease Control and Prevention (Jeremy Hess, Josephine Mililay and Alan Parkinson), scientists considered the ways in which the concept of place - the sense of human relationship with particular environments - will play a key role in motivating, developing, and deploying an effective public health response. In "Climate Change: the Importance of Place", they highlight the concepts of community resilience and risk management, key aspects of a robust response to climate change in public health and other sectors.
Mark Woolhouse, faculty at University of Edinburgh in infectious disease epidemiology, reports in "Emerging Diseases go Global" that novel human infections continue to appear all over the world, but the risk is higher in some regions than others. Identification of emerging-disease 'hotspots' will help target surveillance work. In Global trends in emerging infectious diseases, Kate E. Jones et al. provide a basis for identifying regions where new EIDs are most likely to originate (emerging disease 'hotspots'). Currently, they claim that global resources to counter disease emergence are poorly allocated - the majority of the surveillance effort is focused on countries from where the next important EID is least likely to originate.
John Brownstein, a medical informatics researcher, reports that the existing network of disease surveillance efforts managed by public health institutes has wide gaps in geographic coverage and often suffers from poor and sometimes suppressed information flow across national borders. In "Surveillance Sans Frontieres: Internet Based Emerging Infectious Disease Intelligence and HealthMap project", Brownstein reports that valuable information about infectious diseases is found in internet-accessible information sources such as discussion sites and disease reporting networks. These data must be studied in depth, including false reports and reporting bias. Yet, this information holds potential to provide complementary epidemic intelligence context. This potential is already being realized, as a majority of outbreak verifications currently conducted by the World Health Organization's Global Outbreak Alert and Response Network are triggered by reports from these nontraditional sources.
It is clear that there is a relationship between environmental change and infectious disease, but these links are disease and location specific. Thus, the direct impact on human health is still unclear. Climate change may increase the prevalence of particular infectious diseases in some regions, while decreasing the prevalence in others. Many factors can affect emerging infectious disease, and some of these factors may overshadow the effects of climate. The potential health impacts underscore the need to reinforce response systems for infectious disease outbreaks, including public health preparedness and the capacity necessary to mount effective responses.
There is no natural analog to the rapid increases of human-induced environmental changes that are emerging in the 21st century. The links between rapid environmental change and novel pathogens suggest we are entering a new transition in the history of emerging infectious disease.