VII.6 Managing Infectious Diseases
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VII.6 Managing Infectious Diseases Jonathan A. Patz and Sarah H. Olson OUTLINE 1. Introduction 2. Niche invasion and cross-species transfer of pathogens 3. Why can high biodiversity prevent disease emergence? 4. Harming habitats can harm human health: Tropical rainforest destruction and the rise of malaria 5. Agricultural development, crop irrigation, and breeding sites 6. Conclusions Changes in biodiversity and habitat change affect the transmission or emergence of a range of infectious diseases. These environmental factors include agricultural encroachment , deforestation, road construction, dam building, irrigation , wetland modification, mining, the concentration or expansion of urban environments, coastal zone degradation , and other activities. As a result, a cascade of factors can exacerbate infectious disease resurgence, such as forest fragmentation, disease introduction, pollution, poverty, and human migration. Subsequent biological mechanisms of disease emergence that are affected include altered vector breeding sites or reservoir host distribution, niche invasions or interspecies host transfers, changes in biodiversity (including loss of predator species and changes in host population density), human-induced genetic changes of disease vectors or pathogens (such as mosquito resistance to pesticides or the emergence of antibiotic-resistant bacteria), and environmental contamination of infectious disease agents. GLOSSARY emerging disease. As defined by the Centers for Disease Control and Prevention, emerging infectious diseases are diseases of infectious origin whose incidence in humans has increased within the past two decades or threatens to increase in the near future. In general, an emerging disease can be a completely new disease or an old disease occurring in new places or new populations or that is newly resistant to available treatments. gonotrophic cycle. The complete cycle of time between a mosquito’s blood feeding and subsequent laying of eggs. reservoir host. A reservoir host can harbor human pathogenic organisms without acquiring the disease , and so serves as a source from which the infectious disease may spread. vector-borne disease. Infectious diseases spread indirectly via an insect or rodent. Often, part of the pathogen’s life cycle occurs within the insect vector. Examples include malaria, dengue fever, West Nile virus, Lyme disease, plague, and Hantavirus. zoonotic disease. Any disease that is spread from animals to people. These are also called ‘‘zoonoses’’ (as opposed to ‘‘anthroponoses,’’ which are diseases transmitted directly from person to person). Examples of zoonotic diseases include rabies, Lyme disease, and bat-borne Nipah and Hendra viruses. 1. INTRODUCTION Widespread deforestation and habitat destruction not only threaten biodiversity worldwide, but land use change influences a range of infectious diseases. Anthropogenic (human-created) drivers that especially affect infectious disease risk include destruction or encroachment into wildlife habitat, particularly through logging and road building; changes in the distribution and availability of surface waters, such as through dam construction, irrigation, or stream diversion; agricultural land use changes, including proliferation of both livestock and crops; deposition of chemical pollutants, including nutrients, fertilizers, and pesticides; uncontrolled urbanization or urban sprawl; climate variability and change; migration and international travel and trade; and either accidental or intentional human introduction of pathogens (table 1 and figure 1). Ecological changes can affect specific biological mechanisms of disease transmission. Several biological mechanisms have been identified and are reported in the Millennium Ecosystem Assessment. These include niche invasions or interspecies host transfers; changes in biodiversity (including loss of predator species and changes in host population density); altered vector breeding sites or reservoir host distribution; humaninduced genetic changes of disease vectors or pathogens (such as mosquito resistance to pesticides or the emergence of antibiotic-resistant bacteria); and environmental contamination of infectious disease agents. 2. NICHE INVASION AND CROSS-SPECIES TRANSFER OF PATHOGENS Many widespread diseases of today originally stemmed from domestication of livestock. For example, measles, smallpox, and tuberculosis resulted from the domestication of wild cattle. Pathogens that are currently passed from person to person (anthroponotic), including some influenza viruses and human immunodeficiency virus (HIV), were formerly zoonotic but have diverged genetically from their ancestors that occurred in animal hosts. Rapid population growth and population movements have quickened the pace and extensiveness of ecological change over the past two centuries. New diseases have emerged even as some pathogens that have been around for a long time have been eradicated or rendered insignificant, such as smallpox. Ecological change,pollutants,thewidespreadlossoftoppredators, persistent economic and social crises, and international travel, which drive a great movement of potential hosts, have progressively altered disease ecology, affecting pathogens across a wide taxonomic range of animals and plants. According to estimates, nearly 75% of human diseases are zoonotic and...