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...Theory Epidemiologic Transition, comparing the burden of infectious diseases in the developed world versus the developing world. The advent of modern sanitation and hygiene practices, effective vaccines, and antibiotics have significantly diminished the burden in the developed world, but infectious diseases remain the most common cause of death worldwide. The persistence of this disease burden has been due to a failure to employee effective strategies and to unforeseen developments, such as the emergence of HIV and the re-emergence of malaria and tuberculosis driven by newly developed drug resistance. The challenge in accurately assessing infectious disease burden and developing effective interventions is reviewed along with the most common diseases and current intervention strategies.
Keywords: epidemiology, epidemiologic transition, infectious disease, developing world, developed world, HIV, malaria, tuberculosis, diarrhea, tropical disease
Introduction
For most of recorded history, infectious diseases have been among the leading causes of suffering and death throughout the world. However, from the middle of the 19th century through the middle of the 20th century, deaths from infectious diseases declined rapidly throughout the developed world (1-4). In the late 1960s and early 1970s, many leaders in public health were confident that they were witnessing the end of infectious diseases as a major health threat, so much so that US Surgeon General William H. Stewart is frequently quoted as saying it was time to "close the book on infectious diseases" and focus on more chronic ailments such as cancer and heart disease (it appears that Dr. Stewart may not have really made this statement, but it certainly reflects the sentiment of the day) (5,6). Unfortunately, this victory over infectious diseases was largely only true in the developed world. Current estimates and future projections show that infectious diseases remain the number one cause of death in developing countries, currently accounting for > 25% of deaths worldwide, and over 40% of deaths in developing countries, significantly more than in developed countries (7-9) (See Figure 1). However, the success over infectious diseases was not maintained even in the developed world. Due to emerging and re-emerging diseases, death rates from infections in the United States increased 58% between 1980 and 1992 (10).
[FIGURE 1 OMITTED]
In 1971, Abdel Omran proposed the theory of an "epidemiologic transition" to account for the tremendous progress in health seen in industrialized countries over the 19th and 20th centuries (11). He suggested that all societies progress through three stages of disease: "the age of pestilence and famine", characterized by high, fluctuating mortality rates with life expectancies under 30 years; "the age of receding pandemics", characterized by rising life expectancies to over 50 years, but a persistent heavy burden from infectious diseases; and the "age of degenerative and man-made diseases" during which life expectancy increases further, the burden of infectious diseases declines considerably, and degenerative diseases, such as cancer and cardiovascular disease, become more prevalent. It seemed clear that industrialized countries had moved from the second to the third stage, and it was assumed that developing countries would soon share in a similar decline in infectious disease burden. While progress has certainly been made in many areas of the world, especially when measured simply by mortality rates, the transition is far from complete in many developing countries. Furthermore, mortality is not the only important measure of the burden of infectious diseases. Other important measures of disease burden, such as cognitive and developmental delays or economic impact can reveal an even greater persistent difference in the epidemiologic transition stages between the developed and developing worlds.
[FIGURE 2 OMITTED]
The hope that the 20th century would see the end of a significant burden of infectious diseases and a convergence of health between developed and developing countries was impeded, and in many cases reversed, by several factors. There has simply been a failure to fully employ many of the same strategies in the developing world that were so successful in controlling infectious diseases in the developed world. For instance, immunization programs have been among the most successful and cost-effective public health programs, accounting for the prevention of 2 million child deaths in 2003 alone (12). However, there were still an estimated 2.5 million childhood deaths from vaccine preventable diseases, most of which occurred in developing countries (see Figure 2) (12). There has also been an emergence of new diseases. During the 1970s, no one expected that HIV, which was initially recognized in 1981, would emerge and cause a pandemic that to date has affected 65 million people and has resulted in 25 million deaths (a vast majority of which are in developing countries) (13) (see Table 1). There has also been a re-emergence and spread of other diseases endemic to developing countries, such as tuberculosis (TB) and malaria, often driven and complicated by newly emerged drug resistance (14).
The emergence of HIV and other infectious diseases, such as Ebola and Marburg hemorrhagic fevers, bovine spongiform encephalopathy ("mad cow disease"), severe acute respiratory syndrome (SARS), West Nile virus, and avian influenza, has resulted in an increased awareness of infectious diseases in the developed world (14,15). Furthermore, a growing recognition among developed countries that they are vulnerable to diseases that might emerge from developing countries, as well as genuine humanitarian concern for the condition of those in need, has led to a greater interest in diseases endemic to the developing world (14-16). Tremendous progress has been made against several diseases, including the development of anti-retroviral therapy which has dramatically cut the number of deaths from AIDS (17,18), and the development of new vaccines against devastating infections, but these developments have not always been translated to the developing world (13,19). Rotavirus is the most common cause of severe childhood diarrhea in the world, accounting for 37 to 231 infant deaths per year in the United States and European Union (20,21) but between 400,000 and 500,000 deaths worldwide (22,23). Vaccines for rotavirus have been recently approved and are quickly being incorporated into pediatric vaccine schedules in developed countries but not yet in most developing countries (22,23). Human papillomavirus (HPV) vaccine has also become available and is rapidly being widely utilized in developed countries, but in the developing world, where HPV-related morbidity and mortality from cervical cancer primarily occurs, there are great challenges to making it available (24-26).
Fortunately, numerous philanthropic governmental and nongovernmental organizations are making great efforts to provide financial and technical support to increase the availability of such life-saving measures to developing countries. Examples include the Expanded Program on Immunization (EPI), which was developed in the 1970s by the World Health Organization (WHO) in collaboration with the United Nations Children's Fund (UNICEF) and others to provide basic vaccine coverage to children throughout the world (12), the Global Alliance for Vaccines and Immunization (GAVI), a public-private global health partnership created in 1999, which provides financial assistance for vaccines to children in the poorest countries of the world (27), the Global Fund to Fight AIDS, TB, and Malaria, which provides financing for projects designed to fight those diseases (28), and the President's Emergency Plan For AIDS Relief (PEPFAR), a 5-year US$15 billion initiative to rapidly augment AIDS treatment and prevention programs (29). Each of these programs has had both successes and failures and are justly critiqued (29-31), but the focus of governments on these health issues and the tremendous infusion of resources from private organizations, such as the Bill and Melinda Gates Foundation, has resulted in renewed optimism (12,27,31).
In this article, we will explore the current status of the "epidemiologic transition," discussing the methods credited for achieving this transition, some of the barriers that have slowed the transition in the developing world, and some of the difficulties in even measuring the burden of disease and the progress of the transition. We will also review several of the most common diseases, including strategies for prevention and efforts to establish surveillance systems to monitor for emerging diseases or to target appropriate interventions.
Historical perspective
In the United States in 1900, infectious diseases accounted for 44% of all deaths, led by pneumonia, tuberculosis, and enteric infections (diarrhea and typhoid fever) (32,33). The burden of illness fell especially hard on the young, with 30.4% of all deaths occurring in children less than five years old (32). Remarkable progress was made over the early 20th century to alleviate this burden, so that by 1936, only 18% of deaths were due to infectious diseases (see Table 2) (33) , and by 1997, when only 1.4% of deaths were in children less than five, the leading infectious causes of death, pneumonia, influenza, and HIV, only accounted for 4.5% of deaths overall (32). This progress was largely due to interventions started in the 19th century with the development of the "germ theory", an understanding that microorganisms are the cause of many diseases (34). Following the development of the germ theory, there were significant advances in three main areas which led to dramatic declines in death rates due to infectious diseases: sanitation and hygiene, immunizations, and antibiotics (32,34).
Sanitation and hygiene
In the 19th century, there was a steady migration from rural areas to urban areas as the Industrial Revolution created more economic opportunities in cities across Europe and the United States (33,35,36). The increased populations quickly overwhelmed the basic infrastructure of those cities, causing overcrowding and increased exposure to contaminated food and water, leading to increased mortality, an effect termed the "urban penalty" (33,36). (This same "urban penalty" is currently being described in developing countries experiencing similar urban migrations) (37-39). In the late 1800s in the United States, infant mortality was 140% higher and life expectancy was 10 years shorter in urban areas than in rural areas (33). The situation was similar in Poland (36). Although the germ theory was just being developed, people recognized that inadequate sanitation and hygiene were associated with disease, often attributing the cause of illness to "miasmas," poisonous vapors that were offensive to smell and were thus thought to emanate from contaminated water and filthy areas (33). This recognition led to many efforts, some focused on the individual, some municipal, and some international, to improve sanitation and hygiene (32,33,40).
In 1850, the Report of the Sanitary Commission of Massachusetts 1850, also called the Shattuck Report after the chairman of the commission, Lemuel Shattuck, was published with comprehensive recommendations to improve and protect health (41,42). The Shattuck Report made broad recommendations for both municipalities and individuals for the promotion of health, reflecting the prevailing attitude that the personal aspects of hygiene involved proper considerations of personal cleanliness, food, water, clothing, work, exercise, and other personal habits (41-43). Concurrently, numerous organizations were emerging dedicated to improving personal hygiene, some as part of educational curricula (43), some as part of Christian cleanliness movements, (44) and often simply to improve health (33). Many of these campaigns focused on food preparation (hand washing, boiling milk, etc.), breast-feeding, and personal hygiene, and there is evidence that these actions by individuals and households had some success in decreasing disease (33).
At the same time, large scale interventions were being implemented by municipal governments, including the institution of animal and insect control programs, which were very successful in reducing malaria and eliminating the dog-to-dog transmission of rabies; milk pasteurization and meat inspection, dramatically cutting rates of food-borne illnesses, including tapeworms and trichinosis; and sanitation services, refuse management, and clean water technologies (33). Each of these interventions was beneficial, and it is difficult to separate out the effect of each intervention, but it appears that the advent of clean water technology, filtration and chlorination, was the most important public health intervention of the 20th century (33). Cutler and Miller recently published an elegant analysis of mortality data in 13 major U.S....
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