Kristin Harper; George Armelagos

“ The last transition in this model, the third, is the one that we are entering now. It is characterized by the continued prominence of chronic, non-infectious disease now augmented by the re-emergence of infectious diseases. Many of these infections were once thought to be under control but are now antibiotic resistant, while a number of “new” diseases are also rapidly emerging. The existence of pathogens that are resistant to multiple antibiotics, some of which are virtually untreatable, portends the possibility that we are living in the dusk of the antibiotic era. ”

“ As populations aged thanks to the reduced infectious disease threat, they began to experience a concomitant rise in chronic disease. Deaths caused by chronic conditions such as heart failure, cancer, and diabetes became much more common during this time period; as more and more of the world experiences this transition, the burden of mortality and morbidity increasingly shifts to such causes. ”

“ In this way, epidemiologists may be able to use the model to anticipate future trends in the areas they study. For example, as the age of onset of chronic diseases is delayed in newly high-income countries experiencing the third epidemiological transition, it should be possible to plan for shifting disease burden by allocating more funds to research and health care provision for geriatric disorders. Similarly, by recognizing the distal causes that shape the epidemiological transitions, it should be possible to craft health policies that simultaneously address multiple outcomes. ”

“ Globalization is not a new phenomenon, but it is one that is continuously intensifying. William McNeal has described its centuries-old roots and its prominent role in driving change in the global disease-scape [119] . Perhaps the most dramatic disease consequence of early globalization occurred after European explorers initiated contact with the Americas; it is estimated that millions of Native Americans died of diseases such as measles and smallpox as a result. Today, dramatic examples of the spread of disease linked to globalization abound. ”

“ In addition to considering evolutionary factors in the emergence of disease, we will incorporate the spread of drug resistance and explore the implications of globalization in expanding the threat posed by some diseases. We also discuss how the epidemiological model allows us to examine the recent rise of certain classes of disease, such as the frequency of allergies and autoimmune diseases, in high-income countries. Finally, we discuss the ways in which increased attention from epidemiologists may clarify and improve the model. ”

“ Often, examples of infectious diseases crossing the Atlantic garner the most attention. For example, SARS spread rapidly from its epicenter in East Asia to North America, as ill passengers went about their travel plans. Many of the most important results of globalization, with regard to disease burden, attract much less notice however. For example, lung cancer rates are increasing in many low-income countries just as they are decreasing in many high-income countries, as cigarette manufacturers in the latter nations shift their marketing focus [120] . ”

“ The importance of economic disparities is illustrated through a comparison of health statistics from the Americas and Africa. Combined, North and South America employ 37% of the world’s health workers, spend more than 50% of the money expended on health worldwide, and host 10% of the global burden of disease. ”

“ This could be achieved by manipulating the factors that underlie the overnutrition associated with the second epidemiological transition, for example, to reduce the incidence of diseases like diabetes, stroke, heart failure and cancer.
Like any model, this one is continuously being modified, and there is no doubt that it could benefit from greater attention from epidemiologists. The model rests on our understanding of the increase and decrease in different types of diseases. But dividing diseases into “types,” such as infectious vs. degenerative or human-made, is a difficult exercise. ”

“ As the model of epidemiologic transition became increasingly popular and drew more attention, modifications to it began. Both the demographic transition and Omran’s epidemiological transition theories assumed that the earliest human populations were rife with disease, resulting in communities in which extremely high mortality was compensated for by maximum fertility. ”

“ In many of the disciplines mentioned above, in which the epidemiological transition model has been most influential, investigators are primarily concerned with health writ large or all-cause mortality rather than individual diseases. ”

“ In the second stage, populations experience a decrease in mortality rates while fertility rates remain high, resulting in a rapid increase in population size. In the third stage, mortality rates are low and birth rates begin to decline, resulting in slowed population growth. In the last stage, low mortality and fertility rates result in no increase in population size. Epidemiologic transition theory explains an important component of the demographic transitions by describing the varying patterns of disease that are important contributors to mortality. ”

“ For this reason, Farmer emphasizes that even the more holistic of ecological perspectives, those that consider human behavior as well as microbial changes in seeking to understand the emergence of novel pathogens, fail to place the process of disease emergence in a political-economic context. ”

“ The First Epidemiological Transition 4.2.
The disease-scape changed dramatically after the adoption of agriculture. New proximity to domestic animals created many opportunities for novel pathogens to infect, and eventually adapt, to humans. It has long been thought that many of our most feared diseases (anthrax, tuberculosis, Q fever, brucellosis, smallpox, measles, etc.) emerged at this time, evolving from progenitors contracted from goats, sheep, cattle, pigs, and fowl. ”

“ The disease ecology of contemporary hunter-gatherers also provides a model for that of Paleolithic foragers. For example, from studies of contemporary groups we understand that the population size reached by foraging groups would be too small to support continuous transmission of many of the “crowd” diseases. For example, infections such as influenza, measles, mumps, and smallpox require too large a host population to have been present in the Paleolithic disease-scape [60] . ”

“ Meredith Turshen [33] describes these ecological model as “bourgeois” because of their failure to incorporate economic factors such as poverty and inequality, which bring individuals into the path of disease vectors. In this view, these models focus on the proximate cause of disease (the pathogen) without considering the ultimate causes of disease: sociocultural factors [34] . For example, such models fail to consider the way in which large-scale social forces influence economically disadvantaged individuals [35] . ”

“ Anopheles minimus, the malaria vector in this area, has a flight pattern of about three feet above ground and, thanks to the stilts, contents itself with feeding on the livestock kept below the homes. When mosquitoes do stray above the usual three feet, the smoke in the house serves as a further deterrent to keep them away from its inhabitants [49] .
The example of the highland Vietnamese scenario demonstrates how culture can act as a buffer to disease transmission, shielding the population from insults that originate in the environment. ”

“ This shift occurred as a result of higher standard of livings and the introduction of medical and public health practices in high-income nations undergoing industrialization. This epidemiologic transition, according to Omran, occurred in three successive phases, as described above, and took about two centuries until completion in Western Europe and the United States. Omran’s model focused on a major change in the pattern of health and disease and explored how these patterns were influenced by demographic, economic, and sociological factors. ”

“ It is possible that yaws, a non-sexually transmitted disease caused by a bacterium related to the one that causes syphilis, is also an heirloom disease; it infects our closest relatives, chimpanzees and gorillas [69] , [70] , and genetic analyses indicate that it has an ancient origin in humans [71] , [72] .
In contrast to heirloom species, “souvenir” species are those that are “picked up” along the way, as hominids carry out their daily activities. Souvenir species are usually zoonotic pathogens whose primary hosts are non-human animals; they infect humans only incidentally. ”

“ While the decline in competitor-infections may serve as a partial explanation for the prominence of STIs in the communicable disease burden of high-income countries, the stigma associated with seeking treatment as well as the difficulty in making the type of behavior changes that reduce risk must certainly also play a role. ”

“ The hygiene hypothesis argues that in high-income nations the lack of childhood exposure to infectious pathogens and symbiotic microorganisms increases susceptibility to allergy and similar diseases caused by alterations in the immune system [96] . The theory holds that during the Paleolithic, the impact of infectious diseases was rather minimal but foragers were exposed to many saprophytic microorganisms present in the soil and in decaying plant matter. ”

“ Which habitat represents the environment from which we emerged has important implications for understanding our earliest disease profile, since that of a tropical grassy woodland would be quite different from that of a savannah environment. Inhabitants of a grassy woodland, for example, may have been more likely to encounter the Anopheles mosquito that transmits malaria. ”

“ The Pleistocene disease profile is even more difficult to reconstruct than diet. There are relatively few skeletal remains from this period, and even fewer bear osseous signatures of disease. For this reason, we have to rely primarily on theoretical reconstructions based on our knowledge of hominine habitat, the pattern of disease found in contemporary forager populations, and genomic analyses of humans and their pathogens. ”

“ Thus, in epidemiological studies emerging diseases are usually conceptualized as individual entities that can be attributed to particular proximate causes, rather than members of a suite of diseases that are increasing simultaneously due to common, ultimate causes. ”

“ In addition, establishing whether there is an increase or decrease in the incidence of a disease is a difficult problem well known to epidemiologists, who are used to considering factors such as ascertainment bias. ”

“ Changes in nutrition and food handling would also have altered disease risk. The shift to agriculture resulted in a reduction of the dietary niche, which would have predisposed many individuals to dietary deficiencies uncommon in the Pleistocene. ”

“ As discussed above, small population size would have precluded sustained transmission of many bacteria and viruses, though Cockburn [68] has argued that some viral diseases found in non-human primates could have been easily transmitted to early hominids. Malaria may also fall into this category. If malaria was contracted by humans in the Pleistocene, it likely would have been in isolated incidences. ”

“ In addition to an increase in the chronic disorders listed above, many researchers have noted an apparent increase in allergies and diseases of the immune system in the last half of the 20th century [94] , [95] . ”

“ Alkire, NL; de Queiroz, A & Jones, A (2001) , “Out of Africa: origins of the Taenia tapeworms in humans”, Proc. R. Soc. Lond. B Biol. Sci 268: 781–787 ↑ Laird, M & Service, MW (1989) , “Vector-borne disease introduced into new areas due to human movements: A historical perspective”, Demography and Vector-Borne Diseases (Boca Raton, FL, USA: CRC Press Inc.) ”

“ Aranaz, A & Behr, M (2002) , “Genomic deletions suggest a phylogeny for the Mycobacterium tuberculosis complex”, J. Infect. Dis 186: 74–80, 12089664, pmid:12089664 ↑ Audy, JR & May, JM (1961) , “The ecology of scrub typhus”, Studies in Disease Ecology (New York, NY, USA: Hafner Publishing) : 389–432 ↑ Livingstone, FB (1958) , “Anthropological implications of sickle-cell distribution in West Africa”, Am. ”