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AEI-Brookings Joint Center Policy Matters 03-07 It is not the first such outbreak, and it will not be the last. Before SARS, human immunodeficiency virus, or HIV, rose from obscurity in the 1970s and ‘80s to become a global public health crisis, leaving millions of orphans in its wake. Outbreaks of ebola, of hantavirus pulmonary syndrome, and of Escherichia coli O157:H7 in food and water have also appeared in recent years. And, of course, the flu still has surprises in store, such as the avian (“bird flu”) strains that have infected humans in recent years. How do we get perspective on this terrifying new epidemic? Perhaps the first goal is to demystify the process by which SARS and other outbreaks occur. Understandably, these events––”new” infections that appear suddenly and unexpectedly––seem frightening and random. In ages past, when plagues appeared, they were thought of as portentous acts, retribution for past sins. More recently, we’ve had the “Andromeda strain” of plagues from outer space as a variation on this theme of capricious and unpredictable visitations of disease. Yet while these events seem random and unpredictable, they are not. Emerging infections have identifiable sources. Many (including all the examples in the second paragraph) already existed in nature and have simply gained access to new host populations. Often this is a result of changed ecological or environmental conditions that place humans in contact with previously inaccessible pathogens or the natural hosts that carry them. I coined the term “viral traffic” (or, more generally, microbial traffic) to describe these processes. Major demographic changes (such as population migrations) will often precipitate emergence. Many of these changes are caused (usually inadvertently) by human activities, such as clearing land for agriculture. Think of emergence as a two-step process: the introduction of novel infection into a human population, followed by its dissemination. The many examples of infections originating as zoonoses (infections transmitted from animals to humans) suggest this is an important and potentially rich source of emerging pathogens. HIV is one probable example of an interspecies transfer to humans. Although we do not yet know the sources of the SARS virus, it may well have entered the human population like many other novel infections––as a result of new opportunities for contact with infections that are carried by other species. For example, the hantaviruses that cause hantavirus pulmonary syndrome in the Americas are all natural infections of common wild rodents. Hantavirus pulmonary syndrome––which is characterized by acute respiratory distress and is fatal in 40-60% of cases––was first recognized in a notable outbreak in the southwestern U.S. in 1993. Scattered cases had occurred for years throughout North America, but they went unrecognized until the larger outbreak, initially diagnosed among Navajos, focused attention on the disease. It was later learned that changing environmental conditions had led to a rodent population explosion and thus increased human contact with the animals and the viruses they carried. Once a pathogen has been introduced into a population, its success depends on its establishing itself and disseminating. Both chance and the evolutionary potential of the pathogen play a role here. Many zoonotic introductions cause serious disease. Fortunately, however, few newly emergent infections spread readily from person-to-person: The hantavirus, for instance, depends on contact with rodents or their droppings for each new human case. On the other hand, any infections that do spread readily from person-to-person should be followed very carefully and controlled before they can gain a foothold. SARS has been notable in its ability to spread, at least under conditions of close contact, and is therefore worthy of the attention it is receiving from the world public health community. We don’t yet know when or how SARS first entered the human population. The most likely scenario, however, is that it has been circulating in another species in southern China, and human beings came in contact with it this past autumn, perhaps in an agricultural setting. It is interesting that this part of Asia is the same geographic area from which most known influenza pandemics have arisen. Every few decades, influenza A undergoes a major antigenic change in one key protein and a pandemic results. The process involves a reassortment of viral genes from different influenza strains. It has recently been found that the greatest diversity of influenza viruses is in wildfowl, such as ducks. A number of virologists believe that pigs are an important “mixing vessel” in the next step of the process, allowing a novel influenza virus to make a transition from birds to humans. South China has long practiced a unique traditional form of agriculture that places pigs and ducks in close contact. Writing in the journal Nature, Christoph Scholtissek and Ernest Naylor have suggested that this form of agriculture may facilitate the development of new influenza reassortants by placing ducks––which are the reservoir of influenza diversity––and pigs, which are the possible “mixing vessels” for mammalian influenza strains, in close proximity with each other and with humans. There are still many tantalizing questions about the sudden appearance of SARS. Why now, and not earlier? What is its original source in nature? Why did SARS first appear in this geographic area––are there any ecological similarities between the mechanisms responsible for SARS emergence and those driving influenza emergence (the proximity of certain key species in close contact with humans, for example)? Had SARS been circulating in the human population in the past, before venturing beyond its initial boundaries? The unfolding scientific and field investigation should provide valuable insights into many of these questions. We need this information in order to develop better early assessments of potential disease risks and find strategies for preventing new introductions. Once hantavirus pulmonary syndrome was recognized as coming from rodents, health authorities could suggest some simple ways to reduce risk, for example by making homes less hospitable for rodents and by taking precautions, such as covering the mouth and nose, while cleaning out rodent nests. Once the sources of the SARS virus are known, it may well be possible to develop similar protective measures. If SARS is maintained in nature, as is likely, however, it can be reintroduced periodically from its natural sources. Only by understanding the ecology of diseases like SARS can we act to prevent repeats of these outbreaks in the future. This article was originally published in the Wall Street Journal on April 22, 2003. |