Risk parameters associated with urbanization

How urban social ecology affects emerging disease risk, especially the underlying mechanisms and dynamics involving environmental, human behavioral, and other factors, is poorly understood. However, a number of the parameters affecting risk are known. The risk for urban infectious disease outbreaks is greatest not only where the population density is highest, but also where people, public infrastructure, and public services are poor, and where access to medical care and basic public health programs does not keep pace with population growth. This includes cities in many of the same countries in the low- to moderate-income category just described, which typically are severely overcrowded. Poor neighborhoods lack safe and adequate housing, as well as reliable clean water, sewage disposal, and waste management. Public health is usually underfunded, and surveillance is non-existent or primitive. Generally, the supply of trained medical and public health professionals is insufficient to meet basic public needs. And because all the districts and neighborhoods in a large urban area are linked by modern transportation systems, pathogens can circulate with ease. Ironically, the construction of modern transportation systems intended to support modernization and economic development ensures the mixing of infected and susceptible people at a historically unprecedented rate. Many large urban complexes in developing Asia, for example, where highway and mass transit systems were lacking a decade ago, either now have them or soon will. Moreover, these are linked to transportation networks connecting even the most distantly separated urban centers. So the possibility now exists that an infectious disease outbreak within a neighborhood will spread readily not only throughout the city and to the surrounding areas, but also across the country and beyond national borders. This is of course exactly the scenario describing the near pandemic of SARS in 2003. Once this novel pathogen, whose source may have been fruit bats in Southern China, emerged near the rapidly growing Guangzhou in Guangdong Province (China), it easily spread to Hong Kong by surface transportation, and to Singapore, Vietnam, and even Toronto by infected air travelers, in barely a few days. In 2004, approximately a billion people traveled by air, with an ever-increasing number of them going to and from fast-growing cities in the tropical developing world. From 1983 to 1994, the number of air passengers leaving the United States doubled from about 20 million to 40 million, and more than half went to tropical countries. With modern laboratory analyses, it has been possible to track these epidemics across the urban hubs of common world travel and trade routes.

Although the global spread of epidemic pathogens tends to occur episodically, serious disease-causing pathogens now regularly erupt in cities and spread outward to the surrounding less densely populated areas of the country. In today's booming urban economic centers of Asia, like Guangzhou (approximately 9.5 million in the metropolitan area), Ho Chi Minh City (about 5.4 million), and Bangkok, much of the labor force is almost constantly on the move to and from cities and the country. The magnitude of human movement in and out of cities in developing countries cannot be overstated. A significant proportion (often the majority) of the urban population is of non-native origin and migrants. These people immigrating to the urban centers seeking economic opportunity still call "home" the small cities, towns, and villages of their origin, often because it is where their wives, children, parents, and grandparents live. On holidays like the New Year they travel from the city to their homes en masse. Each individual who travels home is capable of spreading a disease contracted in the city, and vice versa. Not long after the SARS outbreak in Southern China, and when the pathogen was apparently not yet contained, tens of millions of people took a holiday from work to travel home for several weeks during the Chinese New Year. Even with its highly centralized authority, the Chinese Government was largely powerless to control this annual mass migration. Fortunately, in this case, the disease was nonetheless contained or self-limiting.

As already mentioned, a critical population size is required to sustain an epidemic and for diseases to become endemic, the size depending on characteristics intrinsic to a pathogen. For example, this figure is about 250,000 for measles (Black, 1966; Anderson et al., 1992). For this and a number of other common diseases, relatively isolated cities of less than a few hundred thousand or fewer people cannot generate a sufficiently large and constant flow of susceptible (immunologically naive) people to fuel epidemics. Those that do arise as the result of an imported pathogen quite simply "burn out" quickly. For example, a number of Pacific Islands with small populations and limited tourism have had only limited outbreaks of dengue even though they have the Aedes mosquito vectors. Of course today, given the exponential growth in human mobility provided by modern transportation, even geographically isolated cities or human populations are becoming less and less biologically isolated. As this level of connectivity lowers the barriers to pathogen dispersal, the population size thresholds that once limited their continual transmission effectively cease to exist.

Even with effective family planning programs in place, demographic momentum ensures growth in most developing countries will continue until the middle of this century and, as illustrated by the Chinese New Year holiday example above, the capacity to restrict human movement is quite limited in most countries even after an epidemic has started, let alone before. Human population growth and mobility will remain a potent factor underlying disease emergence and re-emergence, as it has for the past three decades. This is clearly suggested for dengue in most Asian countries, where the mosquito vectors have been generally widespread and abundant.

Improved vector control could alter this situation in the future. However, eliminating the mosquitoes like A. aegypti, the primary vector for the pathogens responsible for several important emerging diseases, across large geographic areas or even single cities, likely is no longer possible. Today's urban conglomerations consist of tens of thousands and in some cases even millions of households, virtually every one potentially harboring this highly domestic mosquito. Just as with human mobility, the movement of commodities - between villages, provinces, cities, and countries - consisting of materials capable of harboring eggs, larvae, or adult mosquitoes has grown exponentially. Even if a mosquito population can be extinguished within a semi-urban village on a city's outskirts, or eliminated from an urban district, its absence will be only temporary. The constant influx of mosquito propagules insures the "empty patch" of domestic habitat will not stay unoccupied and unexploited for long. For example, a recent study of A. aegypti movement in Thailand and Puerto Rico (Harrington et al., 2005) showed that individual mosquitoes commonly dispersed actively only as far as adjacent households. Inter-village dispersal (that is, active flight of up to half a kilometer) was found to be rare. Yet passive long-distance dispersal via hitch-hiking on human transport was deduced to be common, based on genetic evidence demonstrating the ecological connectivity between populations.

In light of these and other recent research findings, it's clear today's sprawling and globally interconnected urban landscapes, and the limited effectiveness and ecological risks associated with insecticides, requires new approaches to mosquito vector control. Fortunately, disease control, and even prevention, can be accomplished without completely eliminating the vector, so long as its abundance and biting success is reduced. Keeping mosquito abundance below certain levels by managing the environment and influencing the frequency with which humans are bitten through various efforts can decrease epidemic frequency and severity, as well as preventing pathogens like dengue from expanding its endemic cycle geographically. Disease ecology theory suggests the rate of spread of an infection, including whether this rate is high enough to initially spark an epidemic, is sensitive to both mosquito density and herd immunity in the human population. In disease endemic regions, pathogens like dengue viruses circulate via "silent transmission" in the human population, erupting episodically due to the interaction of a variety of dynamic factors, including the proportion of susceptible people, seasonal changes in mosquito abundance and survival, and viral evolution.

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