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| Home > 125th > Articles > USGS Science -- Yesterday, Today, and Tomorrow | September 22, 2008 | |||||||||
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Geographic Analysis of Disease By Stephen Guptill Health problems caused by emerging infectious diseases are a growing concern worldwide among governments, scientists, the news media, and the public. With geographic distance and isolation no longer meaningful barriers, the opportunities for once-isolated diseases to spread have never been greater. The emergence of so many new human diseases in recent years is a result of our densely populated, highly mobile, and environmentally altered world. Dealing with emerging diseases requires the ability to recognize pathogens when they first appear and to act appropriately. Zoonoses are human diseases that are shared with wildlife species; wildlife can act as a carrier or reservoir of human infection, often through an invertebrate vector such as mosquitoes. Since outbreaks often are evident in the nonhuman components of the environment before people are affected, understanding environmental health is a prerequisite to protecting human health. Today, geographic information systems, remote-sensing satellites, and other technologies are providing scientists with the tools and the data to make clear the geographic relationships between environmental habitats of disease hosts, vectors and pathogens, and the occurrence of disease. USGS can monitor and model environmental and habitat changes, such as altered land-use patterns and urban growth, which increase the rate of human exposure to zoonotic or vector-borne infections. While health professionals can effectively analyze the incidence and direct cause of illness, they may lack the information and expertise to relate the occurrences of diseases with the environment. Public health agencies need information on the source, occurrence (in space and time), transport, transformations, and fate of the pathogens that cause public health problems. They need to know what influencing factors such as human population density and distribution, land and resource use, soil type, climate, animal reservoirs, and vectors affect the risk of disease spread.
The recent West Nile virus (WNV) outbreaks in the United States highlight the shortcomings in our understanding of the complex geographies of vector-borne diseases. This epidemic illustrates our limited ability to detect the presence of pathogens in the environment and to measure and predict their extent and virulence. It is clear that research is needed to obtain a better understanding of bird migration as a mechanism of virus dispersal, vertebrate host/vector relationships and range, and the current and future geographic distribution of WNV. Scientists from the USGS and the Centers for Disease Control and Prevention have formed a collaborative team to address these research challenges and test various spatial hypotheses about the WNV outbreak. We are using advanced techniques of ecological and spatial analysis to describe, model, and develop forecasting techniques for such events. We are most intensively studying the various host systems of the disease process and the environments in which vector/host interactions occur. A 3-year program of active avian surveillance (capture and testing of migratory birds) began in spring 2001 along the Atlantic Flyway; the Central/Mississippi Flyway was added in spring 2002 as WNV moved west. Birds were captured at a series of sites located on Federal property (e.g. parks, wildlife refuges, military bases) spaced along avian flyways. This system, based on the ubiquitous presence of birds and their potential -- through exposure -- to indicate pathogen diffusion across the eastern American landmass, provides a mechanism to detect novel pathogens in the environment, their geographic extent, and linkages to the landscape.
In addition to blood samples, feather samples were taken from each bird. This will allow us to test for naturally occurring stable isotopes in the biological material. Analysis of the various isotopes of hydrogen, carbon, and nitrogen along with trace metals may enable us to determine the location of the bird during the breeding season. If this is possible, it will provide important information regarding the movement of the birds and hence the movement of WNV. The surveillance system provides the basic information on the "geography" of the virus. Beginning in Spring 2002 a program of long-term systematic mosquito surveillance (weekly collections at seven sites) was begun in St. Tammany parish in Louisiana. This complements the avian collections done at Bogue Chitto and Big Branch National Wildlife Refuges in the parish. The collection sites have been placed in various ecological habitats. Three traps are used at each site. In addition, data loggers record temperature, humidity, and rainfall information for each location. Data from this surveillance activity will be used to study the distribution and number of mosquito species in relation to land cover, weather conditions, and presence or absence of WNV in the area.
Results from the survey studies will be used with modeling procedures to develop an integrated process model of the disease that can predict the presence and abundance of WNV for the region. Geospatial databases will provide information on land cover, land use, soil characteristics, terrain, demographics, and socio-economic factors. Weather information (temperature, precipitation) will be obtained for the study sites as factors that may influence the abundance of reservoirs and vectors, locally, when combined with the land characterization data. Land characteristics of known WNV positive and negative sites along with environmental characteristics will be used to estimate the distribution and frequency of WNV in vector populations in unsampled areas. While this effort is focused on WNV, the methods and techniques being developed are not unique to this virus. We expect that the knowledge, experience, and procedures will be applied to other pathogen/vector/host systems to help protect the health of people and environments across the United States. |
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