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The task of the epidemiologist is to determine the causative and associative factors of a disease or phenomena, identify elements affecting its rate of incidence and establish a means of control. A tool that is becoming more widely used in accomplishing this is the geographic information system, with its capabilities for complex mapping and spatial analysis. GIS enables epidemiologists to analyze associations between environment, location and disease; map the geographic spread or containment; pose hypothetical questions; and identify risk factors and spatial patterns that might otherwise go unnoticed.
FIRST MAPPING IN EPIDEMIOLOGY
The first known application of mapping in epidemiology was the classic study by English physician, John Snow, who discovered that cholera is transmitted by contaminated water. During the London epidemic of 1854, Snow mapped the locations of cholera deaths and saw that nearly all were in the vicinity of Broad Street, in the Golden Square district of the city. Further investigation revealed that nearly all of the victims had drunk water from the community pump on Broad Street. At Snow's insistence, the handle of the pump was removed, and the epidemic that took nearly 600 lives in five days ended.
PROBLEMS OF MODERN EPIDEMIOLOGY
The fight against emerging infectious diseases today is often global and exponentially more complicated. Expanding populations, communities displaced by wars and shifting economies, jet travel, the shipment of products across oceans and continents -- all contribute to the spread of infectious diseases and the dissemination of new strains.
Some of the more noticeable are AIDS, the resurgence of measles (about 38 percent imported from Europe, 39 percent from Mexico); TB in the form of multiple, antibiotic-resistant strains, and more recently the appearance of a strain of Ebola virus (Ebola Reston) in Texas that kills only monkeys ... for now. As more lethal forms appear, and new, resistant strains emerge, the ability to quickly and accurately analyze and predict their spread over space and time becomes a critical factor in public health protection. The emerging role of GIS in epidemiology is helping to accomplish that.
SHARED RESOURCES PROVIDE ADVANTAGES
Equally important in the fight against infectious disease is the sharing of information, methods and resources among public health agencies, universities and companies working with GIS. Such efforts often accelerate solutions, produce new approaches and insights, and lower operating costs. In Southern California, for example, the San Bernardino County Department of Public Health and Loma Linda University's School of Public Health joined in a cooperative effort with ESRI to enable an intern to conduct a GIS analysis of the county's 1989-91 measles epidemic. The study produced new insights, and a database that will assist the department in controlling future outbreaks.
THE SAN BERNARDINO EPIDEMIC
As an intern in research epidemiology, William Hoffman was particularly interested in the contributing factors of the epidemic, which coincided with the sudden resurgence of measles nationwide. From 1989-91, the number of cases in San Bernardino County jumped to over 2,000 (4 percent of all cases in the U.S.), up from a total of 28 cases for the period 1981-85; a seemingly disproportionate representation for a county of desert and mountains nearly three times the size of New Jersey, but with a population of only 1.4 million. However, 95 percent of the cases occurred in the cities of Barstow, Fontana and San Bernardino, where most of the population lives.
According to Hoffman, the objective of the study was to demonstrate the utility of GIS as a planning tool for the prevention and control of future measles epidemics in San Bernardino County. Since the department did not have GIS until 1995, it had conducted an investigation of the epidemic using conventional, descriptive statistics.