Sunday, July 09, 2006

Public Health Surveillance

Dr. Snow had done public health surveillance for many years, beginning with the days when disease reports arrived on postcards or by telephone at the Newtopia Health Department, and were further evaluated if they seemed to represent unusual numbers or types of cases. Shoeboxes full of cards were counted by hand at the end of the year for the annual report to the national level.

The reportable disease system functioned in the same way after computers arrived, but counting the reports no longer required hash marks and surveillance secretaries. The list of reportable diseases was still comprised of 50 to 60 mostly rare conditions that are a tiny fraction of the total disease burden, although their role as preventable and potentially epidemic diseases assures their place in the surveillance system.

Dr. Snow takes her job seriously, and often thinks about the 33,000 deaths per year in Newtopia as her responsibility to prevent. She sees patterns in disease by assigning time windows and applying filters, looking first for patterns of disease and then for risk factors that might be related and could be controlled.

Google Earth has evolved to the point where every building in Newtopia is visible, now that airliners carry cameras that feed the digital world image. Of course, data on disease are confidential, and she treats geographic coordinates, names and addresses, with respect. Data from death certificates are less sensitive in most states.

This morning Dr. Snow reviews deaths and hospitalizations for the past day, week, and month by applying filters of different colors and sizes that she has previously configured. The display is Google Earth showing Newtopia, but floating above the earth are "buildings"of various sizes made in Google SketchUp, but configured programatically. The volume of a building reprsents the number of cases or episodes, and its height represents a rate (cases per population specified by the population filter). There is a very tall thin building over a town called Haight's Corners (pop 58) where one death occurred, but a more substantial structure hovers over Newtopia's capital, with a population of 1 million and many deaths per day.

The display is clearest when a single set of conditions such as "Head Injury" and "past 3 days" is selected, but the buildings can also assume colors and textures to display more than one condition. On one side of the computer screen is a control "building" for choosing scale and changing the significance of the dimensions and colors. Data sources on hospital admissions, emergency visits, deaths, and insurance claims are shown as choices. In the adjacent piece of "land", one can choose filters for the denominator population to represent particular age and sex groups, smokers, people with credit cards, telephones, etc. For many of the factors, estimates are only available for large areas, but the program does the best it can to estimate. New databases are discovered frequently and added to those available as "risk factor filters."

The top of each building has a statistical "tower" representing the height at the 95th or 99th percent confidence limit, with the corresponding lower limit being that far below the top. Various other options are available for the statistical displays.

Dr. Snow sends a couple of views to her assistant for further investigation, but today there is nothing dramatic in the surveillance overview.

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