A 3D plot shows the nighttime brightness of areas of Niger over the course of a year. Image courtesy Science/AAAS.
The intensity of light shining from cities at night could help identify hot spots where outbreaks of infectious disease are likely to take place.
A team of researchers tracked satellite images of three cities in Niger and found that fluctuations in nighttime brightness were strongly correlated to measles incidence, according to results published in this week's Science.
Measles outbreaks tend to fluctuate with the seasons, usually spiking in the dry season in Niger from September to May. Migration patterns in many of the countries where the disease is still widespread also vary with the seasons, as people spread out to work on rural farmlands. When the rains stop and less work is available in rural areas, people return to more concentrated communities in search of other employment.
The movement of populations has long been hypothesized as part of the measles outbreak pattern, but tracking seasonal population migration is difficult. That's where the satellite images come in -- changes in concentration of nighttime light reflected changes in population more accurately than past data tools like cell phone records or census estimates.
"We turned to this technique because there is really no other way to get any idea of how populations are changing in a place like Niger," said lead author Nita Bharti, a Princeton University evolutionary biology researcher. "That's true throughout most of sub-Saharan Africa and a lot of other places in the world."
The research team, supported by the Bill and Melinda Gates Foundation*, used satellite images taken by the U.S. Department of Defense between 2000 and 2004 and data from Niger's Ministry of Health to connect the two patterns.
The same tracking of nighttime light could be used for other diseases as well, the team wrote, and could help public health officials plan for emerging epidemics and predict outbreaks.
"Measurements of fluctuations in population density provide important information to guide decisions on disease control strategies, international aid and humanitarian responses, and assessments of economic development," the authors concluded.
Measles are highly contagious and are still a leading cause of death for children around the world, despite the availability of an effective vaccine.
The satellite images could also be useful for planning vaccine campaigns, the authors pointed out, because they are not always timed to the height of the seasonal population growth in cities. They cited one example in 2004, when a mass vaccine campaign was launched just as nighttime brightness, and thus population, was decreasing in the city.
The quick availability of recent satellite imagery could allow public health officials to revise their plans for prevention and response to infectious disease outbreaks in the future.
*For the record the PBS NewsHour's global health coverage is funded by the Bill and Melinda Gates Foundation.
