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Satellite images of nighttime lights, which normally are used to detect population centers, also can help keep tabs on diseases in developing nations, according to new research. An international research team that includes Matthew Ferrari, an assistant professor of biology at Penn State, found that the new technique accurately indicates fluctuations in population density -- and thus the corresponding risk of epidemic -- that can elude current methods of monitoring outbreaks. The research is expected to help medical professionals to synchronize vaccination strategies with increases in population density.

Ferrari and his team used nighttime images of the three largest cities in the West African nation of Niger to correlate seasonal population fluctuations with the onset of measles epidemics during the country’s dry season, roughly from September to May. Because many pathogens that cause epidemics flourish in areas where the population density is the greatest, satellite imagery showing brighter areas -- indicating greater numbers of people -- then can be used to pinpoint disease hot spots. The images, taken between 2000 and 2004 by a U.S. Department of Defense satellite, were compared to records from Niger’s Ministry of Health of weekly measles outbreaks during the same years in Maradi, Zinder, and Niger’s capital, Niamey.

In many agriculturally dependent nations, such as Niger, people migrate from rural to urban areas after the growing season, explained Nita Bharti, a postdoctoral researcher at Princeton University and the first-listed author of the research paper. As people gather in cities during the dry-season months when agricultural work is unavailable, these urban centers frequently become hosts to outbreaks of crowd-dependent diseases such as measles. Because temporary and seasonal migrations are very hard to measure directly, the night lights are an important source of data for Africa and Asia, especially, where other sources of data are sometimes absent.

The team found that measles cases were most prevalent when a city’s lighted area was largest and brightest. "We found that seasonal brightness for all three cities changed similarly," Ferrari said. "Brightness was below average for Maradi, Zinder, and Niamey during the agriculturally busy rainy season, then rose to above average as people moved to urban areas during the dry season. Measles transmission rates followed the same pattern -- low in the rainy season, high in the dry season." The team members also found that the relationship between brightness and measles transmission appeared even clearer at the local level, as did the potential value of the researchers’ technique in providing medical treatment.