Without Borders; and Ali Djibo of the Niger Ministry of Health.

The trouble with tracking migratory populations

Deborah Balk, a professor at the City University of New York and associate director of the university's Institute for Demographic Research, said the researchers' use of city-scale nighttime-lights imagery to examine the spread of disease is "pathbreaking" and offers significant advantages over more common techniques.

Images of nighttime lights have typically been used to study urbanization and economic development, as well as physical-science questions, said Balk, who is familiar with the project but had no role in it. In this case, Bharti and her collaborators use the nighttime brightness data to illustrate seasonal population swings, information that other types of satellite data such as images of housing density cannot detect, she said.

"Temporary and seasonalfor each city during the agriculturally busy rainy season, then rose to above average as people packed urban areas during the dry season. Measles transmission rates followed the same pattern – low in the rainy season, high in the dry.

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. In Niamey, measles cases were recorded daily for three districts, or communes, during the 2003-04 dry season. Brightness and measles infection both peaked early in the first and second communes in February and March of 2004.

A two-week mass-vaccination campaign was launched in March and April, but population density, as determined by light brightness, had already started to decline in communes 1 and