Tracking an epidemic with the glow of the night sky

A Princeton researcher is using satellite images of the night sky to track the transmission of measles in Niger.

Postdoctoral researcher Nita Bharti said it is hard to get good seasonal population data for the West African country.

So, Bharti turned to images of the night sky. She said comparing images in three large cities with government measles data supported her hypothesis that population shifts were causing seasonal outbreaks.

“What we found was that the brightness levels in each of the cities increased drastically during the dry season and decreased during the rainy cities,” Bharti said. “Which exactly matched what measles transmission was doing in big cities.”

Many people in the country move to cities from the countryside during the dry season, when agricultural work is unavailable.

Pej Rohani, professor of infectious diseases at the University of Michigan, said in rapidly moving populations like these, it can be hard to get good population numbers.

“In less mobile populations, it’s more straightforward to rely on standard census methods to know how many people might be present,” said Rohani, who is unaffiliated with the research. “But when you have mass-movement effects, that happen quickly, then it’s difficult to know how many people might be present.”

Bharti said the method could help public health officials plan vaccination campaigns.

Though measuring light cannot give researchers absolute population numbers, Bharti said more nighttime light can show relative population growth, the important factor in the spread of measles.