In West Africa, changes in population density – measured by the glow of nighttime lights seen from space – can help predict the spread of diseases.
Cases of the measles were the most prevalent when a city's lighted area was largest and brightest. The new satellite imaging study could help health officials plan vaccination efforts to better control a major cause of childhood death in West Africa.
Epidemics of infectious diseases can fluctuate from one season to the next, but infections tend to drop at the start of the rainy season, spiking in the dry season – which might be due to changes in population density during different seasons.
Cities swell up during the dry season when agricultural work is unavailable, but thins out when it starts to rain and people move back to farm in rural areas.
Princeton’s Nita Bharti and colleagues looked at seasonal fluctuations in human population density – based on the brightness of light sources like electric lighting and fires.
The images of 3 cities in Niger – Niamey, Maradi and Zinder – were taken between 2000 and 2004 by a US Department of Defense satellite, part of the Defense Meteorological Satellite Program's Operational Linescan System.
"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,” Bharti says. “That's true throughout most of sub-Saharan Africa and a lot of other places in the world.”
Combining those with disease models and 10 years worth of measles reports, the researchers show a high correlation between measles transmission rates and population density.
Urban centers frequently become host to outbreaks of crowd-dependent diseases. Cities had below average brightness during the agriculturally busy rainy season, then rose to above average as people packed into urban areas during the dry season. Measles transmission rates followed the same pattern: low in the rainy season, high in the dry.
(Pictured here, a 3D plot of Niger showing the brightness over the course of a year.)
Migratory populations are notoriously difficult to track, Bharti says, making it more difficult to carry out large-scale vaccinations.
Measles kills around 164,000 unvaccinated children per year. New Scientist reports:
Between 2000 and 2008 an aggressive vaccination campaign in poor countries cut deaths by 80% globally, and by a massive 92% in Africa. Yet in 2008 economic cutbacks hit vaccination, and measles – the most contagious human virus known – began to bounce back, with a resurgence in west Africa seen as a particular threat.
"These diseases are geographically specific, for the most part, to areas where this would be a useful technique,” Bharti adds. “These are places that are not so industrialized that they will always be saturated with brightness and where there may be some level of agricultural dependence so that there are detectable labor migrations."
The study was published in Science this week.
This post was originally published on Smartplanet.com