The movement of millions of people, monitored through their cellphones, has shed new light on how malaria is transmitted via human hosts in Kenya.
Malaria is usually associated with the bite of infected female mosquitoes. But once humans contract the disease, they can act as a vector if they are bitten by uninfected insects, which then spread the parasite to other people.
To track the spread of such infections, Amy Wesolowski at Carnegie Mellon University in Pittsburgh, Pennsylvania, and her colleagues used anonymised location data gathered from the cellphones of nearly 15 million Kenyans – out of a total population of close to 40 million – as they travelled around the country between June 2008 and June 2009. They combined this information with maps of population distribution and malaria prevalence over the same period to create, for the first time, a map that correlates large-scale trends in movement to the spread of the disease.
Previous attempts at correlating disease transmission with human movement have relied on smaller data sets gathered through monitoring how people given GPS tracking devices move over a period of time, for example. By taking location information directly from the logs of a national phone-service provider, the team was able to map every call or text to the cell tower which routed it.
The analysis offers explanations of malaria hotspots in Kenya’s western highlands and in the suburbs of Nairobi. Both are areas with low populations of infected mosquitoes. But, crucially, they each receive large amounts of human traffic from the coast of Lake Victoria, which possesses the highest concentration of infected mosquitoes in the country.
In general, the researchers found that the parts of Kenya with the highest malaria risk contained people that travelled the most. For example, people living in the 10 per cent of settlements with the highest risk of catching malaria made an average of 29 trips in or out during the year. The average for the other 90 per cent of the country was 20 trips.
The findings suggest that efforts to fight the spread of malaria must look beyond merely controlling the mosquito population. “To eliminate malaria, we’ve got to start thinking about transmission,” says team member Caroline Buckee of the Harvard School of Public Health in Boston. “There’s a big danger of people damaging control-programmes by travelling and bringing parasites in with them.”
Tom Scott of the Mosquito Research Laboratory at the University of California, Davis, says the work represents “cutting edge science” and provides “previously unattainable insights into long distance movement of malaria parasites”. Scott said such data would be critical in finding and targeting the human transmission routes of malaria-causing parasites.
Philip Bejon, a malaria vaccine researcher at the Kenya Medical Research Institute in Kilifi, who was not part of the study, says that prior to this work there was very little reliable information on how people move around the country. “If most cases in a given area are imported rather than acquired at home, then intensifying local malaria control isn’t going to achieve anything,” he says, “but screening travellers might.”
Journal reference: Science, DOI: 10.1126/science.1223467
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