The findings from researchers at the Kenya Medical Research Institute (KEMRI) Wellcome Trust Research Program, Harvard School of Public Health, and six other institutions, indicate that malaria largely emanates from Kenya’s Lake Victoria region and spreads east, chiefly toward the capital, Nairobi.
Professor Caroline Buckee, Harvard School of Public Health and lead author of the study, said: “This is the first time that such a massive amount of cell phone data – from millions of individuals over the course of a year – has been used, together with detailed infectious disease data, to measure human mobility and understand how a disease is spreading.”
Professor Bob Snow of the KEMRI-University of Oxford-Wellcome Trust Collaborative Programme, based in Nairobi, said: “Kenya is one of the countries in Africa where even the most rural inhabitants have a cell phone. Kenya is also a country where people move between regions, cities and rural areas for business, home-leave and funerals.
“As Kenya begins to succeed in reducing malaria transmission in some areas but not others, cell phone mapping of human movement between high- and low-risk regions becomes a valuable planning tool.”
Between June 2008 and June 2009, the researchers mapped every call or text made by each of 14 816 521 Kenyan mobile phone subscribers to one of 11 920 cell towers located in 692 different settlements. Every time an individual left his or her primary settlement, the destination and duration of each journey was calculated.
Then, using a malaria prevalence map to estimate the disease’s prevalence in each location being studied, the researchers inferred each resident’s probability of being infected and the daily probability that visitors to particular areas would become infected.
“To estimate malaria’s potential spread, it is important to factor in not only information about the location of the mosquitoes that carry the malaria parasite but also the behaviour of the people who might be infected,” explained Professor Buckee. “Since many infected people have no symptoms, they can unintentionally carry the parasite during their travels and infect hundreds of others.”
The study enabled the researchers to build a map of parasite movements between ‘source’ areas, which mostly emit disease, and ‘sink’ areas, which mostly receive disease.
The team found that a surprisingly large fraction of ‘imported’ infections (infections carried by people moving from one place to another) wind up in Nairobi. Infected residents return there after journeys to spots such as Lake Victoria or the coast.
Dr Abdisalan Noor from the KEMRI-Wellcome Trust Research Programme noted that imported malaria infections contribute to the risks of epidemics in very low transmission areas. He said: “The analysis provided in the paper in ‘Science’ shows how we might use this information to mitigate and prepare areas subject to the highest imported infection risk.”
“The information available from these new types of analyses holds promise for helping public health officials decide where and how to control imported cases of malaria. For instance, officials could send text message warnings to the phones of people traveling to high-risk areas, suggesting that they use a bednet,” added Professor Buckee.
Malaria kills about 1 million people each year and threatens 40 million globally. Of those affected, 95 per cent are children under five in sub-Saharan Africa.
The study is published today in the journal ‘Science’.
Image credit: Hugh Sturrock, Wellcome Images.
Wesolowski A et al. Quantifying the impact of human mobility on malaria. Science 2012.