Mosquitoes can fly just a few miles at a time, limiting their ability to spread insect-borne diseases such as malaria. Humans are much better long-distance disease transmitters; through our movements, we help malaria get much farther, and move faster than parasite-ridden insects could. However, because human movement can be difficult to quantify, especially in third-world countries where malaria is prevalent, few studies have been able to understand how travelers alter the movements of malaria over large spatial scales.
But the recent proliferation of mobile phones in African countries offers a creative way to track human travel and inform our understanding of malaria dispersal. In this week’s issue of Science, a group of researchers use cell phone records to help map out networks of malaria transmission in Kenya, identifying hotspots where the disease is likely to be contracted and transmitted.
The study used records from nearly 15 million mobile phone users in Kenya, tracking them for one year; the cell phone company provided coded ID numbers to identify each user, as well as the location, time, and date of each call or text. The researchers determined where each person spent most of their time based on the location of the majority of their call and text records. After assigning individuals' home bases, the researchers could study each trip that each person took within Kenya.
Then, based on knowledge of malaria rates across Kenya, they assigned each town a “malaria endemicity class,” which measured the probability that a person there would become infected. Between the travel information and the likelihood of infection, the researchers developed a model that predicted the transmission of malaria around the country.
With this data, the researchers determined which towns and cities were “sources,” or areas responsible for new infections, and which were “sinks”—areas likely to receive parasites from elsewhere. Sources and sinks tended to occur in close proximity; the east coast of Kenya and settlements around Lake Victoria on the western edge of Kenya were major sources of malarial parasites, and the neighboring areas inland of these hotspots ended up with much of the new infections. Nairobi, Kenya's capital, was also a major parasite sink.
Sinks were characterized by residents that traveled often, since these people were likely to come in contact with malaria and bring the parasite back home; residents of the top 10 percent of sinks took about 29 trips per year, while residents of other areas took about 20 trips per year. Areas that received high numbers of visitors from malaria-prone regions were also more likely to be sinks than those that had fewer visitors from these areas.
While cell phone data offers new insights and information about human movement in Kenya, there are shortcomings to this approach as well. Obviously, data can only be collected from people with mobile phones and in areas with functioning cell phone towers. Furthermore, the researchers could only collect data in Kenya, so cross-border travelling cannot be monitored; this is especially important to note in this study, since areas bordering Lake Victoria—which straddles Kenya, Uganda, and Tanzania—were major sources of malaria.
Despite these limitations, the researchers believe that using cell phones to track human movements contributes greatly to our understanding of human connectivity and malaria transmission, both in Kenya and elsewhere. By identifying disease sources and sinks, public health efforts can target areas that put the population at the greatest risk, and can determine which control methods to use in areas with different transmission patterns. Different types of public initiatives and malaria control programs may be needed in areas with high-volume traffic, for example, compared to areas prone to transmitting disease locally.
According to recent estimates, malaria kills more than a million people each year, so every bit of knowledge helps.