An Asian malaria-carrying mosquito that has adapted to urban life has the potential to spread to dozens of cities across the African continent, a new modeling study suggests. That could put more than 100 million additional people at risk of the deadly disease, including many who were never before exposed to it and have no immunity.
The mosquito species, Anopheles stephensi, poses a serious new threat for African cities, says Francesca Frentiu, a geneticist at the Queensland University of Technology who was not involved in the research. She praises the work as “an important effort, underpinned by robust methods.”
Malaria, which kills more than 400,000 people per year—most of them African children—is caused by Plasmodium parasites and spread by several mosquito species. In Africa, the most important one is A. gambiae, which thrives in rural settings. But recently, scientists have also spotted A. stephensi, which is well adapted to city life and has long spread malaria in urban environments in Asia. A. stephensi hopped from Asia to the Arabian Peninsula between 2000 and 2010 and then made another jump to the Horn of Africa; scientists first discovered it in Djibouti in 2012, then later in Ethiopia and Sudan.
To gauge its potential to spread farther, Janet Hemingway, an insect molecular biologist at the Liverpool School of Tropical Medicine, and colleagues used data about every place where A. stephensi is now known to occur—including variables such as annual mean temperature, rainfall seasonality, and human population density—to produce maps of the places in Africa where the mosquito might take up residence next.
The results are disconcerting. Out of 68 African cities with a population of more than 1 million, 44 seem suitable habitats for A. stephensi, the team reports this week in the Proceedings of the National Academy of Sciences. Together, those cities—from Casablanca, Morocco, to Durban, South Africa—are home to 126 million people, including 20.5 million in the greater Cairo area alone and another 19 million in Lagos, Nigeria.
If A. stephensi continues its incursions, there is “a very real possibility of mass outbreaks,” that could be “catastrophic,” the researchers write. The fact that countries in North Africa are susceptible is particularly concerning, as they currently have very little or no malaria and people there have no immunity.
The World Health Organization has warned Africa about A. stephensi, calling for active mosquito surveillance. The findings suggest cities across the continent should take these warnings to heart, says Marianne Sinka, a zoologist at the University of Oxford who led the research.
The maps the team created will be useful in tracking and combatting malaria, says Tamar Carter, a biologist at Baylor University who was not involved with the study. Still, Carter says more research is needed to figure out just how big of a threat A. stephensi poses to African cities—and how best to allocate limited resources to fight it.