The migrations of large African mammals across Tanzania’s Serengeti and the southern Sudan are true wildlife spectacles, but high in the night sky over the Sahel (the transition zone that belts the continent between the Sahara Desert and tropical savanna), another mass movement of animals immensely important to humans has gone undetected—until recently, that is. A dogged band of researchers revealed that millions of malaria-vectoring Anopheles mosquitoes ride high-altitude winds that spread them (and potentially the disease) far and wide, including to areas from which malaria has been eradicated.
Reported last year, the finding that mosquitoes can traverse hundreds of kilometers contradicted the accepted scientific assumption that mosquitoes in flight hug the ground and are stay-at-homes, with a lifetime range of no more than five kilometers. Now, in a paper published in July in the Journal of Medical Entomology, the same research team suggests that the high-flying little bloodsuckers can actually transmit malaria after the battering, buffeting, and desiccation of the strong winds that whoosh them through the atmosphere. Before the research was published, it was not known whether they could go about their routine business of feeding and reproducing after being blown about at altitudes up to 290 meters during an overnight flight.
To make the experiments more challenging, the study was carried out during the transition from wet to dry seasons in October and November, when relative humidity drops, wind speeds rise, and conditions are worst for mosquito survival. It turned out that not only can mosquitoes survive the rigors of high-altitude flight during this worst of times, but also that the females can lay eggs and take blood meals afterwards. Assuming that the single-celled Plasmodium parasite that causes malaria is viable after flight, the wind-borne mosquitoes can start new populations that are wellsprings of the disease.
“Our new results, based on a larger sample size, demonstrate that mosquito migrants at high altitude can indeed survive, lay eggs, and thereafter take a new blood meal, thus enabling a new transmission encounter with the host after their migration,” write the researchers, who have been studying mosquito ecology among the mud-brick agricultural villages of semi-desert Mali for more than a decade. “We conclude that similar to other high-altitude migrating insects, mosquitoes are able to withstand extended high-altitude flight and subsequently reproduce and transmit pathogens by blood feeding on new hosts.”
The new finding is vindication, in a sense, for Tovi Lehmann, Ph.D., of the National Institute of Allergy and Infectious Diseases, who heads a team that also includes scientists from Mali’s University of Bamako. According to many of his colleagues, he has pulled off a Star Trek-quality achievement, literally going where no researcher has gone before.
“He has tackled one of the very biggest questions in vector biology that was long considered ‘the graveyard of postdoctoral fellows’ because they serially tried and failed,” says Nora Besansky, Ph.D., of the University of Notre Dame, whose use of genomics to study environmental relationships of malaria mosquitoes earned her an appointment to the National Academy of Sciences.
Demonstrating that persistence does indeed pay off, several years of research by Lehmann and his colleagues solved the mystery of how various species of malaria mosquitoes suddenly repopulate areas of the Sahel that lack the surface waters required by larvae for three to eight months of the year. One species appears to aestivate in the Sahel (at least locally), but it also migrates, perhaps over shorter distances than the others. At least two species hitchhike south on prevailing winds to areas with year-round water as the dry season begins in the Sahel; then they head back north when the winds do an about-face with the approach of the wet season.
This finding, says Besansky, has a profound impact on long-standing attitudes toward mosquito movement. High-altitude migration of many insects that are agricultural pests, including locusts, is well documented, but Lehmann’s work is a first for mosquitoes. “It is not often that paradigms shift,” she says. “Tovi has succeeded in forcing a shift.”
Lehmann and his team collected mosquitoes around local villages. Each female sent aloft was placed in a polypropylene tube 5 centimeters long and 3 centimeters wide, the ends covered by netting or cloth. With more wind resistance, it later turned out that a cloth covering promoted mosquito survival. Groups of five to 10 tubes were taped to ropes tethered to helium balloons, which were launched into the night sky. “Using the balloons for high-altitude sampling,” says Lehmann, “was a shot in the dark,” untried until then for such research in Africa. Obtaining the helium and transporting it and other equipment to the extremely remote research site was expensive, he adds. Insect migrations can be tracked by radar and satellite, but the results are not as specific as actual sampling by using balloons.
The balloons were retrieved in the morning. Tubes with surviving mosquitoes were placed in water to test whether the insects could lay eggs, which many did. Placed on chickens, they fed as well. Potential flight distances for the hours spent aloft by the mosquitoes were estimated by using models developed and utilized by the National Oceanic and Atmospheric Administration to track entities moved by wind.
The implications of the research are profound. Says Lehmann, “Changing our mosquito dispersal paradigm as it relates to the rapid and long-range spread of vector-borne infections, including arboviruses such as Rift Valley fever, as well as adaptive traits such as insecticide resistance by wind-borne mosquitoes, would allow us to up our ability to predict outcomes and mitigate them more successfully.”
Lehmann and his colleagues are continuing his studies not only on mosquitoes but also on other insects that travel the winds. Their research on immense migrations of trillions of Sahelian insects is about to be published. Lehmann has become an apostle of research on long-distance migration of insects that impact public health, conservation, and the security of food. He envisions scientists of many disciplines forming consortiums that monitor such insect movements on a continent-wide scale. For further developments, stay tuned.
Credit: Entomology Today