Mosquito Migration: Study Finds More High-Altitude Dispersal of Disease Vectors in Africa
While the world focused on the march of COVID-19 in 2020, scientists working in Africa were clinching a case for how mosquitoes can spread malaria and several other dreaded diseases fast, far, and perhaps even overnight by a process previously thought impossible: riding high-altitude winds to new locales, where they land ready to have young, feed, and potentially spread pathogens at journey’s end.
Not only that, but new research suggests such windborne mosquitoes are not merely casting about looking for a quick meal or mate or fleeing the dry season for wetter places. Some, at least, are actually migrating to colonize new areas favorable for survival, according to a study published in April in the Journal of Medical Entomology.
Conducted in Kenya, the study builds on other research by researchers at the National Institute of Allergy and Infectious Diseases (NIAID) and a bevy of other institutions over the past few years. The previous series of studies, a step at a time, put together a picture of mosquitoes using high-altitude winds to escape the dry season in Mali’s Sahel, the arid belt between the Sahara Desert on its north and tropical savanna to the south. The newest study shows that mosquitoes in mid-equatorial Kenya also ride the winds, not to escape drought but to expand their range.
The study shows that mosquitoes and other insects of medical importance are dispersing at high altitude in a region that already has the basics they require to live, suggesting the movement is not caused by scarcity of resources. These mosquito migrants could challenge existing mosquito surveillance and control measures in the new regions where they land.
“This will compromise the already found gains achieved in such a region on malaria control,” says Harrysone Atieli, Ph.D., research scientist at Kenya’s Tom Mboya University College and the International Center of Excellence for Malaria Research (ICEMR) and lead author on the new study. He was joined on the study by colleagues from Tom Mboya University College; NIAID; the University of California, Irvine; Case Western Reserve University; and Mali’s Malaria Research and Training Center.
What scientists are hailing as a “paradigm shift” in their understanding of mosquito movements began with a 10-year-study, published in 2014, of mosquito populations in Mali hinting that at least one species, which transmits malaria, escaped the Sahelian dry season by moving south. Prevailing wisdom at the time was that mosquitoes in flight stay low and close to home, traveling no more than five kilometers in a lifetime. Far from it, revealed follow-up research in the Sahel, published in 2019 and 2020. It suggested that perhaps 50 million malaria-vectoring Anopheles mosquitoes annually travel hundreds of kilometers at up to 290 meters altitude, toughing it out against the battering winds on which they hitchhike.
Moreover, the high fliers included blood-fed females carrying eggs, indicating that on landing they could lay eggs to colonize and potentially spread pathogens with their next meal. When winds shifted north with the approach of the wet season, the mosquitoes did an about face as well. The studies leave no doubt that mosquitoes can survive harsh climatic conditions experienced at high altitude such as strong wind and rain yet still maintain their viability to lay eggs and reproduce. This means that they can spread disease far from their place of origin.
After the discovery that mosquitoes can travel long distance on high-altitude winds, new research adds to the scope of such migrations and the species engaged.
The discovery was enabled by the use of a new research tool: glue-daubed net traps hanging below helium-filled balloons. The same technique was used in the Kenya study, which produced another startling finding. It was not surprising that the mosquitoes in Sahel were seasonally moving to wetter surroundings from dry origins, but those studied in mid-equatorial Kenya migrated between habitats in the Lake Victoria Basin with year-round high humidity and other resources contributing to mosquito survival. The most common mosquitoes sampled were of the Culex genus, several members of which can spread the West Nile virus, filariasis, and types of encephalitis. Others were Aedes, some of which are vectors of chikungunya fever, Zika, and dengue fever, and Anopheles, among which some are key malaria vectors.
The research suggests “that windborne dispersal activity of mosquito vectors and other diseases occur on a broad scale in sub-Saharan Africa” as well as to the north, the authors write. The researchers contend the mosquito movement aloft is a full-fledged migration, not a haphazard search for better living conditions. “The presence and diversity of these high-altitude migratory insects in both perennially resource-endowed (Kenya) and seasonal areas (Sahel), confirms the fact that migration is a deliberate individual behavior specific to particular insects,” the researchers write. “It is characterized as a persistent flight by insects to a new environment.”
Winds may be carrying mosquito vectors of disease over vast areas of Africa, warn the researchers. The implications on transmission of pathogens and genetic resistance to insecticides in insects could be profound.
“Mosquitoes’ wide-spread insecticide resistance has long been reported in multiple countries in Africa,” says Atieli. “This is attributed partially to the uncoordinated use of different insecticide classes for both vector and pest control across Africa. Dispersal of vectors at high altitude to new regions is likely to introduce new colonies that are already genetically resistant to insecticides used in their new home, leading to successful spread of the resistance gene in this region. This colony will thrive in the new habitat, pose a challenge to existing control tools, and lead to enhanced transmission. ”
Atieli and colleagues are looking to extend their studies to other parts of Africa, and they hope to discover the role of high-altitude mosquito dispersal in the sudden occurrence of malaria in various spots, notably, Ethiopia and Kenya.
Journal of Medical Entomology
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.