Decades old mystery solved: why malarial mosquitoes appear out of nowhere

Decades old mystery solved
Why malaria mosquitoes appear out of nowhere

The “dry season malaria paradox” has been the subject of speculation for nearly a century: Malaria mosquitoes should not survive seven months without rain. And yet, after the end of the dry season, they come back out of nowhere. Researchers are now finding an explanation for this.

Malaria mosquitoes can survive the dry season in the African Sahel, which lasts about seven months, in a dry sleep. Scientists discovered this by spotting mosquito larvae in two villages in Mali. About 18 percent of Anopheles coluzzii mosquitoes most likely bridged the time between rainy seasons through dry sleep. The researchers led by Roy Faiman of the National Institute of Allergy and Infectious Diseases in Rockville, Maryland, present their approach and results in the journal “Nature Ecology & Evolution.”

Normally, Anopheles mosquitoes only live for a few weeks. Due to the harsh environmental conditions, they are unlikely to survive the long dry season in the Sahel zone, which sees no rain for months. However, when the rainy season usually starts in June, the mosquitoes are already present in large numbers after a few days. This time is far too short to hatch from the eggs and get past the larval stage. This “dry season malaria paradox” has puzzled experts for nearly a century, the study authors write. To this day it is not clear whether the mosquitoes spend the dry season elsewhere and then fly back or whether they fall into dry resting state (also called summer rest or aestivation) on site.

Experiment with heavy water

mosquito larvae in the laboratory.

(Photo: NIAID/dpa)

To clarify the question, Faiman and colleagues conducted a large-scale experiment in the Malian villages of Thierola and M’Piabougou: They enriched the open ponds with heavy water — that is, water with deuterium instead of hydrogen. In later lab tests of captive Anopheles coluzzii mosquitoes, the researchers were now able to determine whether the mosquitoes spent their larval stage in one of the ponds, which dry up during the dry season. The measurements showed that shortly before the start of the dry season, about 33 percent of the mosquitoes in both villages had elevated deuterium levels.

Also during the dry season, at the end of March or the beginning of April, many malaria mosquitoes appear for a short time. During this time and at the beginning of the rainy season, the researchers collected mosquitoes and determined the deuterium value. They calculated that about 18 percent of the buzzing mosquitoes had not left the area around the two villages during the dry season. The other mosquitoes come from other regions, for example from the rice area of ​​Niono, 140 kilometers away.

Possible help with mosquito control

“This survival strategy could impact mosquito control and malaria elimination campaigns,” Faiman’s team writes. Mosquito repellents can also be used during the dry season, even if the mosquitoes are not active or visible during this time. When using genetically modified mosquitoes to reduce the population, the dry sleep of mosquitoes must also be taken into account, the researchers emphasize.

The researchers’ findings represent an important advance in understanding the dry-season malaria paradox, writes Peter Armbruster of Georgetown University in Washington (District of Columbia, US) in a commentary, also in Nature Ecology & Evolution. They clearly showed that dry sleep is a mechanism used by Anopheles coluzzii mosquitoes to survive the dry season. An elusive but important goal must now be to find the places where the mosquitoes keep their dry sleep in order to kill them there.

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