Sterilized bugs might assist combat mosquito-borne illnesses Horizon: the EU journal for analysis and innovation
While the Asian tiger mosquito bite is little more than a pinprick, it causes tens of thousands of deaths worldwide each year.
The tiny aggressive insect, named for its striped appearance, carries a number of nasty viruses that cause diseases like yellow fever, dengue fever, chikungunya, zika, and Japanese encephalitis. While these are primarily viewed as tropical diseases, the spread of the mosquitoes that transmit them has raised fears that the viruses could be more prevalent in Europe as well.
The European Centers for Disease Control (ECDC) predicted ten years ago that tiger mosquitoes would spread across Europe, and climate change is now threatening to make their spread even more likely. “Even the southern part of Sweden may be climatically suitable for this mosquito, although it has not yet got there,” said Professor Jan Semenza, who heads an ECDC department that assesses the threat of infectious diseases.
The striped pest, also known as Aedes albopictus, is native to Southeast Asia but came to Albania in the 1970s before reaching Italy in the 1990s. It first colonized the Mediterranean coast, then expanded steadily to the north and is now found in large parts of France, Greece, Bosnia, parts of Spain, southern Portugal and Germany. It was even found in greenhouses in the Netherlands. During the summer, the mosquitos became a nuisance in some places.
But illness can come with mosquitos. So far, outbreaks have been relatively limited and low in number, but there have been cases of dengue in Croatia, France and Spain. In Italy, hundreds of people contracted chikungunya in 2017. In 2019, there were two cases of locally captured Zika in southern France.
The diseases can be transmitted to Europe by people infected with the virus from countries in South America and Asia where they are endemic. Brazil, for example, was a hotspot for dengue fever this year. If these people in Europe are bitten by a tiger mosquito, the insect can transmit the virus to other people, who it bites.
“We have noticed an increase in vector capacity due to climate change,” warns Prof. Semenza. In warmer temperatures, the biting insects can survive through the winter. ‘Dengue fever has an enormous burden of disease worldwide. It can turn into a life threatening condition so we are concerned about it getting to Europe. ‘
Dengue fever was endemic to Greece in the early 20th century but has been eradicated. “We do not want this type of disease to recur in Europe,” added Prof. Semenza.
But an increasing number of mosquitoes that can transmit disease makes it more likely that these viruses could re-establish themselves in Europe. Another pathogenic insect – Aedes aegypti, also known as the yellow fever mosquito – is also threatened with a return to Europe after being wiped out there in the 20th century. Originally from Africa, it is now present near the Black Sea on the Portuguese island of Madeira and northeastern Turkey.
However, with many insecticides banned in Europe due to their toxicity and greater environmental pollution, there are fewer ways to control the mosquitoes.
Birth control in insects
Dr. Jérémy Bouyer, biologist and mosquito expert at the French Agricultural Research Center for International Development (CIRAD) in Montpellier, predicts that Europe could face an uncontrolled dengue epidemic in the next five to ten years if no more is done to combat mosquito populations.
“The Aedes tiger mosquitoes are very difficult to control,” said Dr. Bouyer. The insects tend to breed in relatively small locations, making them difficult to attack, he said. “Instead of ponds or lakes, they like man-made habitats.”
But we may not be entirely defenseless against these mosquito pests. Dr. Bouyer is developing a new approach to combating mosquitoes as part of a research project called REVOLINC. Over the next few years it will be releasing tens of thousands of sterile male yellow fever mosquitoes on Reunion Island, a French territory in the Indian Ocean.
Once released, the males should mate with wild females and produce sterile eggs to suppress the number of mosquito larvae. However, the males are also coated with a secret weapon – a biopesticide called pyriproxyfen, which mimics hormones in insects and restricts their growth. The males transmit this biopesticide to the female when trying to mate with them, and it contaminates their habitat for eggs and larvae. This means that eggs fertilized by non-sterilized males cannot mature from larvae to adult mosquitos either.
“Even if the sterile males do not succeed with the females, they still transmit the biopesticide,” said Dr. Bouyer. “We have shown that this can increase the effects of control by 10 to 100 fold, so we may have to release fewer male mosquitoes.”
In order to raise enough insects, Dr. Bouyer and his colleagues at the International Atomic Energy Agency (IAEA) facility in Seibersdorf, Austria, have developed technologies for real mosquito factories, in which the larvae are raised on stacks of dishes submerged in water, each containing 18,000 of the wrigglers. The larvae are then sorted by sex as they pupate before the young males are sterilized with a finely balanced dose of radiation – enough to achieve near-complete sterility, but they remain healthy enough to be able to mate upon release. The sterile males can then be shipped in refrigerated cardboard boxes.
“The Aedes mosquitoes are very difficult to control … instead of ponds or lakes, they like man-made habitats.”
-Dr Jeremy Bouyer, CIRAD, Montpellier, France
Male mosquitoes were previously released in canisters on the ground on Reunion Island. But 90% of the insects will not be more than 100 meters from this point.
To improve the spread of sterile male insects, Dr. Bouyer is also involved in a project called MOSQUAREL, which uses drones to release mosquitoes from the air. He has already used a 12kg drone in Brazil to release 50,000 sterile male mosquitoes per flight and hopes to test a lighter 900g drone that can release 30,000 mosquitoes at a time. The advantage of this smaller drone is that it can fly over residential areas in Europe.
“To treat a city with sterile insects, you have to drive a vehicle on roads, stop every 100 meters, and release a box of mosquitos,” said Dr. Bouyer. “It takes two hours for two vehicles to cover 30 hectares.” A drone fired from the back of a truck could treat the same area in ten minutes, he said, is faster, cheaper, and spreads the insects more evenly over an area.
The new drone is also being tested in Valencia, Spain, working with Spanish state rural development company Tragsa to release sterile tiger mosquitoes in a citrus tree production area.
Dr. Bouyer also hopes the drone will release sterile yellow fever mosquitoes in Reunion Island, which are coated in biopesticides and target three small mosquito populations.
But while Reunion has a few isolated bags of yellow fever mosquitos that Dr. Bouyer wants to wipe out tiger mosquitoes are a bigger problem on the island. The reunification has seen a surge in dengue fever cases since early 2018, and one particularly dire outbreak this year resulted in a number of deaths. Tiger mosquitoes can thrive in warmer temperatures and humid conditions, so Dr. Bouyer to tackle these insects next. However, releasing sterile men just coated with biopesticides does not work.
Both Aedes mosquito species only need tiny amounts of water for their larvae, which often develop on garbage, in discarded plastic or in old tires. Reducing such artificial larval habitat in urban areas will therefore also be necessary to ensure that the drone drops can have a greater impact.
“If we suppress it in one place, we can prevent dengue fever there,” said Dr. Bouyer. “This is the best achievement we can hope for – protecting people.”
The research in this article was funded by the EU. If you enjoyed this article, you can share it on social media too.