Using mosquitoes to eliminate mosquito-borne dengue fever? That's right, a super-strong bacterium will help scientists overcome this medical puzzle!
It was so itchy that even the buzzing in the middle of the night disturbed people who couldn't close their eyes, and the mosquitoes really got away from us as much as possible. However, in recent years, in some parts of the world, some people have actually farmed mosquitoes and released them into nature! In fact, this is a project organized by the World Mosquito Program. The experiment was first carried out in Cairns, Australia, in 2011, and then in other countries: in Vietnam in 2013, in Indonesia and Brazil in 2014, in Colombia in 2015... By 2019, the experiment was also carried out in the city of Noumea, the capital of French New Caledonia, 16,000 kilometers from mainland France. Since July of that year, scientists from the project team have often been seen carrying plastic pipes through the streets of Noumea city, dropping mosquitoes bred in laboratories. Every 100 meters or so, they open the tubes in their hands and release 100 to 150 mosquitoes at a time, allowing them to multiply and grow in all corners of the city.
Sounds absurd? Not really. Scientists say mosquito stocking could help eradicate dengue, an infectious disease.
A staff member of the World Mosquito Programme is briefing local children in New Caledonia on dengue eradication measures.
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390 million new cases are added each year
Dengue is potentially fatal and causes a range of complications such as fever, vomiting, bleeding, joint and muscle pain, and more. It is extremely contagious, infecting nearly 390 million people each year and killing more than 25,000 people. Drugs and treatments have not yet been found to be effective in combating the disease.
Dengue fever is caused by a viral infection. Dengue virus is transmitted to humans through the Aedes mosquitoes, especially the terrible Aedes aegypti mosquito. The Aedes aegypti mosquito has white stripes on its legs and a thorax similar to that of the lira, and is commonly found in tropical regions around the world, including French overseas territories such as the French Antilles, French Guiana, Mayotte and New Caledonia.
The hospital in Bangladesh is packed with dengue patients (as many as 2,000 new infections per day in August 2019).
How does Aedes mosquito transmit dengue fever?
This is the Aedes aegypti mosquito, which transmits the dengue virus. During the breeding season, female mosquitoes (male mosquitoes do not bite people) rely on massive blood sucking to accumulate energy. If a female mosquito sucks the blood of a dengue carrier, it also inhales the virus at the same time. The virus multiplies inside the digestive system of mosquitoes and then enters the salivary glands. It has no effect on the mosquito itself, but is transmitted to humans through mosquito bites. The role of Wolbachia is to destroy the virus and avoid its reproduction, thus interrupting transmission.
Residents of Noumea opened their courtyard doors to greet the Aedes aegypti mosquito. Assists in the propagation of mosquitoes that transmit dengue virus? This sounds counter to eliminating dengue fever. In fact, these laboratory-bred Aedes mosquitoes have more of the secret weapon of Wolbachia than their wild brethren.
In 1924, two American scientists discovered Wolbachia in Culex auricules. This bacterium survives by lurking in the host. Once left from their hosts and left alone in soil or water, they die immediately. Since Wolbachia do not bring any disease to the host, their presence is almost ignored.
In the 1990s, scientists discovered that Wolbachia had extraordinary abilities! First, they naturally parasitize most animals, such as spiders, crustaceans, and nearly 60 percent of insects. Wolbachia has managed to lurk in many hosts because it manipulates the reproduction of hosts. The female host automatically passes the bacterium on to the offspring, while the male host does not have the function. If a male host mated with an uninfected female, Wolbach would kill the egg directly! Thus, Wolbachia can spread on a large scale with the change of generations by parasitizing only a few individuals of an animal population. Not to mention that Wolbachia can also protect the host: in order to quickly parasitize the entire population, Wolbachia will never allow other microorganisms to seize their place, or harm or even kill the host... After injecting mosquitoes with different kinds of viruses, the researchers found that mosquitoes carrying Wolbachia were more tenacious than other individuals! Although the mechanism is not yet clear, Wolbachia can obviously prevent other viruses from multiplying in their hosts, and will also annihilate the virus in one fell swoop!
By the end of the 2000s, scientists were considering using these two properties of Wolbachia to block the spread of dengue virus. Specifically, female mosquitoes infected with the bacteria are allowed to mate with wild mosquitoes, thereby passing the bacteria on to their offspring. Due to the rapid spread of Wolbachia, more and more mosquitoes will be "resistant" to dengue virus, which can greatly reduce the probability of transmission of the virus to humans.
This idea is perfect... However, although Wolbachia naturally resides in 60% of insects, Aedes aegypti belongs to the other 40%. Don't worry, there are always more ways than problems! Scientists have invented a technique that can transfer Wolbachia from its natural host to the Aedes aegypti mosquito. Subsequently, dengue virus was injected into the mosquitoes carrying Wolbachia, and sure enough, the invading virus was eliminated.
Insurmountable dengue fever
More than 40% of the world's population lives in areas at high risk of dengue infection. Currently, there is neither a drug nor an effective vaccine to fight the dengue virus. The only way to prevent infection is to minimize the possibility of being bitten by mosquitoes. General recommendations include the installation of screens (Aedes aegypti mosquitoes are active during the day), the wearing of long-sleeved trousers whenever possible, the use of insect repellents, and the removal of stagnant water containers or devices near the residence that are conducive to mosquito spawning. Health and epidemic prevention departments sometimes spray mosquito repellent in large quantities in mosquito breeding areas, but this practice is not very environmentally friendly, and it will kill other insects while killing mosquitoes. Not to mention that the above measures can only reduce infections, but cannot really stop the spread of the virus. At present, all hope is pinned on Wolbachia.
In Kathmandu, the capital of Nepal, people were only able to remove mosquitoes by spraying insecticides in large quantities during the pandemic.
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Mosquito eggs in capsules
So the researchers set out to grow virus-resistant mosquitoes and put them in plastic tubes to release them into dengue-plagued areas, such as New Caledonia. To track the reproduction of Wolbachia, project scientists set traps in the drop-off area, regularly collected captured mosquitoes, and inferred the proportion of mosquitoes in the population that carried the bacterium based on the number of individuals infected with Wolbachia. "In Noumea, about half of the Aedes aegypti mosquitoes were infected with Wolbachia a year and a half after the first stocking." Dale Amtberg, the group's senior media adviser, said.
Researchers are testing whether the collected mosquitoes are infected with Wolbachia (blue part of the image on the right).
In some areas where transmission is less than ideal, scientists will re-launch it in 3 to 6 months. The new delivery has all the improvements compared to the previous one. They would place capsules containing mosquito eggs in the waters. The effects of this new method have been demonstrated in Indonesia: mosquitoes that grow up in the "wild" state have a higher survival rate; Some of the mosquitoes raised in the laboratory died on their way to the release site.
The goal of the project scientists is to infect all Aedes aegypti mosquitoes in Noumea with Wolbachia within one to two years, thus completely eliminating the dengue outbreak in the city. This is not delusional. The trials in Cairns were a success: since its first launch in 2011, dengue cases have decreased by 97% in 8 years, and no new cases of viral infection have been detected since then.
Such an inconspicuous bacterium can achieve such a great feat! What's more to be expected is that researchers have already targeted two other viruses transmitted by the Aedes aegypti mosquitoes , Zika virus and chikungunya virus , and Wolbachia may be able to make a new contribution.
Written by Alexane Roupioz
Compiled by Zou Lingli
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