"Nansha Science Popularization Lecture Hall" is a new scientific and cultural communication brand founded by Guangzhou Nansha District Science and Technology Association for young families. Each lecture hall invites four speakers from different fields to share their research results and back stories, and bring the audience a feast of knowledge that blends science, culture and art through engaging language and immersive live atmosphere. The "Nansha Science Popularization Lecture Hall" was hosted by the Guangzhou Nansha District Science and Technology Association, and the second phase of the event was organized by the Secretariat of the Greater Bay Area Science Forum.
Nansha Science Lecture Hall
Phase 2 | July 20, 2024
Wu Yu
Associate Professor, Sun Yat-sen Medical College, Sun Yat-sen University
Hello everyone, I'm Wu Yu from Sun Yat-sen Medical College, Sun Yat-sen University, and my job is to use "nuclear" technology to sterilize mosquitoes.
The deadliest animal in the world
We're all familiar with mosquitoes, but did you know that mosquitoes are the deadliest animals in the world? More than 700,000 people die each year from bloodborne diseases caused by mosquitoes, 10 times more than from venomous snake bites.
At present, more than 3,000 species of mosquitoes have been recorded worldwide, while more than 300 species have been found on the mainland, among which the most important vectors of disease transmission are the three genera of mosquitoes shown in the figure below, Aedes mosquitoes, Culex mosquitoes and Anopheles mosquitoes. They mainly transmit more than a hundred diseases such as malaria, dengue fever, Japanese encephalitis, etc.
What is the difference between these three genera of mosquitoes? Active in outdoor places such as parks during the day, the main thing that bites us is the spotted mosquito, that is, Aedes aetus; Mosquitoes that bite us indoors at night, mainly Culex mosquitoes, or house mosquitoes; Anopheles mosquitoes are mostly found in rural areas, and the mosquito species we see in urban areas are mainly Aedes mosquitoes and Culex mosquitoes.
Malaria is transmitted by the bite of Anopheles mosquitoes and affects about 250 million people worldwide each year, causing more than 600,000 deaths, most of which occur in poorer and less backward areas of Africa, especially children under 5 years of age.
China has made outstanding contributions to malaria prevention and control. Chinese scientist Professor Tu Youyou was awarded the 2015 Nobel Prize in Physiology or Medicine for his discovery of artemisinin, a specific drug for the treatment of malaria. In the sixties and seventies of the last century, the Viet Nam War broke out, and many soldiers on the battlefield suffered non-combat attrition due to malaria infection. The Viet Nam government turned to China for help in developing a specific drug for malaria. The Chinese government has mobilized the efforts of the whole country to organize scientists from all over the country to collaborate in the research and development of specific drugs for the treatment of malaria. In the end, Professor Tu Youyou's team drew inspiration from the classical literature of traditional Chinese medicine and extracted artemisinin from Artemisia annua, saving hundreds of millions of lives around the world.
After more than 70 years of unremitting efforts, China was certified by WHO for malaria elimination on June 30, 2021, so there are now no indigenous cases of malaria infection in the mainland, but there is still a risk of imported malaria cases. Because malaria is still endemic in Africa, Southeast Asia and other countries, people who travel and work in malaria-endemic areas may be infected with malaria if they are bitten by the local Anopheles mosquito, and the onset will become an imported malaria case after returning home. What are the clinical signs of malaria? Malaria attacks, commonly known as "swinging", have three steps: chills, high fever, sweating and fever. The Chinese program has made outstanding contributions to malaria prevention and control, and the Gates Foundation has also invested a lot of money, playing an important role in the global malaria elimination process.
The second most common mosquito-borne disease is dengue fever, which also has more than 100 million cases worldwide each year. There are also cases of dengue fever infection in Guangzhou every year, and posters on the community bulletin board can often be seen to eliminate stagnant water and prevent dengue fever, which shows that Guangzhou attaches great importance to the prevention and treatment of dengue fever. As of July 19, 2024, it has been reported that 9 of the 11 districts in Guangzhou have reported local or imported cases of dengue fever, so we need to start from the small things around us to actively remove stagnant water and prevent dengue fever.
The mosquito mosquito (Aedes ae) shown here is a vector of dengue fever that we are familiar with. In China, dengue fever is mainly transmitted by Aedes albopictus, and in South America, Africa and other countries, Aedes Egypt albopictus is the main vector of dengue fever.
If you look at the two mosquitoes in the picture, can you tell the difference between a female and a male mosquito? Looking at the head structure of the mosquito, there is a needle-like structure in the middle of the head of the female mosquito on the left, called the "beak". When the female mosquito bites and sucks blood, she inserts this needle into the host blood vessel to suck blood, and at the same time, the salivary glands secrete saliva and inject saliva into the blood vessels, if the saliva carries pathogens, it can transmit diseases. Therefore, sucking blood is the main way for female mosquitoes to transmit pathogens.
Observe that the male mosquito on the right has a needle-like structure in the middle and two brushes on both sides. Because the beak of the male mosquito is degraded and not enough to cut the skin, the male mosquito cannot suck blood and can only survive by sucking nectar and plant juice.
In the laboratory, we feed male and female mosquitoes with sugar water, so the survival of mosquitoes can be maintained by sucking sugar water and nectar. But the female mosquito still needs to suck blood, the ovaries can develop, and can lay eggs to reproduce offspring, if it does not suck blood, it cannot reproduce offspring, so only female mosquitoes suck blood to transmit disease, male mosquitoes do not suck blood and do not transmit disease.
As you can see in the picture, the body color of these two mosquitoes is black, and the male mosquito on the left and the female mosquito on the right can be judged by the characteristics of the head. There is also a white longitudinal stripe on its chest and back, so it is called Aedes albopictus, commonly known as Aedes versicolor, which is the main vector of dengue fever transmission in the mainland.
Breeding ground for mosquitoes
Three days after the female mosquito sucks blood, her ovaries are mature and she will seek out water bodies to lay eggs. The larvae hatch from the eggs, and the body of water where the larvae breed is called the breeding ground. The bottles and jars, such as flower pots and sewers, are breeding grounds for mosquito larvae of different genera.
The tires shown on the picture are where Aedes mosquitoes like to lay eggs. When the tires are waterlogged, the blood-sucking female mosquitoes will lay eggs, and even after the water in the tires is dry, the eggs can survive for several months. Through the trade of tires between countries, the eggs carried in the tires also spread around the world, so the pathogens carried by mosquitoes can also spread over long distances through the eggs.
To sum up, we know that the most fundamental way to control mosquitoes is to carry out environmental management, and removing stagnant water can completely eliminate mosquito breeding sites. In addition, the use of screen doors, window screens, mosquito nets to prevent mosquitoes, and the use of electric mosquito swatters to control mosquitoes are all physical methods of mosquito control.
Communities often have teams to sanitize pesticides, which are chemical control methods. In addition, we raise some mosquito-eating fish in fish tanks, flower pots, and aquatic plant containers, which can devour mosquito larvae, which is a biological control method, and there are also some giant mosquito larvae that can also prey on mosquito larvae. Next, I will describe how our team applied "nuclear weapons" to control mosquitoes.
Sterile Insect Technique (SIT)
Sun Yat-sen University has created a mosquito factory in Dongguan Songshan Lake High-tech Industrial Development Zone, producing hundreds of thousands to millions of mosquitoes every week, when the mosquitoes develop to the pupa or adult stage, they will be treated with radiation, and the radiation will destroy the reproductive organs of the mosquitoes, and the male mosquitoes can not produce normal sperm, and the male mosquitoes are called "sterilized male mosquitoes".
We release the neutered male mosquitoes into the wild, and although the male mosquito sperm is not motile, they can still mate with the female mosquito in the wild, and when the female mosquito lays eggs, the mosquito eggs cannot develop normally because there is no normal sperm insemination. By continuously releasing sterilized male mosquitoes, the number of mosquitoes in the wild can be reduced by reducing the number of mosquitoes, a method of controlling mosquito population density called the "sterile insect technique".
SIT, or SIT, was first successfully applied to agricultural pest control. In the 50s of the last century, a large number of screw flies invaded the Americas, and their larvae can erode animal bodies, causing a lot of economic losses in animal husbandry. In this regard, scientists from the United States Department of Agriculture proposed to use sterile technology to control screw flies. The screw fly population was successfully controlled by irradiating the male flies and releasing the neutered male flies to mate with the female flies in the wild. Through a "carpet" release from North America all the way to Central America, the local screwfly was completely eradicated.
Similar methods are applied in other countries to control different agricultural pests. In China, we use sterile insect technology to control the dengue vector, Aedes mosquitoes. In 2014, the dengue fever epidemic in Guangzhou was very serious, with more than 40,000 dengue fever cases in the country. In view of the need for dengue prevention and control, our team reduces the risk of dengue transmission by controlling dengue vectors. Therefore, at the Dongguan Research Institute of Sun Yat-sen University-Songshan Lake Development Zone, we have built a mosquito factory to produce a large number of sterilized male mosquitoes every week to control Aedes mosquitoes in the wild.
Before understanding the production process of a mosquito factory, let's first understand the growth and development process of mosquitoes. The life cycle of a mosquito consists of stages such as adult, egg, larvae, and pupae.
In the video, we can see that mosquito eggs hatch larvae in the water, and the larvae have four instars, from less than two millimeters to more than ten millimeters, and they grow more and more stout. Every age transition requires the stage of molting. After going through the 1st, 2nd, 3rd and 4th instar larval stages, the 4th instar larvae can turn into mosquito pupa after molting again, and we can see the process of pupation, the epidermis of the 4th instar larvae slowly fades away, forming a pupa that is a bit like a tadpole.
The pupal does not feed, but it can swim actively. After two days of development, the pupae can emerge into adult mosquitoes. We can tell by the fact that it does not have two brushes on its head, but has a distinct beak, and this is a feathered female mosquito.
Let's look at the process of male mosquito emergence, which is also after the pupal skin fades, the adult mosquito spreads its wings and flies out. After a day or two of development and mating, the newly fledged female mosquito needs to suck blood and then look for water to lay eggs. At 26-27 degrees Celsius and 75% humidity, it takes about 7 days for mosquitoes to complete the development of one generation. If the weather is hot, the cycle of mosquito development is shortened, so the hotter the weather, the more mosquitoes will be. The lower the temperature, the less frequent the mosquitoes' activity.
So how do we produce mosquitoes in a short period of time and on a large scale in our factories? First of all, we have to collect a large number of mosquito eggs, we put the mosquito eggs in the incubator at one time, and after it hatches larvae, we put the larvae into the larval breeding rack with multiple trays, and add the larvae food. Do you know what the larvae eat? It is made up of a variety of ingredients, such as yeast, beef liver powder, etc. Under the rearing of these organic substances, the larvae will undergo 4 instars to develop and pupate.
Female and male pupae differ in size and morphology. However, in the larval stage, we cannot distinguish between male and female by the naked eye, but only by genetic testing. In the pupal stage, the male pupae are smaller and the female pupae are larger. According to the size of the chest of male and female mosquito pupae, we designed a male and female separator, the diameter of the mesh screen in the separator can allow small male pupae to pass through, and large female pupae are intercepted on it, through this physical method can separate female and male pupa.
The separated male and female pupae were put into different mosquito cages and waited for them to emerge into adult mosquitoes. The feathered adult mosquitoes are put in the cold storage to freeze and halo, and then placed in the irradiator for irradiation. There is an irradiation cup in the middle of the inner cavity of the irradiation instrument, which is used to place mosquitoes, and the circular area on both sides has built-in X-ray generators, and X-rays are emitted from both sides, and the rotation of the base of the middle irradiation cup can make the mosquitoes evenly irradiated, that is, the dose of radiation for each mosquito is almost equal.
This is the appearance of the irradiator, which emits X-rays when the power is turned on and stops producing X-rays when the power is turned off. Because the radiator door contains a lead plate component, which can block the penetration of rays, it is safe to close the door when the irradiator is working. We can also see the warning signs of ionizing radiation when we do CT and X-ray examinations in hospitals, and when we see this sign, we should be vigilant not to expose ourselves to radiation sources or wear appropriate protective equipment to protect our health.
We transport the sterilized male mosquitoes to the wild, because there are both male and female mosquitoes in the wild, so the sterilized male mosquitoes need to compete with the male mosquitoes in the wild for the female mosquitoes in the wild, which is equivalent to "spies" and "undercover". After the sterilized male mosquito is transported to the wild in a frozen state, it is then released at room temperature, and when the temperature rises, the mosquito will revive and fly out on its own, which completes the release of the sterilized male mosquito.
To test the effectiveness of SIT in the control of Aedes mosquitoes
During the long-term release process, we need to evaluate the effectiveness of the sterile insect technique in the control of Aedes mosquitoes. In this regard, we use three methods to comprehensively evaluate, namely egg traps, adult mosquito traps, and human traps.
The first method is to collect mosquito eggs with an oviduct. The ovarian consists of a cup with water and paper filter inside and three round holes in the lid. In the wild, blood-sucking female mosquitoes can fly into the oviduct to lay eggs, and the eggs laid by the female mosquito will collect on the filter paper. We counted the number of eggs on the filter paper to see if the number of mosquito eggs was reduced compared to the area before release, and in areas where it was not released, and whether the proportion of mosquito eggs hatching larvae was reduced. The eggs that hatch are incomplete, and the egg cap will open, so we can judge whether the mosquito eggs hatch or not according to the morphology of the eggs, and calculate the hatchability rate.
The second method is to capture adult mosquitoes with adult traps. The adult mosquito trap is a cylindrical device with two colors, black and white, a black fitting in the middle, a white waterproof cover on the outside, a small electric fan inside, and some attractants. When mosquitoes are lured in and enter through the top of the trap, they can't fly out due to the rotation of the internal fan. Why is this device black and white? Because mosquitoes have underdeveloped vision, devices with strong black and white color contrast are attractive to mosquitoes, and there are some chemoreceptors on the bristles of mosquito antennae that can sense some chemicals volatilized by the human body or attractants for precise positioning.
The third method is human inducement. Staff or volunteers stand in the field for about 15 minutes and use gravids to catch and count mosquitoes that come to bite.
Through these three methods, we can scientifically evaluate the effectiveness of the sterile insect technique in the control of Aedes mosquitoes.
This biological mosquito control method has been successfully applied to the islands of Nansha. In some areas, the number of female Aedes albopictus mosquitoes in the wild has been reduced by more than 90 percent, and the bite rate has also decreased by more than 90 percent, which means that our control methods have achieved remarkable results.
Science and technology are tools for "people", and nuclear technology can also be used to create peace.
Well, thank you!
Source: Nansha District Association for Science and Technology