Is human the most dangerous animal in the world? Check out the correct answer

Figure 1: The deadliest animals. Ranked by estimated number of deaths caused each year[1]

Written by Li Qingchao (Shandong Normal University)

The mosquito tops the list of the deadliest animals because of the pathogens it carries. Tsetse flies, cone bugs, and ticks that frequently appear in the news also endanger human health by spreading diseases. These organisms that can spread infectious pathogens between humans and between animals and humans are called infectious disease vectors, or "vectors" for short.

Vector-borne diseases account for 17% of all infectious diseases and cause more than 700,000 deaths each year[2]. The main vectors include mosquitoes, ticks, lice, fleas, cone bugs, flies, and freshwater snails. This article will introduce you to the incomparable representative of vectors - mosquitoes.

The infamous mosquito

Mosquito (pronounced like the name of a certain type of coffee, but actually means "little fly") is a general term for insects of the family Culicidae in the order Diptera, and Aedes, Anopheles and Culex are the main vectors of infectious diseases.

The genus Aedes has nothing to do with humans. Instead, it comes from the ancient Greek word "ἀηδής", which means "hateful". Aedes mosquitoes are found on all continents except Antarctica and can be spread through human activities. In the 1980s, the trade in used tires spread Aedes mosquitoes to the Americas (question: why can mosquitoes travel to the Americas on used tires?). The most dangerous members of the genus Aedes are Aedes aegypti (also known as the yellow fever mosquito) and Aedes albopictus (also known as the Asian tiger mosquito), which mainly spread yellow fever, dengue fever, Zika fever, chikungunya fever, Japanese encephalitis, etc. Among them, Aedes aegypti can spread 54 viruses and 2 malaria parasites [3].

The hallmark feature of the Aedes mosquito is the black and white patterns on its body and legs, and its wings without any markings. It is commonly known as the black mosquito or the flower mosquito. It usually flies and forages during the day except at dusk and dawn, and it bites very ferociously.

Figure 2: The legs and body of the Aedes mosquito have black and white patterns. The upper side of the picture shows the difference in the back patterns of the two Aedes mosquitoes. Source: Wikipedia

Tips: Dengue fever and yellow fever

Dengue fever is a viral disease mainly transmitted by Aedes aegypti and Aedes albopictus. The main symptoms include high fever, muscle, bone marrow and joint pain, and extreme fatigue. It often occurs in cities and semi-urban areas in tropical and subtropical climates around the world. It also occurs in China, mainly in Hainan, southern Taiwan, Guangxi and parts of Guangdong, and has also spread to Zhejiang and Jiangxi, but has not spread throughout the country.

Dengue fever is estimated to infect 100 to 400 million people each year, 80% of whom have mild or asymptomatic symptoms, while severe dengue fever carries a risk of death. There are four serotypes of dengue virus, and none of them are strong enough to induce a long-lasting protective immune response, so a person may be infected with four different dengue viruses[4]. Currently, the development and application of dengue vaccines have not been smooth, and there is no specific drug. The main way to prevent deaths caused by severe cases is to provide effective clinical supportive treatment.

Yellow fever is an acute viral hemorrhagic disease mainly transmitted by the Aedes mosquito. Its main symptoms include fever, headache, jaundice, muscle pain, nausea, vomiting, and fatigue. Yellow fever is mainly prevalent in Africa and Central and South America, causing tens of thousands of deaths each year[5]. The good news is that yellow fever can be effectively prevented through vaccines.

The name of the genus Anopheles comes from the ancient Greek word "ἀνωφελής", which means "doing no good". There are more than 460 species found so far, and more than 100 species can transmit malaria, so Anopheles is also called the malaria mosquito. Anopheles gambiae is the most contagious malaria-transmitting mosquito known. In addition, Anopheles can also transmit lymphatic filariasis (also known as elephantiasis). Anopheles is generally gray-brown, with spots or scales on its wings. It only flies short distances. When stationary, its body is at a 45° angle to the surface of the object (with its butt sticking out), and it is active at night.

Figure 3: Anopheles gambiae[6]

Tips: Malaria

Malaria, commonly known as "shaking the body", is an infectious disease caused by infection with Plasmodium parasites. Symptoms include recurrent fever, fatigue and headaches. In severe cases, jaundice, coma or death may occur. It is estimated that there are 219 million cases worldwide each year, of which 90% occur in Africa, resulting in more than 400,000 deaths in the region each year. Most deaths occur in children under the age of five, accounting for 80% of the total deaths from malaria in Africa[7].

Why are we not sensitive to such a "famous" disease? This is thanks to my country's outstanding contribution to malaria prevention and control. In addition, the invention of artemisinin has made outstanding contributions to the treatment of malaria in the world, saving the lives of millions of malaria patients. Tu Youyou won the 2015 Nobel Prize in Physiology or Medicine for this. In 2021, my country was certified as a "malaria-free country" (at least no malaria transmission in the past three consecutive years) by the World Health Organization's Malaria Elimination Certification Group [8] - you have to know that around 1949, the number of malaria cases in my country was as high as 30 million per year.

Figure 4: Different forms of malarial parasites. The life cycle of malarial parasites is complex. The left picture shows the sporozoites in mosquitoes that are transmitted to humans. The right picture shows the malarial parasites in the form of rings in red blood cells and crescent-shaped gametocytes in the blood. Source: Wikipedia

Culex, whose name comes from the Latin for midge, mosquito or gnat, can transmit West Nile disease, Japanese encephalitis, and filariasis. Culex mosquitoes have transparent wings, strong flying ability (preferring to bite birds), and are also nocturnal.

Figure 5: Culex mosquito. Its wings are transparent and its body is parallel to the ground when at rest.[9]

Tips: Japanese encephalitis

Japanese encephalitis is mainly transmitted by Culex mosquitoes and is the main source of viral encephalitis in many Asian countries, with approximately 68,000 clinical cases each year. It is rare for patients to develop encephalitis symptoms after being infected with the Japanese encephalitis virus, but once encephalitis occurs, the mortality rate can be as high as 30%, and there is a 30-50% chance that survivors will have psychiatric sequelae. There is currently no specific drug for this disease, and the only way to help patients overcome the infection is to alleviate clinical symptoms. Fortunately, there is an effective vaccine available [10].

Japanese encephalitis virus (JEV), dengue virus (DENV), yellow fever virus (YFV), and New Nile virus (WNV) are all members of the Flaviviridae family. They are relatively close on the viral evolutionary tree and are vector-borne pathogens.

Figure 6: Flaviviridae viruses contain a large number of important animal pathogens, including several of the vector-borne viral diseases described in this article. | Author

Well, the introduction of "hateful" and "harmless" mosquitoes is over. Next time there is a mosquito in your bedroom, can you recognize which one it is? Forget it, just swat it to death, don't be too gentle. But maybe you have also seen very big mosquitoes quietly resting on the walls of the community, most of them are members of the Tipulidae family, they don't bite people, and they are big in size.

Figure 7: A giant mosquito with a wingspan of 11.15 cm. “No matter how small a mosquito is, it is still meat.” It is not that small anymore.[11]

A bite is serious

"Don't sing in my ear! If you want blood, just take it. Why give me a bag?" A little blood sucked by a mosquito is really nothing, but the buzzing of mosquitoes is annoying, and the bites are itchy and unbearable. In fact, this may be the result of evolutionary selection: those who can tolerate mosquitoes have already received the "Darwin Award for Evolution".

Why do mosquitoes bite people? Because they need to raise their young. Only female mosquitoes suck blood to obtain nutrients for laying eggs. Female mosquitoes lay their eggs in stagnant water or near the water surface. The hatched larvae live in the water and are called mosquito larvae (if there are mosquito larvae in the stagnant water at home, they must be cleaned up decisively). When the mosquito larvae mature, they form pupae and become mosquitoes after they emerge from the water. Female mosquitoes are attracted by the high heat, strong smell and dark color of the human body. When they get the chance, they use their piercing and sucking mouthparts to suck blood. Once bitten by a mosquito, do not scratch excessively, otherwise new wounds may form and cause infection. You can apply baking soda and water to the affected area, or apply anti-itching drugs such as Fengyoujing, Baihua Oil, and Qingliang Oil.

Figure 8: The life cycle of Culex pipiens. The sequence is egg, larva, pupa and adult (before and after blood feeding) [12]

The mouthparts of a mosquito look like a syringe needle, but actually consist of five structures, namely the upper lip, upper jaw (2 strips), tongue, lower jaw (2 strips), and lower lip, a total of seven structures. The lower lip is like a scabbard, which does not pierce the human body but plays a supporting role. The mouth needle composed of other structures pierces the human skin, cuts the tissue through the upper and lower jaws and searches for blood vessels. Once a blood vessel is found, the upper lip is responsible for sucking blood, while the tongue is responsible for injecting saliva. The saliva of mosquitoes contains anticoagulant and anesthetic components, which can anesthetize our nerves and prevent blood from clotting, so that they can suck blood without attracting our attention.

Figure 9: The piercing-sucking mouthparts of a mosquito[13]

Figure 10: A mosquito inserts its mouthparts into the skin (top) and searches for a blood vessel (middle) to feed on (bottom)[14]

The danger of mosquito bites is not the loss of blood, but the deadly saliva that mosquitoes shed when biting people. When a mosquito bites you, it uses special piercing and sucking mouthparts to pierce your skin and inject saliva into your skin. Your body will have a hypersensitive reaction to saliva, resulting in lumps and itching.

Mosquitoes that bite infected people will acquire the pathogens, and when they bite other people, they will inject the pathogens into the human body along with the saliva and infect other people. It should be noted that this process is not a simple transfer or transportation, but a process of "pathogens infecting mosquitoes and multiplying and amplifying" - vector-borne pathogens first infect the midgut of mosquitoes, multiply and amplify, then enter the saliva through the salivary glands, and finally inject into new individuals along with the saliva. The specific process of different infectious diseases varies. For example, malarial parasites hatch oocysts in the midgut of mosquitoes and release sporozoites, which then pass through the mosquito circulatory system to the salivary glands and enter the saliva, while the yellow fever virus can replicate directly in the salivary glands. Non-vector-borne blood-borne diseases, such as hepatitis viruses and HIV, can enter the mosquito through the blood, but cannot infect mosquitoes and amplify, let alone use the salivary glands to enter the saliva, so it is almost impossible to be transmitted through mosquitoes.

Figure 11: The transmission process of mosquito-borne pathogens. The pathogens are multiplied in the mosquito. [15]

Eliminating mosquitoes is a great virtue

Eliminating mosquitoes is an effective means of eliminating mosquito-borne infectious diseases. Generally speaking, there are three types of measures: physical, chemical and biological. In terms of effectiveness, they can be roughly divided into three levels: avoiding mosquitoes (mosquitoes are there, but they can't bite you), repelling mosquitoes (driving away mosquitoes), and eliminating mosquitoes (killing mosquitoes).

Physical means: Mechanical blocking, screens, mosquito nets, long sleeves and long pants to prevent mosquitoes from getting close to you and biting you. A palm leaf fan or fan can also blow away mosquitoes. Remember to take a palm leaf fan when you go out for a walk. By cleaning up stagnant water to destroy the growth environment of mosquito larvae, you can also achieve the effect of controlling mosquitoes. Mosquito repellent lamps that use ultraviolet light and other light sources to attract and kill mosquitoes are not very effective and are likely to attract other insects. Slap to death, simple and crude, relieves anger.

Chemical means: Use mosquito repellent containing DEET, chlorpyrifos and other ingredients to repel mosquitoes; use pyrethroid insecticides to kill mosquitoes. You can also use sugar, lactic acid, octenal or commercial traps to trap mosquitoes.

Biological means: protect the natural enemies of mosquitoes, use pathogens that can infect mosquitoes for biological control, or use genetically modified mosquitoes to interfere with the mosquito breeding process. If you are going to enter an area where insect-borne infectious diseases are prevalent, you need to take precautions and get the available vaccines. For relevant information, please visit the website of the Chinese Embassy [16].

Tips: Silent Spring

"Silent Spring" is a landmark book in environmental protection. The wave of public opinion it set off led to a reversal of the United States' pesticide use policy and ultimately led to the establishment of the Environmental Protection Agency.

In 1939, Swiss chemist Paul Hermann Müller discovered that dichlorodiphenyltrichloroethane (commonly known as DDT) had a good insecticide effect. In the second half of World War II, DDT was widely used to kill insect vectors of malaria and typhus. DDT became a major contributor to malaria control, and Müller also won the Nobel Prize in Physiology or Medicine in 1948[17]. Almost at the same time, Rachel Carson paid attention to the impact of pesticides on the environment, especially birds, and eventually published the book Silent Spring. The book listed the various hazards caused by the abuse of pesticides in various places, prompting the public to pay general attention to pesticides and environmental pollution.

Figure 12: DDT was once widely used to disinfect pests in the environment and on individuals. Source: Wikipedia

Figure 13: Mosquitoes bite people, that’s bad! [18] (Question: What kind of mosquito is the cartoon character in the picture?)

Check to see if there are mosquito larvae in your hydroponic tank at home-_-||

References

[1] https://www.gatesnotes.com/Health/Most-Lethal-Animal-Mosquito-Week

[2] https://www.who.int/zh/news-room/fact-sheets/detail/vector-borne-diseases

[3] https://www.wrbu.si.edu/vectorspecies/mosquitoes/aegypti

[4] https://www.who.int/en/news-room/fact-sheets/detail/dengue-and-severe-dengue

[5] https://www.who.int/zh/news-room/fact-sheets/detail/yellow-fever

[6] https://phil.cdc.gov/Details.aspx?pid=18764

[7] https://www.who.int/zh/news-room/fact-sheets/detail/malaria

[8] https://www.who.int/zh/news/item/30-06-2021-from-30-million-cases-to-zero-china-is-certified-malaria-free-by-who

[9] https://euresisjournal.org/difference-between-culex-and-anopheles-mosquito

[10] https://www.who.int/zh/news-room/fact-sheets/detail/japanese-encephalitis

[11] http://www.xinhuanet.com/politics/2018-04/24/c_1122732440.htm

[12] https://www.cdc.gov/mosquitoes/about/life-cycles/culex.html

[13] https://cronodon.com/BioTech/Insects_nutrition.html

[14] https://www.natgeomedia.com/science/article/content-6424.html

[15] https://www.nytimes.com/interactive/2016/health/what-is-zika-virus.html

[16] https://www.mfa.gov.cn/ce/cesr/chn/zytz/t1528594.htm

[17] https://en.wikipedia.org/wiki/DDT

[18] https://www.cdc.gov/mosquitoes/mosquito-bites/symptoms.html

Produced by: Science Popularization China

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