The secret weapons and cultivation tips of mosquitoes丨World Mosquito Day

August 20 is World Mosquito Day. This is not a holiday for mosquitoes. The main reason for setting up this commemoration day is to raise people's awareness of mosquito-borne infectious diseases such as malaria. Mosquitoes can carry many dangerous pathogens, which can be called the "secret weapons" of mosquitoes. So how do they infect people? In recent years, scientists have taken great pains to understand the vector efficiency of mosquitoes and have learned a lot of mosquitoes' "cultivation secrets".

Written by Chen Lu (Department of Basic Medical Sciences, School of Medicine, Tsinghua University) , Liu Jianying (Institute of Infectious Diseases, Shenzhen Bay Laboratory) , Cheng Gong (Department of Basic Medical Sciences, School of Medicine, Tsinghua University)

As a common blood-sucking insect, mosquitoes have always been regarded as a nuisance in summer, but the health threats hidden behind them are far more than people know. Mosquitoes are the spreaders of many important diseases, and it is no exaggeration to call them "wings of death". Mosquito-borne infectious diseases refer to diseases that are transmitted through mosquito bites. Among them, malaria, dengue fever, yellow fever, Zika virus disease, etc. are typical examples that people are familiar with. The number of deaths and illnesses caused by these diseases each year remains high, seriously threatening global public health security. In order to raise people's awareness of mosquito-borne infectious diseases such as malaria, August 20th of each year is designated as "World Mosquito Day". Today, let's take a look at how mosquitoes become the "lifelong enemy" of humans.

Malaria: The oldest mosquito-borne disease

Malaria is undoubtedly the most deadly mosquito-borne disease to humans. In many countries where malaria is prevalent, it is the leading cause of illness and death. Its typical symptoms are recurrent chills, fatigue, vomiting, and headaches. If not treated promptly, the disease may progress to jaundice, splenomegaly, anemia, epilepsy, and even death. The global mortality rate for malaria is 0.3%-2.2%, while the mortality rate for severe malaria can reach 30%. The breadth and depth of the impact of malaria has become a global health problem. An estimated 200 million cases of malaria occur each year, resulting in hundreds of thousands of deaths[1] . Even more shocking, historians speculate that malaria may have caused the deaths of approximately 6 billion people since the emergence of humans [2].

Malaria is one of the oldest diseases in human history. Ancient documents show that malaria swept through major civilizations from China to Mesopotamia, from Egypt to India. As early as the ancient Greek period, people noticed that people living in swampy areas often had fevers and enlarged spleens. At that time, people generally believed that this was caused by breathing the "miasma" produced in the swamps. The word "malaria" comes from "mala" (bad) + "aria" (air).

It was not until the end of the 19th century that scientists began to gain a deeper understanding of the disease. In 1880, Dr. Charles Louis Alphonse Laveran discovered a very special organism in the blood of malaria patients - Plasmodium (Figure 1). He observed that this organism was not only able to move, but also multiply in the host, eventually causing malaria. Since then, mankind has begun a scientific fight against malaria.

Figure 1. Plasmodium drawn by Alphonse Laveran[3]. Image source: Reference [3]

The transmission mechanism of malaria parasites once puzzled scientists. Although it was known that mosquitoes could transmit the parasite filariasis, the specific transmitter of malaria parasites remained a mystery. Ronald Ross, a military doctor stationed in India, examined thousands of mosquitoes in areas with high incidence of malaria, but could not find any trace of malaria parasites. However, when he tried to feed the blood of malaria patients to different types of mosquitoes, he found malaria parasite spores (sporozoites) in a certain Anopheles mosquito. In 1899, Ross successfully used Anopheles mosquitoes infected with Plasmodium vivax to infect his medical student son and a volunteer, thus confirming that Anopheles mosquitoes were the vectors of malaria [4]. Later, he called the day he discovered the malaria parasite spores - August 20, 1897 - "Mosquito Day". This day is now known as "World Mosquito Day" and people will always remember Ross's contribution.

It was also discovered that malaria can only be transmitted by mosquitoes, and it was not until 1957 that the life cycle of the Plasmodium parasite in the human body was fully understood. When an Anopheles mosquito infected with Plasmodium parasites bites a human, the Plasmodium sporozoites in its salivary glands enter the human body along with the saliva. The sporozoites then quickly transfer to the liver through the blood circulation, infect the liver cells, and mature and reproduce there. This stage is considered the incubation period and there are no clinical symptoms. Once the Plasmodium parasites have completed their proliferation and replication, a large number of merozoites are released from the infected liver cells and invade the red blood cells. At this time, the infected person will begin to develop clinical symptoms and may even die [5].

The two key antimalarial drugs currently in use are derived from two important plants: artemisinin from the Artemisia annua plant and quinine from the Cinchona plant. Quinine and artemisinin are the most effective antimalarial drugs today. At this point, the story of human exploration of Plasmodium is finally basically complete. In the more than 120 years since the Nobel Prize was established, malaria-related research has won four awards: in 1902, it was awarded to Ronald Ross for proving that Anopheles is the vector of malaria and elucidating the developmental history of Plasmodium; in 1907, it was awarded to Alphonse Laveran for discovering Plasmodium in blood cells; in 1965, it was awarded to Robert Burns Woodward for the first artificial synthesis of quinine; and in 2015, it was awarded to Tu Youyou for isolating a new antimalarial drug, artemisinin. In the long struggle between humans and malaria, they have played a milestone role in the history of medicine.

Yellow fever: the oldest mosquito-borne viral disease

In addition to parasites, viruses are also key factors in causing mosquito-borne diseases. Yellow fever virus was the first virus confirmed to be transmitted by mosquitoes. Historical data show that there were records of yellow fever outbreaks in Mexico as early as 1648. In the following 200 years, yellow fever was one of the most deadly and feared infectious diseases, causing massive casualties in Africa and the Americas [6].

Yellow fever virus infection presents with different clinical features, such as self-limited illness with fever, muscle pain, headache, nausea and vomiting, and other symptoms similar to mild flu; in most cases, the symptoms disappear after 3 to 4 days. However, a small number of patients will enter a second, more virulent stage within 24 hours, during which patients may experience recurring high fever, bleeding, jaundice, dark urine, liver and kidney failure, etc. Half of the patients who relapse die within 7-10 days [7] (Figure 2).

Figure 2. Four illustrations showing the progression of yellow fever. Image credit: Etienne Pariset and André Mazet. 1820. Four illustrations showing the progression of yellow fever.

Before the 19th century, people had no idea about the cause and transmission of yellow fever. It was not until 1881 that Cuban doctor Carlos Juan Finlay proposed the "mosquito hypothesis" based on epidemiology that mosquitoes could transmit yellow fever, laying the foundation for subsequent scientific research on yellow fever. In 1901, Walter Reed conducted a study using mosquitoes biting human volunteers and confirmed that Aedes mosquitoes were the main vector of yellow fever. Subsequent mosquito-borne intervention measures implemented in Cuba did control the incidence of yellow fever. In addition, Reed also observed that yellow fever was caused by substances in the patient's blood that could pass through extremely small pores, indicating that the pathogen of yellow fever was much smaller than bacteria. However, it was not until 1927 that the yellow fever virus was isolated, becoming the first human virus isolated in history [8] . Later, Max Theiler discovered that after the yellow fever virus was propagated multiple times in animals, the toxicity of the virus would gradually decay. After years of experiments, Theil finally isolated an attenuated strain called 17D. 17D has very low virulence, but it can induce a protective immune response. Theil developed a yellow fever vaccine based on this, and the immunity after vaccination lasts for 30-35 years. It is considered one of the most effective vaccines in history [9]. Max Theil also won the Nobel Prize in 1951 for this.

Dengue fever: a potential killer in the tropics

Dengue fever is an acute tropical viral disease caused by dengue virus, which has four serotypes. It is mainly transmitted through the bites of Aedes aegypti and Aedes albopictus. The disease is widely spread in tropical and subtropical regions. Dengue fever has occurred in Guangdong, Fujian, Yunnan and Taiwan in my country. Most people infected with dengue virus are asymptomatic, while symptomatic infections often present with flu-like symptoms such as high fever, headache, muscle and joint pain, and rash. In severe cases, it may cause bleeding accompanied by shock and even death[10].

In 2019, the World Health Organization (WHO) listed dengue fever as one of the top ten potential threatening diseases. Globally, approximately 3.5 billion people are at risk of infection by the dengue virus. It is estimated that dengue cases increased by 600% between 1999 and 2019 [11] . Due to the global mobility of the population, climate change, and the continued expansion of urbanization, the prevalence of dengue disease will continue to expand. In July 2023, the WHO warned that the number of dengue fever infections worldwide this year may reach a record high because global warming is conducive to the growth of mosquitoes and the spread of mosquito-borne diseases.

Japanese encephalitis: Asia's invisible threat

Unlike dengue virus, Japanese encephalitis virus (JEV) is usually transmitted by Culex mosquitoes and the natural cycle of the virus involves multiple vertebrate hosts. Pigs and waterfowl are considered to be the two most important hosts for JEV amplification. Although they are usually asymptomatic after infection, they can develop high viremia, sufficient to transmit the virus to mosquitoes. Humans and horses are considered to be accidental hosts of JEV and are not important sources of mosquito infection with JEV. After infection with JEV, humans only develop low-level and transient viremia, but less than 1% of infected people will still show symptoms of fatal encephalitis [12] . JEV is mainly prevalent in Asia, including China, South Korea, Japan and Thailand, and is the main cause of viral encephalitis in these countries. JEV can cause severe neurological disease. Symptoms of the acute encephalitis stage may include neck stiffness, hemiplegia, convulsions and high fever. It is a very serious disease with a mortality rate of up to 30% in patients with encephalitis. 30% to 50% of surviving patients may suffer from permanent intellectual, behavioral, or neurological disabilities, such as deafness, paralysis, and inability to speak [13].

Zika virus disease: a new public health threat

Zika virus was first isolated from a sentinel macaque in the Zika Forest of Uganda in 1947. Prior to 2007, Zika virus had been circulating quietly in many parts of Africa and Asia without causing severe disease or major outbreaks. Most Zika infections are relatively mild and may include fever, joint pain, rash, and conjunctivitis. However, in 2015-2016, Zika virus attracted widespread global attention. In 2015 alone, millions of cases of Zika infection were reported in Brazil. Although infection with Zika virus is rarely fatal, it can cause a rare immune disease called Guillain-Barré syndrome. If the infected person is pregnant, the virus can also cause congenital microcephaly in the fetus and miscarriage [14, 15].

Research on the transmission mechanism of mosquito-borne viruses

Everyone knows that mosquitoes transmit pathogens to animals when they suck blood, but how do viruses work in mosquitoes?

In the field of mosquito-borne diseases, this issue mainly focuses on the study of mosquito vector competence, that is, the ability of mosquitoes to acquire, maintain and transmit insect-borne viruses. First, we should understand that only some species of female mosquitoes need to suck blood for nutrition during the egg hatching cycle, while most mosquitoes feed on nectar and plant sap.

Studies have found that after mosquitoes suck blood containing the virus, the virus enters the midgut and establishes stable replication in the mosquito's midgut epithelial cells; the virus is then released into the mosquito's hemolymph and spreads to the mosquito's systemic tissues, such as fat bodies, hemolymph cells, muscles, salivary glands, and nervous tissues; the virus is then enriched in the salivary glands, and during the next blood-sucking process, the pathogen will enter the next host along with the anticoagulants and allergens in the saliva (Figure 3). Mosquito saliva has been confirmed to promote the transmission of mosquito-borne viruses to hosts and is associated with the development of related diseases.

Figure 3. The process by which mosquitoes are infected and spread viruses. Image source: Snodgrass, Robert Evans. 1959. "The anatomical life of the mosquito." Smithsonian Miscellaneous Collections, 139, (8), 1–87.

Obviously, mosquitoes can only acquire the virus and continue to spread it after feeding on infected people, so why can mosquitoes easily find infected people? Human odor is a key factor in regulating mosquito behavior. Dengue virus and Zika virus can regulate the skin microorganisms of infected people and reshape the smell of infected people, thereby affecting the olfactory perception of mosquitoes, allowing mosquitoes to efficiently locate infected people and suck blood containing the virus[16].

Recent studies have found that components in the host's blood (such as iron ions [17] and secreted viral nonstructural protein NS1 [18] ) can regulate mosquitoes' acquisition of viruses. In addition to host blood components, mosquitoes' intestinal symbiotic bacteria also play an important role in the acquisition of viruses. There are a rich variety of intestinal microbial flora in the mosquito's intestines. Studies have found that there is a kind of Serratia marcescens in the intestines of Aedes mosquitoes that can assist viruses in infecting the mosquito's intestines, significantly enhancing the susceptibility of Aedes mosquitoes to mosquito-borne viruses [19] . In a recent study, researchers found that a salivary protein can significantly enhance the infection of mammalian immune cells by Zika virus and dengue virus, proving that it is a key factor that assists the transmission of mosquito-borne viruses [20]. These studies not only reveal the interactive relationship between hosts, vector mosquitoes and viruses, but also provide new intervention targets and ideas for the prevention and control of important mosquito-borne viruses.

In recent years, the progress made in the field of mosquito-borne virus infection and transmission is exciting. A large number of studies have revealed the intricate relationship between hosts, vector mosquitoes and viruses. Although researchers have rapidly expanded their understanding of the interactions between mosquitoes, viruses and hosts, there are still many puzzling mysteries that need to be studied in depth, such as how mosquitoes tolerate viral replication without producing serious pathological reactions; why different viruses prefer different mosquito species for transmission; how genetic background and environmental differences affect the vector efficiency of mosquitoes, etc. After hundreds of millions of years of evolution, mosquitoes have always accompanied the evolution of humans. It is foreseeable that they will continue to affect our survival. Therefore, how to effectively reduce the threat of mosquitoes to humans and how to coexist peacefully with mosquitoes will be an important topic for our continued exploration in the future. We need to further explore the interaction mechanism between mosquitoes and pathogens in order to develop more effective control and prevention strategies. In addition, strengthening public health education and raising people's awareness of mosquito-borne virus transmission are also the key to prevention. In the face of these challenges, scientists, doctors and all sectors of society play an important role. Cooperation and innovation will be the key to solving these problems. Through joint efforts, we hope to establish a more harmonious coexistence relationship with mosquitoes while protecting human health.

References

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[2] Whitfield, J. (2002). Portrait of a serial killer. Nature 3.

[3] Bruce-Chwatt, LJ (1981). Alphonse Laveran's discovery 100 years ago and today's global fight against malaria. Journal of the Royal Society of Medicine 74, 531-536.

[4] Cox, FE (2010). History of the discovery of the malaria parasites and their vectors. Parasites & vectors 3, 1-9.

[5] Varo, R., Chaccour, C., and Bassat, Q. (2020). Update on malaria. Medicina Clínica (English Edition) 155, 395-402.

[6] Barrett, AD, and Higgs, S. (2007). Yellow fever: a disease that has yet to be conquered. Annu. Rev. Entomol. 52, 209-229.

[7] Douam, F., and Ploss, A. (2018). Yellow fever virus: knowledge gaps impeding the fight against an old foe. Trends in microbiology 26, 913-928.

[8] Staples, JE, and Monath, TP (2008). Yellow fever: 100 years of discovery. Jama 300, 960-962.

[9] Monath, TP (2005). Yellow fever vaccine. Expert review of vaccines 4, 553-574.

[10] Pierson, TC, and Diamond, MS (2020). The continued threat of emerging flaviviruses. Nature microbiology 5, 796-812.

[11] Organization, WH (2022). Virtual Meeting of Regional Technical Advisory Group for dengue and other arbovirus diseases, New Delhi, India, 4-6 October 2021. World Health Organization. Regional Office for South-East Asia.

[12] Misra, UK, and Kalita, J. (2010). Overview: japanese encephalitis. Progress in neurobiology 91, 108-120.

[13] Campbell, GL, Hills, SL, Fischer, M., Jacobson, JA, Hoke, CH, Hombach, JM, Marfin, AA, Solomon, T., Tsai, TF, and Tsu, VD (2011). Estimated global incidence of Japanese encephalitis: a systematic review. Bulletin of the World Health Organization 89, 766-774.

[14] Mlakar, J., Korva, M., Tul, N., Popović, M., Poljšak-Prijatelj, M., Mraz, J., Kolenc, M., Resman Rus, K., Vesnaver Vipotnik, T., and Fabjan Vodušek, V. (2016). Zika virus associated with microcephaly. New England Journal of Medicine 374, 951-958.

[15] Musso, D., Ko, AI, and Baud, D. (2019). Zika virus infection—after the pandemic. New England Journal of Medicine 381, 1444-1457.

[16] Zhang, H., Zhu, Y., Liu, Z., Peng, Y., Peng, W., Tong, L., Wang, J., Liu, Q., Wang, P., and Cheng, G. (2022). A volatile from the skin microbiota of flavivirus-infected hosts promotes mosquito attractiveness. Cell 185, 2510-2522. e2516.

[17] Zhu, Y., Tong, L., Nie, K., Wiwatanaratanabutr, I., Sun, P., Li, Q., Yu, X., Wu, P., Wu, T., and Yu, C. (2019). Host serum iron modulates dengue virus acquisition by mosquitoes. Nature Microbiology 4, 2405-2415.

[18] Liu, J., Liu, Y., Nie, K., Du, S., Qiu, J., Pang, X., Wang, P., and Cheng, G. (2016). Flavivirus NS1 protein in infected host sera enhances viral acquisition by mosquitoes. Nat Microbiol 1, 16087. 10.1038/nmicrobiol.2016.87.

[19] Wu, P., Sun, P., Nie, K., Zhu, Y., Shi, M., Xiao, C., Liu, H., Liu, Q., Zhao, T., and Chen, X. (2019). A gut commensal bacterium promotes mosquito permissiveness to arboviruses. Cell host & microbe 25, 101-112. e105.

[20] Sun, P., Nie, K., Zhu, Y., Liu, Y., Wu, P., Liu, Z., Du, S., Fan, H., Chen, C.-H., and Zhang, R. (2020). A mosquito salivary protein promotes flavivirus transmission by activation of autophagy. Nature communications 11, 260.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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