Biology

Mosquitoes in America and Asia are more susceptible to being infected with Zika

The Zika virus (ZIKV) was first isolated from a primate in Uganda in 1947, and a few years later its presence in humans was detected in Uganda and Tanzania. Despite its African origin, there have only been sporadic outbreaks on the continent, while in Asia, the Pacific and the Americas there have been major outbreaks since 2007, the worst of which was the 2016 epidemic that spread to more than 60 countries and caused some 5,000 cases of congenital microcephaly among the babies of women who were infected with the virus during pregnancy. Why has Zika caused so much devastation outside of Africa and not on its home continent? A new study offers an explanation to the puzzle.

The difference would be in the mosquito that transmits it. In all cases, it is the yellow fever mosquito, Aedes aegypti, the main vector involved in the transmission of old dengue diseases and yellow fever, but also emerging diseases such as chikungunya or Zika. But within the yellow fever mosquito there are two subspecies. One of them is Aedes aegypti aegypti, with mosquitoes adapted to living in humanized environments, it reproduces in artificial water containers in urban areas and has a preference for biting humans as explained in a previous post. This subspecies originated in West Africa about 5,000 or 10,000 years ago and from there, the slave trade was responsible for introducing it into America and later Asia. The mosquito adapted to urban environments is now widely distributed throughout the world. In contrast, in most of the African continent, there is another subspecies, Aedes aegypti formosus, which inhabits both wooded and urban areas, breeds in holes in trees and does not feel a special attraction to human blood, feeding on a great variety of vertebrates, including people.

Until now, it was believed that this affinity for living close to humans and their blood was the main explanation why Zika had caused more havoc outside Africa than on the continent, but the study provides another piece of information: the related mosquitoes, Aedes aegypti formosus, are less likely to contract and therefore transmit the Zika virus.

 

The changes suffered during adaptation to human environments were accompanied by a greater ability to become infected

The results suggest that the changes that mosquitoes underwent thousands of years ago when adapting to human environments were accompanied by an intrinsic ability to become infected with this type of virus. To reach this conclusion, the researchers conducted an experiment with mosquitoes from eight populations collected in Africa, America, and Asia. In the laboratory, they were fed blood infected with different strains of the Zika virus to study how they became infected. They observed that the mosquitoes of America and Asia required less viral load in the blood to become infected than the African mosquitoes (Fig. 1).

Fig. 1.  Proportion curves of mosquitoes that become infected with the Zika virus (ZIKV) when feeding on blood at different concentrations of the virus. Each box is the same experiment carried out with different strains of the virus, the colors indicate the origin of the mosquitoes. Source: Mosquito Alert CC-BY based on the original by Aubry et al. 2020. Science 370: 991-996

 

They designed a second experiment to measure the susceptibility to infection of a larger number of African populations, including those on the west coast that are related to those in America and Asia. Those of the coast of Senegal gave similar results to those of America and Asia, with low doses of the virus in the blood they become infected. They found that in mosquitoes of the Aedes aegypti aegypti subspecies it was easier to find the virus in their salivary glands. Not only are they more susceptible to becoming infected with the virus, but they are more likely to transmit it when they feed on a person and inject their saliva.

All the results establish a strong link between the mosquito domestication process that took place around 10,000 – 5,000 years ago and the transmission capacity of viruses such as Zika. The difference in susceptibility between the two subspecies may contribute to the absence of major outbreaks in Africa, although other factors may exist, including mutations in the virus that have allowed it to specialize and become more infectious in populations outside of Africa.


References:

Aubry F, Dabo S, Manet C, Filipović I, Rose NH, Miot EF, Martynow D, Baidaliuk A, Merkling SH, Dickson LB, Crist AB, Anyango VO, Romero-Vivas C, Vega-Rúa A, Dusfour I, Jiolle D, Paupy C, Mayanja MN, Lutwama JJ, Kohl A, Duong V, Ponlawat A, Sylla M, Akorli J, Otoo S, Lutomiah J, Sang R, Mutebi JP, Cao-Lormeau VM, Jarman RG, Diagne CT. Faye O, Faye O, Sall AA, McBride CS, Montagutelli X, Rašić G, Lambrechts L. 2020. Enhanced Zika virus susceptibility of globally invasive Aedes aegypti populations. Science 370: 991-996

Kotsakiozi P, Evans BR, Gloria-Soria A, Kamgang B, Mayanja M, Lutwama J, Le Goff G, Ayala D, Paupy C, Badolo A, Pinto J, Sousa CA, Troco AD, Powell JR. 2018. Population structure of a vector of human diseases: Aedes aegypti in its ancestral range, Africa. Ecology and Evolution 8: 7835-7848

Rose NH, Sylla M, Badolo A, Lutomiah J, Ayala D, Aribodor OB, Ibn N, Akorli J, Otoo S, Mutebi JP, Kriete AL, Ewing EG, Sang R, Gloria-Soria A, Powell JR, Baker RE, White BJ, Crawford JE, McBride C. 2020. Climate and urbanization drive mosquito preference for humans. Current Biology 30: 3570-3579.e6

Vampires, mosquitoes and other bloodsuckers

When feeding on blood, it is called hematophagy (from the Greek αἷμα haima “blood” and φάγειν phagein “eat”). It may seem like a strange way of feeding, but the truth is that this strategy is widely spread in the animal world, from mammals to insects, fish and birds. Feeding on the blood of others has evolved independently in various taxonomic groups, perhaps up to 100 different times, giving rise to around 30,000 blood-sucking species. From vampires, to mosquitoes, butterflies, birds , leeches, fleas, ticks, and a myriad of animals that have adapted to these diets.

In order to feed on blood, animals have had to undergo changes in their physiology, morphology and behavior. Blood is rich in protein but poor in vitamins and carbohydrates, which requires special adaptations, both genetic and with communities of bacteria that help them do so. In addition, the blood is extremely rich in iron, which in excess can be toxic, requiring mechanisms to block it and avoid poisoning.

Being a bloodsucker carries great dangers, the main one being detected by the host and ending up crushed by it. To put ourselves in their place, we should imagine how we would approach an animal whose weight is 35 million times our own. We must approach this gigantic mastodon and bite him hard enough to make him bleed. It’s easy to die trying. This relationship is the one that exists between a human weighing 70 kilos and a mosquito weighing 2 milligrams.

Finding the host from which to extract a few drops of blood is not easy either. In a forest, there are possibly hundreds of meters between one mammal and another. On our scale that would be like traveling several kilometers to look for food. City mosquitoes have it easier because we live in high densities. In any case it is a risky activity, and everything and so has evolved multiple times giving rise to a variety of bloodsuckers much greater than that of fictional vampires.

Vampires, draculae and lampreys

Some groups are obligate blood-suckers, that is, they need to feed on blood to survive. This is the case of the so-called vampire bats (Desmodontinae), a group of three species of bat from the American continent that feed only on blood. They eat birds and mammals, including cattle and humans. Once they have located a host, they have an infrared radiation sensor that allows them to detect areas where blood flows close to the skin. With their sharp and sharp incisors they cut into the skin, but they do not cut veins or arteries. Their saliva, like that of mosquitoes, contains blood thinners that prolong bleeding from the incision. However, they do not absorb blood, as mosquitoes do, but instead suck it up. These are the real vampires.

Vampires don’t absorb blood, they lick it

The name “vampire” was used in French to describe them in 1810, in reference to the Slavic folklore that swarmed the Balkans. At the end of the 16th century, the Slovenian writer Janez Vajkard Valvasor documented the existence of a vampire in Istria. The Serbian voice “wampira” (wam = blood, pir = monster) designated the dead man who returned to feed on the blood of his acquaintances. The term entered the German language when Austria gained control of part of the Balkans, and from there the word jumped and became popular in Western Europe, to the point of giving the name of the myth to the American bats that fed on blood. In a script twist, later, the bats ended up impersonating the folk vampire from the hand of the novel “Dracula” by Bram Stoker. In reality, blood-sucking monsters like Dracula, present in many cultures, have nothing to do with true blood-sucking but with our internal fears, of the ancient panic of imagining something taking away our vital energy.

In the waters of the sea there is another group of vertebrates that feed on blood. These are lampreys, fish with an elongated, cylindrical body, without scales, gelatinous, very slippery. They lack a jaw, instead having a circular suction cup mouth with several concentric circles of teeth. With it, they hook onto their prey, scrape their tissues and suck their blood. Its hosts are various sharks, salmon, cod and even marine mammals.

Even among birds we find species that use blood to feed. The vampire ground finch (Geospiza septentrionalis) of the Galapagos Islands is famous for its unusual diet. Occasionally it feeds by drinking blood from other larger birds, pecking at them with its sharp beak until blood spurts. A behavior that possibly evolved from their behavior of cleaning the parasites of these birds, to feeding directly on the birds.

In Africa, Bupaghus erythrorhynchus also occasionally feed on the blood of mammals for which they normally hunt ticks and lice. It has been seen that when allowed to choose between blood and ticks, these birds opt for blood.

Mosquitoes: life beyond blood

Like vampire ground finches, many other organisms are not obligate blood-suckers, being able to obtain nutrients from resources other than blood. This is the case with mosquitoes. These insects digest blood, but they also consume other substances such as nectar or pollen from plants. In fact, male mosquitoes never feed on blood, only females bite, and not in all species, nor is it always the case. Females require blood for the development of their eggs. Their blood doses are higher when previously they have not been able to feed on the sugars provided by the nectar of the flowers.

Anopheles gambiae mosquitoes increase their frequency of bites if they have not had access to sugars before. When they combine both sources of food they live longer, as has been observed in the case of the common mosquito (Culex pipiens), Anopheles sergentii and Anopheles claviger. Despite the change in the frequency of bites, it is unknown whether it implies a reduction in the transmission of pathogens.

The tiger mosquito has a preference for laying eggs in containers with flowers nearby

But there is evidence that in areas where there are a large number of plants rich in sugars, the number of female Anopheles sergentii mosquitoes can be four times higher than in areas without such plants. Although the presence of plants with sugars reduces the number of bites, it also increases their longevity, survival and allows larger populations of mosquitoes, demonstrating the importance of this food source for mosquitoes. And offending an opportunity to regulate mosquito populations by controlling the plant community of an area.

A work carried out with the tiger mosquito (Aedes albopictus) showed that females preferred to deposit their eggs in containers with flowers nearby, than in containers without flowers. Where there were flowers with sugars, there were more eggs. It is suggested that the selection of flowering breeding sites is to provide a safe source of food for the next generation.

Vampires that feed on vampires

Even animals that feed on blood do not escape other animals that feed on their blood. Even mosquitoes are hosts for other organisms that draw blood from them. Bitting midges, small dipterans, have been documented to parasitize mosquitoes, sucking blood from a mosquito’s swollen stomach.

Vampire bats have more parasites than the average of bats, including those known as “bat flies”, which are all related to flies and mosquitoes, they have the appearance of a spider, with a flattened body, without eyes or wings, specialized in sucking the blood of bats.

The practice of hematophagy by itself is not deadly, the animals that practice it never kill their hosts, as fictional vampires do with their victims, but its practice carries a risk: the possibility of transmitting diseases. Malaria, rabies, bubonic plague, dengue, Zika, West Nile fever, typhus and a large number of diseases are transmitted involuntarily by the various animals that feed on the blood of others. It has been seen that some pathogens even modify the smell of the infected organism to make it more attractive to blood-suckers, thus manipulating the vector, but we will talk about that later.

 


References

Barredo E, DeGennaro. 2020. Not just from blood: mosquito nutrient acquisition from nectar sources. Trends in Parasitology 36: 473-484

Davis TJ, Kline DL, Kaufman PE. 2016. Aedes albopictus oviposition preference as influenced by container size and Buddleja davidii plants. Journal of Medical Entomology 53: 273-278

Ebrahimi B, Jackson BT, Guseman JL, Pzrybylowicz CM, Stone CM, Foster WA. 2017. Alteration of plant species assemblages can decrease the transmission potential of malaria mosquitoes. Journal of Applied Ecology 55: 841-851

Fernandes L, Briegel H. 2005. Reproductive physiology of Anopheles gambiae and Anopheles atroparvus. Journal of Vector Ecology 30: 11-26

Greenhall AM. 1972. The biting and feeding habits of the vampire bat, Desmodus rotundus. Journal of Zoology 168: 451-461

Müller G, Schlein Y. 2005. Plant tissues: the frugal diet of mosquitoes in adverse conditions. Medical and Veterinary Entomology 19: 413-422

Plantan T, Howitt M, Kotzé A, Gaines M. 2012. Feeding preferences of the red-billed oxpecker, Buphagus erythrorhynchus: a parasitic mutualism? African Journal of Ecology 51: 325-336

Sterkel M, Oliveira JHM, Bottino-Rojas V, Paiva-Silva GO, Oliveira PL. 2017. The dose makes the poison: nutritional overload determines the life traits of blood-feeding arthropods. Trends in Parasitology 33: 633-644

Stone CM, Taylor RM, Roitberg BD, Foster WA. 2009. Sugar deprivation reduces insemination of Anopheles gambiae, despite daily recruitment of adults, and predicts decline in model populations. Journal of Medical Entomology 51: 1327-1337

Looking for the source of the yellow fever mosquito

It has always been thought that the yellow fever mosquito, Aedes aegypti, evolved on the African continent. It was there that some of their populations adapted to living together in human settlements and with predilection to bite people. It was also from West Africa that European slave traders introduced the species to the American continent less than five centuries ago. Mosquitoes stowed away inside slave ships from one end of the Atlantic to the other. It is on the African continent where today there is still a greater genetic diversity of the species. Everything suggested that the continent, the cradle of humanity, was the place of origin of the mosquito, but a new study has denied this. A genetic analysis indicates that its origin is in the islands of the Indian Ocean, off the southeast coast of the African continent.

To date, the Aedes aegypti populations that inhabited the Indian islands, such as Madagascar, Reunion or Mauritius, had practically been little studied. Genetic analyzes estimated that Madagascar’s populations of the mosquito had separated from the continental ones by 10 million years. In this group of islands, together with the yellow fever mosquito, there are two species related to it: Aedes marcarensis and Aedes pia, described in 1924 and 2013 respectively.

To clarify the origin of the yellow fever mosquito, researchers have carried out a genetic study of various populations of Aedes aegypti, as well as related species that inhabit the islands near the continent. With all this they have been able to estimate the evolutionary history of the group and obtain a surprising result.

Diversification of the group from island to island and colonization of Africa

Although the Stegomyia group, to which Aedes aegypti belongs, originated and diversified in Africa, 16 million years ago a line of them colonized the islands of the Indian Ocean, and diversified as new islands were formed. Aedes pia formed isolated on the island of Mayotte 20 million years ago. Aedes mascarensis originated when occupying Mauritius no more than 9 million years ago. The oldest group of Aedes aegypti is also found there, it differed from the other species 7 million years ago in eastern Madagascar (Fig. 1).

Fig. 1. Location of the islands of the Indian Ocean included in the study. The species of the Stegomyia group were diversifying as they colonized the new islands that were formed.

 

The results suggest that Aedes aegypti inhabited the oceanic island for millions of years, until between 85,000 and 50,000 years ago it jumped to Europa Island, halfway between Madagascar and Mozambique, and to the African continent. But it was not until the end of the last glacial maximum, between 25,000 and 17,000 years ago, that the species was able to spread widely across the continent. During this period, forests once again occupied a large part of the East African coast, offering the ideal habitat for the species. About 6,000 years ago the forests reached their maximum extent covering much of central Africa, facilitating the dispersal of mosquitoes from the east coast to the west. At that time, two lines were created: Aedes aegypti aegypti and Aedes aegypti formosus.

6,000 years ago, human settlements began to offer them everything they needed: food and places to reproduce

The origin of this division is speculated to be related to human activity. Numerous human settlements were established in sub-Saharan Africa, with new techniques of storing water to overcome the dry season. Some mosquitoes changed the holes in the trees, which carried off rainwater, to reproduce, by the water reservoirs of people. The various mammals of the forests were replaced by people. Human settlements began to offer them everything they needed: food and places to reproduce.

 


References

Soghigian J, Gloria-Soria A, Robert V, Le Goff G, Failloux AB, Powell JR. 2020. Genetic evidence for the origin of Aedes aegypti, the yellow fever mosquito, in the southwestern Indian Ocean. Molecular Ecology, first view doi.org/10.1111/mec.15590

 

Mosquitoes like flowers

Some species of mosquitoes are important transmitters of viral diseases such as dengue, Zika or malaria, to the point of being considered one of the most deadly animals on planet earth. For this reason, many scientific studies have focused on understanding the mosquito-human interaction and, in particular, the sensory responses generated by humans.

Although it is hard to believe mosquitoes not only feed on human blood, in fact flower nectar is their main source of food.

A team of scientists, led by the University of Washington in the United States, has discovered the chemical signals that lead these insects to pollinate a species of orchid particularly irresistible to them. Knowing what you find irresistible could help you develop less toxic and more effective repellents in the future.

Mosquitoes have a very sensitive olfactory system that they use to locate important sources of nutrients, among them the nectar of different flowers or our presence to bite us. While male mosquitoes need nectar to survive, in females the sugar of flowers helps them to increase their life expectancy, survival rate and increase reproduction.

“For male mosquitoes, nectar is their only food source, and females feed on nectar for almost every day of their lives,” says Jeffrey Riffell, professor of biology at the American university

In the study, they examined the neural and behavioral processes of mosquitoes when exposed to flower aromas for which they feel preferences.

  • Pollination studies in the orchid species Platanthera obtusata by mosquitoes of the Aedes group.
  • Analysis of the floral aromatic compounds that attract various species of mosquitoes.
  • Recordings of the electrical activity of mosquito antennas showing how these aromas and compounds are represented and affect their neural activity.

His favorite flower

Some Aedes mosquitoes show affinity for the Platanthera obtusata orchid, being effective pollinators for this species. This association between mosquitoes and orchids provides a unique opportunity to identify sensory mechanisms that help mosquitoes locate sources of nectar.

Orquídea

Platanthera obtusata (Banks ex Pursh) Lindl. 20090624.96 Mount Stearns, Willmore Wilderness, Alberta

During the study they observed more than 581 Platanthera obtusata flowers during 47 hours, in which they were able to register up to 57 times Aedes mosquitoes feeding on them.

The observations were made in northern Washington state, where the Platanthera group orchids and mosquitoes abound.

 

Platanthera obtusata has an aroma reminiscent of grass, while other orchids in the environment, which are less attractive to mosquitoes, have a sweeter fragrance. The height and green coloration of the flowers make this plant difficult to distinguish from neighboring vegetation, yet the mosquitoes still manage to orient themselves and zigzag towards their flowers.

When the researchers covered the plants with bags, to prevent mosquitoes from seeing the flowers, the insects kept trying to reach the plants through the bag. This simple experiment made it possible to verify that the orchid generates a great olfactory attraction on mosquitoes, which led experts to analyze the chemical compounds in its aroma.

“The scent is actually a complex combination of chemicals, that of a rose, for example, consists of more than 300, and mosquitoes can detect the different types of chemicals that make it up,” says Riffell.

The orchids of the genus Platanthera differ in their floral essences

Using the gas chromatography and mass spectrometry technique, they analyzed the aromatic essences of six different orchids. They were able to identify a quarantine of chemical products of the different species of orchids of the Plathantera genus, observing that Platanthera obtusata, unlike the others, has a large amount of a compound called nonanal, as well as small amounts of another compound, lilac aldehyde.

The researchers analyzed the reactions of different mosquito species (Aedes canadensis, Culsette sp., Aedes dianteaus and Aedes cinereus) to the identified chemical compounds, by recording the electrical activity of their antennas. Despite the fact that not all native Aedes species reacted with the same magnitude to chemicals, the responses were consistent, leading the team to examine whether other mosquitoes, not native to the orchid habitat, would react the same to their compounds chemicals.

They verified it with Anopheles stephensi, one of the species that spreads malaria and Aedes aegypti that spreads viruses of diseases such as dengue, yellow fever or Zika.

Both Aedes aegypti and Anopheles stephensi were attracted to the scent of orchids when lilac aldehydes were included. But by removing the lilac aldehyde from the aroma, both these species and the native Aedes lost interest in the flower or were repelled by the resulting odor.

Knowing how mosquitoes process complex odors to detect attractive food sources or others, as well as those odors that are repulsive to them could be used in the future to develop more effective repellents.


References:

Lahondère C, Vinauger C, Okubo RP, Wolf GH, Chan JK, Akbari OS, et al. The olfactory basis of orchid pollination by mosquitoes. Proc Natl Acad Sci USA. 2020;117:708–16

Jurassic Park and its mosquitoes

If there is a famous fictional mosquito, that is the Jurassic Park mosquito. The plot of the film is well known to all, a team of scientists manages to bring dinosaurs back to life from DNA recovered from prehistoric mosquitoes. The researchers manage to extract blood from the “last supper” of one of the mosquitoes trapped millions of years ago in amber, a blood sample that belonged to a dinosaur (Fig. 1), from which they manage to sequence its genome to clone one of the dinosaurs that will later eat the researchers themselves.

The origin of all the tragedy written by Michael Crichton lies in a mosquito. We know that genetic material does not survive so many years as to make the Jurassic Park story a reality. And although molecular techniques have improved and managed to recover ancient DNA in ways we couldn’t even imagine a few years ago, until we obtained the complete genome of a horse that lived 700,000 years ago, the dream of seeing dinosaurs walk again continues. being that: a dream.

Fig. 1. The plot of the book and the films that followed revolves around the possibility of recovering the DNA of a dinosaur from a mosquito that previously had bitten one of them. Source: Mosquito Alert CC-BY 2.0

 

If one day molecular techniques made it possible, the research team that would carry it out should hire an entomologist to avoid the mistakes made by the Jurassic Park film teams. After all, without a mosquito there is no dinosaur. Or rather, without dinosaur blood inside the mosquito there is no dinosaur. And it is that the mosquito that appears in the film when they draw the blood of the dinosaur is one of the few mosquitoes that do not bite and do not feed on blood.

The mosquito used in the movie Jurassic Park is one that does not feed on blood, it could never contain dinosaur DNA in its body

Not all mosquitoes feed on blood

The mosquito on the scene belongs to the genus Toxorhynchites, the largest mosquitoes known, hence its common name “elephant mosquito”, and also one of the mosquitoes that are not blood-sucking (that is, they do not feed on blood) . In the film frame you can see the long proboscis formed by its mouthparts, but also that it is bent downwards (Fig. 2). This angle helps these mosquitoes feed on the nectar of the flowers, not through the skin of any animal.

Fig. 2. Frame from Jurassic Park (1993) where the mosquito is seen, to which they will extract the blood from the abdomen. Something impossible since the mosquito is of the genus Toxorhynchites, note its downward bent proboscis that feeds on flowers. Furthermore the individual is a male, appreciate his feathered antennae. Males, whatever their species, never sting.

 

But not only was the species wrong, but the individual made the failure even worse. The mosquito is a male, it is recognized by its feathered antennae that allows them to detect females when mating. Males of any species feed on blood, ever. Only females do. Well, in the species used in the movie, neither the females.

Not everything that looks like a mosquito is a mosquito

In 1993 they made that mistake, but in 2014 with the premiere of Jurassic World, the advice of some entomologists failed them again. Once again a mosquito trapped in amber allows new dinosaurs to be brought back to life, but this time the insect from which they obtain DNA is not even a mosquito. It is a typula or crane fly, that although their confusion with mosquitoes is common, they have nothing to do with them, and they do not bite or feed on blood (Fig. 3). In fact, many of them even feed in their adult phase. So we could hardly find dinosaur blood in his abdomen.

Fig. 3. Still from the Jurassic World movie (2014) of one of the “mosquitoes” from which DNA is recovered to bring dinosaurs back to life. If you don’t see the proboscis with the mosquitoes’ own stylet, it is because it is not a mosquito, but a harmless crane fly.

 

If the people in charge of the film had sent a photo of the “mosquito” they were going to use in the film to the Mosquito Alert app, the team of entomologists would have notified them that it was not a mosquito, thus avoiding the error. The Mosquito Alert participants would not have done so. For once mosquitoes are the protagonists of a story, and it turns out that the individuals used are not mosquitoes.

 


References:

Millar CD, Lambert DM. 2013. Towards a million-year-old genome. Nature 499: 34-35

Orlando L, Ginolhac A, [..], Willerslev E. 2013. Recalibrating Equus evolution using the genomes sequence of an early Middle Pleistocene horse. Nature 499: 74-78

Who is the tiger mosquito feeding on?

Mosquitoes are a recurring topic of conversation on summer evenings. We talk about them when they bite us or when we detect their presence around us and we anticipate that sooner or later their sting will arrive. Mosquitoes bite us, we are aware of them, but do they bite other non-human animals? Just as they bite us, they can bite our pets, or the bird that usually perches on the cables that cross the street. That it bites only us or other animals may seem an irrelevant detail but it is not.

Over the past decades, the burden of emerging infectious diseases has increased worldwide, to become a major threat to the health, safety and maintenance of economies. Covid-19 is a clear example of the health, social and economic impact that the appearance of a new infectious disease can generate. The Zika crisis in 2015 is another example. 75% of emerging diseases have an animal origin, they are what are called zoonotic diseases. Zoonotic pathogens can be transmitted from animals to humans, and from humans to animals, directly or indirectly. Indirect forms require an arthropod to jump from one organism to another, it is here that mosquitoes play an important role.

75% of emerging diseases are zoonotic, pathogens that can be transmitted from animals to humans

A mosquito can act as a bridge, allowing a pathogen that circulates among animals to end up jumping into a rural environment with humans and from there to the urban one (Fig. 1). Whether a species of mosquito can act as a bridge depends on its biology and ecology. It must be a species capable of inhabiting different habitats and ecosystems. To be able to reproduce both in a jungle, a forest, an agricultural area, a garden or a city. And also have a preference when it comes to eating varied. There are opportunistic species that bite a large number of different organisms, and others specialized in biting a few organisms. An opportunistic diet increases the probability of being able to transmit a pathogen from one species to another.

Fig. 1. Mosquito species that inhabit areas where animals and humans live together can act as a bridge in the transmission of viruses and other pathogens between species. The figure represents the different cycles described for chikungunya, a jungle one in which it is transmitted between primates, a rural one in which it can jump from primates to humans, and an urban one in which transmission is between humans mediated by the tiger mosquito and the yellow fever mosquito. Source: Mosquito Alert CC-BY 2.0.

Is the tiger mosquito an opportunist?

The tiger mosquito is an invasive species. Its origin is in the tropical forests of Asia. There the mosquito feeds on wild animals and breeds in tree holes, bamboo stumps, or in rock concavities that can hold water. But his ability to reproduce in artificial containers and having eggs resistant to drying has allowed him to colonize all the continents and domesticate himself until he completes his cycle in large cities. It is found in forest, rural and urban areas.

We know that the tiger mosquito is attracted to humans. We endure their bites for months, but why are other animals attracted to them? It is obvious that humans are not their main source of food in their native forests. In rural settings, it may not either. And in urban settings? A study carried out in the city of Barcelona found that 100% of the analyzed samples of blood in the tiger mosquito belonged to people. However, cities also live other animals, in addition to companion animals, rats, mice, pigeons, sparrows, parrots and other species live. Who does the tiger mosquito bite? Can it act as a bridge transmitting pathogens between species?

The tiger mosquito has a preference for humans

The species has a great capacity to reproduce in a wide variety of environments, both in natural environments and in landscapes modified by human activity. A review of the works that analyze the blood samples obtained from mosquitoes confirms the preference of the tiger mosquito for mammals, which represent 90% of the samples. Within mammals, the preference for humans is clear, we represent 60% of the total samples, while the rest of the species are the remaining 30%. The birds are only 7% and the other vertebrates represent 3% (Fig. 2).

Fig. 2. Percentage of animals that the tiger mosquito feeds on from blood samples. That 60% are human shows their preference for people, 30% are other mammals, and the remaining 10% is shared between birds and other vertebrates (reptiles and amphibians). The percentage is the average of various studies, calculated by Pereira-dos-Santos et al. 2020, Pathogens 9: 266. Source: Mosquito Alert CC-BY 2.0.

 

When looking at whether the minced animals are domesticated or wild species, it is obtained that the majority are domesticated animals, both dogs and cats, as well as farm animals and birds, where wild animals only represent 10% of the samples (Fig. 3).

Both studies of the blood found inside mosquitoes captured in the field, and laboratory experiments on the choice of mosquitoes, indicate their preference for humans over other animals. However, there is a clear bias in the studies carried out, most of them carried out in rural or urban areas. There is little information on the species’ behavior and feeding habits in more forest and natural environments, or on the flow of individuals from one environment to another.

Fig. 3. Tiger mosquito preference for humans, domesticated animals or wild animals. The percentage is the average of various studies, calculated by Pereira-dos-Santos et al. 2020, Pathogens 9: 266. Source: Mosquito Alert CC-BY 2.0.

 

Although the knowledge is biased, the set of works indicates that the mosquito can colonize both forest and rural environments, in addition to urban ones, and that it can interact with domesticated animals and wildlife. This opportunistic behavior in tropical regions of the Congo Basin and the Amazon is a risk, in which the species could act as a bridge and contribute to the spread of a disease present in wildlife to people living in and from the villages. to the cities.

The tiger mosquito is capable of colonizing very diverse environments and interacting with both domesticated and wild animals, in addition to humans.

To date, the tiger mosquito has been shown to be competent for many arboviruses, increasing the risk that it can transmit them from one environment and some species to another, although much knowledge is also lacking in this regard. In the capacity of infection and transmission, it is known that there is a great difference between populations within the species, not only of the mosquito but of the virus. The intraspecific variability of both the mosquito and a virus affect the transmission capacity and alter the risk estimate. To understand and better assess risk, it is necessary to undertake studies with a holistic vision that analyze the whole problem in an integrated way.

It is not enough to monitor viruses present in human populations, it is also necessary to monitor viruses that circulate in domestic animals and wildlife. Understand the human-mosquito interaction, but also those with other animals.

 


References:

Jones KKE, Patel NGN, Levy MA, Storeygard A, Balk D, Gittleman JL, Daszak P. 2008. Global trends in emerging infectious diseases. Nature 451: 990–993

Muñoz J, Eritja R, Alcaide M, Montalvo T, Soriguer RC, Figuerola J. 2011. Host-feeding pateras of native Culex pipiens and invasive Aedes albopictus mosquitoes (Diptera: Culicidae) in urban zones from Barcelona, Spain. Journal of Medical Entomology 48: 956-960

Pereira-dos-Santos T, Roiz D, Santos de Abreu FV, Luz SLB, Santalucia M, Jiolle D, Neves MSAS, Simard F, Lourenço-de-Oliveira R, Paupy C. 2018. Potential of Aedes albopictus as a bridge vector for enzootic pathogens at the urban-forest interface in Brazil. Emerging Microbes & Infections 7: 1-8

Pereira-dos-Santos T, Roiz D, Lourenço-de-Oliveira R, Paupy C. 2020. A systematic review: Is Aedes albopictus an efficient bridge vector for zoonotic arboviruses? Pathogens 9: 266

Savage HM, Niebylski ML, Smith GC, Mitchell CJ, Craig GB. 1993. Host-feeding patterns of Aedes albopictus at a temperate North American site. Journal of Medical Entomology 30: 27-34

Vasilakis N, Cardosa J, Hanley KA, Holmes EC, Weaver SC. 2011. Fever from the forest: prospects for the continued emergence of sylvatic dengue virus and its impacts on public health. Natural Reviews Microbiology 9: 532-541

Weaver SC, Reisen WK. 2010. Present and future arboviral threats. Antiviral Reseach 85: 328-345

Whitmee S, Haines A, Beyrer C, Boltz F, Capon AG, De Souza Dias BF, Ezeh A, Frumkin H, Gong, P, Head P, et al. 2015. Safeguarding human health in the Anthropocene epoch: Report of the Rockefeller Foundation-Lancet Commission on planetary health. Lancet 386: 1973–2028

Climate and urbanization drive mosquitoes’ preference for humans

The yellow fever mosquito (Aedes aegypti) is known throughout the world for inhabiting urban areas, biting humans and thereby spreading diseases such as dengue, Zika, chikungunya and the yellow fever that gives it its name. The WHO estimates that this species causes 50 million infections and 25,000 deaths a year. Studies carried out in America and Asia found that 95% of the blood consumed by the mosquito is human, demonstrating its preference for us.

However, in Africa, where the mosquito originates from, most of the populations of this mosquito do not have a preference for humans, but for other animals, mainly mammals such as primates and rodents. But this trend could change in the coming years, a new study warns. The increasing urbanization that the continent is experiencing and climate change, will favor the spread of mosquitoes with a preference for humans. And this will increase your ability to spread disease.

The yellow fever mosquito is native to Africa, where most of its populations have no preference for biting humans, unlike their populations in America and Asia

To reach these conclusions, the study authors analyzed the preference of various populations of African Aedes aegypti and the climatic and environmental variables that could explain the differences between them. They collected eggs from 27 locations in sub-Saharan Africa (Fig. 1), which varied in their climatic conditions, ranging from the semi-arid regions of the Sahel, to seasonal forests and rain forests. The other variable they analyzed was the density of the human population present in the area. The mosquitoes were bred in a laboratory where their biting preference was studied, exposing them to human odors and odors from other non-human animals at the same time. In this way they could observe which of them they were heading and calculate the preference of the different populations.

Fig. 1. Location of the populations used in the study. The size of the circle represents the density of human population present in the area. The color the mosquito’s preference for biting humans (reddish) or other animals (bluish). The figure below shows the preference index obtained by giving them a choice between humans or other animals for each of the populations. The point is the mean and the bar is the 95% confidence interval. Source: Mosquito Alert CC-BY from the original by Rose et al. 2020. bioRxiv 939041

Urbanization and climate determine mosquito preference

Most of the populations preferred the smell of the other animals (blue), some did not show preference for one or the other (violets), and only a few of them showed a clear predilection for human smell (red) (Fig. 1) . The taste for human blood was present in those regions with a higher human density. Suggesting that when the human population is large in an area, the most favored mosquitoes are those that tend to feed on people’s blood.

Another factor that explained the distribution of mosquitoes with this preference for human odor was the climate. These mosquitoes are more abundant in dry regions. In them the natural habitats where they usually deposit eggs, tree holes or concavities in the rocks with rainwater, are scarce. Instead, human containers and tanks proliferate to store water. Humans provide mosquitoes with places to breed in environments that are initially unfavorable to them.

Both morphology and genetics show that mosquitoes with a preference for humans, which today have invaded the American and Asian continents, are related to the populations of the Sahel, the region that makes the transition between the Sahara desert to the north, and the savannah to the south. The term of Arab origin literally means “coast”, in reference to the appearance of the vegetation delimiting the sandy sea of ​​the Sahara. The yellow fever mosquito populations that inhabit northern Senegal, in addition to showing a preference for humans, is the one that is genetically related to the mosquitoes that we find today in America and Asia.

The yellow fever mosquito tamed itself

In these semi-arid environments, mosquitoes depend on human habitats to reproduce. Human-made tanks and water drums allow them to inhabit a region where they find few natural habitats to reproduce. Being forced to live together with human populations, where the most abundant prey is humans, natural selection ended up generating mosquito populations with a preference for feeding on humans. Individuals with the mutations in sensory systems that determine their preference for human odors are the most abundant in these populations.

A drier climate and greater urbanization foresees an increase in mosquitoes with a preference for humans in Africa in the coming decades

The combination of dry seasons and dependence on humanized spaces has not only been important in the evolution of the yellow fever mosquito, but has also shaped the evolution of Anopheles mosquitoes that transmit malaria in Africa.

In Africa, the climate and density of human populations are changing, and it is feared that the change in rainfall and the increase in urbanized areas will favor mosquitoes that have adapted to live and feed on humans. That would mean a considerable increase in human populations on the continent exposed to the diseases that this species can transmit.

 


References:

Crawford JE, Alves JM, Palmer WJ, Day JP, Sylla M, Ramasamy R, Surendran SN, Black WC, Pain A, Jiggins FM. 2017. Population genomics reveals that an anthrophilic population of Aedes aegypti mosquitoes in West Africa recently gave rise to American and Asian populations of this major disease vector. BMC Biology 15: 16

Dao A, Yaro AS, Diallo M, Timbiné S, Huestis DL, Kassogué Y, Traoré AI, Sanogo ZL, Samaké D, Lehmann T. 2014. Signatures of aestivation and migration in Sahelian malaria mosquito populations. Nature 516: 387-390

Kotsakiozi P, Evans BR, Gloria-Soria A, Kamgang B, Mayanja M, Lutwama J, Le Goff G, Ayala D, Paupy C, Badolo A, Pinto J, Sousa CA, Troco AD, Powell JR. 2018. Population structure of a vector of human diseases: Aedes aegypti in its ancestral range, Africa. Ecology and Evolution 8: 7835-7848

McBride CS. 2016. Genes and odors underlying the recent evolution of mosquito preference for humans. Current Biology 26: R41-R46

Rose NH, Sylla M, Badolo A, Lutomiah J, Ayala D, Aribodor OB, Ibn N, Akorli J, Otoo S, Mutebi JP, Kriete AL, Ewing EG, Sang R, Gloria-Soria A, Powell JR, Baker RE, White BJ, Crawford JE, McBride C. 2020. Climate and urbanization drive mosquito preference for humans. bioRxiv: dos.org/10.1101/2020.02.12.939041

Takken W, Verhulst NO. 2013. Host preference of blood-feeding mosquitoes. Annual Review of Entomology 58: 433-453

 

Climate change accelerates the spread of the yellow fever mosquito

The spread of disease-transmitting mosquitoes is primarily a product of globalization, with a long history behind it. The yellow fever mosquito (Aedes aegypti) colonized the American continent from Africa as a stowaway in slave ships that trafficked slaves. With the mosquito came yellow fever to the new continent. Later both would arrive at the most important ports in Europe. The tiger mosquito (Aedes albopictus) has also traveled hidden on commercial ships from Asia to the rest of the world in much more recent times. Globalization has allowed these species originating from tropical regions to invade new areas, but another human phenomenon has also helped: climate change.

The rapid expansion registered by both species in just under half a century has been facilitated by rising temperatures. Climate change has great effects on the distribution of many species by altering the climatic conditions of different regions. Ectothermic (or cold-blooded) organisms, such as mosquitoes, are not capable of generating internal heat, as, for example, we mammals do through various metabolic processes (Fig. 1). This makes your activity dependent on external heat sources. This explains its absence during the cold winter months and its revival in spring. Therefore, a warmer world, like the one we are heading to, is more favorable for tropical species.

 

Fig. 1. Differences in body temperature with respect to the ambient temperature of an ectotherm, such as the tiger mosquito, and an endotherm, such as the tiger. The temperature of the ectotherms varies with the environmental one, unlike the endotherms that maintain a constant temperature regardless of the ambient temperature. Source: Mosquito Alert (CC-BY-NC-2.0)

 

In recent years, studies have appeared that predict that both the tiger mosquito and the yellow fever mosquito will have an easier time expanding to areas that are now temperate as average temperatures rise. The warming will allow these species to find new favorable places where they can complete their cycle. Precisely the last study has been based on the ability of Aedes aegypti to complete its cycle in different climatic conditions to estimate how much and where it will be able to expand in the coming years.

A warmer world will favor the expansion of tropical species such as the yellow fever mosquito or the tiger mosquito

 

Temperature accelerates mosquito development

For this they analyzed how temperature can affect the mosquito in its different stages of development: egg, larva, pupa and adult. The higher the temperature, the faster the transition from one phase to another. For example, from the time a female has fed on blood until she lays her eggs, 8 days pass at 20ºC. But if the temperature is 26ºC the time is reduced to 3 days. Only 2 days if the temperature is 30ºC. Or 4 if it exceeds 35ºC, because the excess temperature also affects them.

Using the temperature values ​​necessary for the animal to survive from one phase to another, as well as its speed as a function of temperature, scientists at Imperial College London and Tel Aviv University have been able to calculate how many times it could complete its life cycle the yellow fever mosquito in every region of the world. A greater number of completed cycles implies more mosquitoes and for a longer time. That directly translates into more likely to transmit diseases such as dengue, Zika or chikungunya.

The more life cycles completed per year in an area, the more mosquitoes and more exposure to the risk of contracting one of the diseases they transmit

Using historical data on world temperatures, as well as future projections under different emission scenarios, they have been able to make a model to predict the efficiency with which the mosquito will reproduce in different places in the coming decades.

In 2030 some regions of the Mediterranean will meet the requirements for the establishment of the yellow fever mosquito

In fact, the mosquito appears to have taken advantage of warming for the past half century. The warming recorded from 1950 to 2000 has allowed the species to expand, both within tropical and subtropical zones, and reaching temperate zones in Asia and America. In 2050 the species will move north more rapidly, especially in China and the United States (Fig. 2). The advance will be between 5 and 6 kilometers a year. It doesn’t seem like much, but it represents a huge annual increase in area and new human populations exposed to the mosquito.

Fig. 2. Predictions of the regions that will meet the climatic requirements in the coming years that may favor the establishment of the yellow fever mosquito. Source: Iwamura et al. 2020. Nature Communications 11: 2130

 

Europe may also be invaded by the species. The Mediterranean region in a couple of decades will have the ideal climatic conditions so that the mosquito can complete its cycle (Fig. 2). The authors estimate that by 2030 regions of Spain, Portugal, Greece and Turkey will meet the climatic requirements for the species to become established. In fact, in some of these areas it was already present in the past. Athens, for example, suffered a major dengue outbreak between 1927 and 1928, although from the 1950s the species began to disappear from the continent. Precisely for this reason they believe that their models underestimate the mosquito’s colonization capacity, and that if in not too distant times they were able to establish themselves in some regions of the Mediterranean and the Black Sea, with global warming their expansion may be greater than the one predicted by the models.

In 2030 the climatic conditions in southern Spain will be favorable to the establishment of the yellow fever mosquito

These regions where it was present in the past may be on the edge of its niche, so control measures and the climate helped to eradicate it, a situation that will change if the climate does not contribute to its control. Warming implies the globalization of mosquitoes, which in turn implies the globalization of the diseases they transmit.


References:

Iwamura T, Guzman-Holst A, Murray KA. 2020. Accelerating invasion potential of disease vector Aedes aegypti under climate change. Nature Communications 11: 2130

Liu-Helmersson J, Rocköv J, Sewe M, Brännström A. 2015. Climate Change may enable Aedes aegypti infestation in major European cities by 2100. Environmental Research 172: 639-699

Messina JP, Brady OJ, Golding N, Kraemer MUG, Wint GRW, Pigott DM, Shearer FM, Johnson K, Earl L, Marczack LB, Shirude S, Weaver ND, Gilbert M, Velayudhan R, Jones P, Jaenisch T, Scott TW, Reiner RC, Hay S. 2019. The current and future global distribution and population at risk of dengue. Nature Microbiology 4: 1508-1515

Ryan SJ. Carlson CJ, Mordecai EA, Johnson LR. 2019. Global expansion and redistribution of Aedes-borne virus transmission risk with climate change. PLoS Neglected Tropical Diseases 0007213

Wearing HJ, Robert MA, Christofferson RC. 2016. Dengue and Chikungunya: modelling the expansion of mosquito-borne viruses into naïve populations. Parasitology 143: 860-873

Can the tiger mosquito transmit the coronavirus?

The pandemic has paralyzed the world. With hundreds of thousands affected, Covid-19 has had a huge impact on the economy and has emptied public spaces across half the planet. The arrival of spring has found a large part of the population confined to their homes and with the cities practically empty. This time of year means that temperatures continue to rise and we have already had days of heavy rain. Conditions are ideal for mosquitoes to reactivate, and with its return, many people wonder if the tiger mosquito, or any other species of mosquito, can transmit the new virus.

Many people wonder if a mosquito can transmit SARS-CoV-2

The World Health Organization has made this clear: “to date, there is no information or evidence to indicate that COVID-19 can be transmitted by mosquitoes.” Furthermore, the WHO insists that “the new coronavirus is a respiratory virus that is spread mainly by contact with an infected person through the respiratory droplets that are generated when this person coughs or sneezes.”

If they transmit dengue, why not SARS-CoV-2, or the common cold?

The doubt is reasonable, because we know that some mosquitoes transmit viruses and other pathogens between people by biting them. If a tiger mosquito that bites a person infected with dengue can then transmit the virus to another person, can’t the same happen with the new SARS-CoV-2?

The same question arose in the late 1980s and early 1990s regarding HIV, until it was finally shown that the virus could not be transmitted by a mosquito. But HIV is not a rarity, viruses are the majority that cannot be transmitted by these insects. In fact, the rhinoviruses that cause the common cold belong to the Coronavirus family, and we will all agree that the cold is not transmitted by mosquitoes. Neither does the flu virus. Neither that of smallpox or Ebola.

It is common to think that if a mosquito has bitten a person infected with a virus, it can directly transmit the virus to its next victim. As if the mosquito was transfusing from one person to another, injecting the blood of its first victim into the next. But such a thing does not happen that way, because when a mosquito takes blood, it injects its saliva, but not the blood of its previous victim, who already digested days ago.

Puede un mosquito transmitir la COV-19

Fig. 1. Can a mosquito transmit COVID-19? There is no evidence of this, nor anything that suggests that this may be the case. Source: Mosquito Alert (CC-BY-NC-2.0)

 

The blood drawn by the sick person’s mosquito goes to their stomach, where digestion destroys viruses. This is the case, for example, with HIV, and with almost everyone else. For a virus to be transmitted by a mosquito, it must overcome a large number of barriers, which is not easy.

Arboviruses: viruses specialized in being transmitted by mosquitoes and ticks

The list of viruses that can be transmitted by mosquitoes may seem long, but it is very short when compared to the millions that cannot. Dengue virus (DENV), yellow fever virus (YFV), Zika virus (ZIKV), West Nile virus (WNV), Saint Louis encephalitis virus (SLEV), and chikingunya virus (CHIKV) stand out. ), the Ross River (RRV), Rift Valley fever (RVFV) among many other viruses. These mosquito-borne viruses are known as arboviruses, viruses carried by arthropods, mainly mosquitoes and ticks.

Viruses that can be transmitted by mosquitoes belong to a few families and are highly specialized. Not any virus can infect and be transmitted by a mosquito

Despite being enough, they represent a small proportion of the great diversity of viruses that exist. All arboviruses known to date belong to a few families of RNA viruses: Flaviviridae, Togaviridae, Bunyaviridae, Reoviridae, Rhabdoviridae, Orthomyxoviridae, and a single DNA virus from the Asfarviridae family. No virus in the Coronaviridae family, to which SARS-CoV-2 belongs, has ever been identified as an insect-borne virus.

The limited number of viruses that can be transmitted by mosquitoes is due to the ecological and evolutionary complexity that this implies from the point of view of the virus. It must not only be able to break through the immune system of a vertebrate organism (humans, for example), but also of the mosquito’s immune system.

Barriers that overcome arboviruses

In order to be transmitted by a mosquito, a virus must cross four main barriers. As it has already been said, the sucked blood, and the viruses that are in it, go to the mosquito’s digestive system (Fig. 2). The virus must survive in that hostile environment and be able to infect and replicate in the epithelial cells of the intestine. If you manage to overcome that barrier, then you still need to overcome the basal lamina that surrounds the intestine. Overcoming this second obstacle, the virus passes to the hemolymph and must be able to reach the salivary glands, which also pose a barrier to many viruses. Only if it can enter and replicate in the salivary glands can it be inoculated as soon as the mosquito bites another person. And it must replicate in large numbers because normally a few viruses are not enough.

Infección de un mosquito por virus

Fig. 2. The route of infection of a mosquito by a virus, showing the main barriers that it must overcome in order to transmit itself through its bite. (1) Survive the digestive system and replicate. (2) Overcome the basal lamina that surrounds the intestine. (3) Tackle the mosquito’s antiviral immune response without killing it. (4) Reach the salivary glands and overcome their barrier. Image modified from the original by Rückert & Ebel 2018, Trends in Parasitology 34: 310-321. Source: Mosquito Alert (CC-BY-NC-2.0)

 

Regardless of all these barriers, the virus must face the mosquito’s antiviral immune response, but without making it sick. The mosquito should not suffer consequences because if the pathogen it carries were to kill it, it could not be transmitted and the virus would harm itself. Only specialized viruses are capable of making this journey through the mosquito without succumbing to any of its defensive barriers.

As we can see, arboviruses are a group of highly specialized viruses that throughout their evolutionary history of millions of years have developed an intimate association, both with a vertebrate host and with a mosquito in order to perpetuate themselves. So this only happens between a few mosquitoes and a few viruses.

To date, none of the coronaviruses, such as SARS-CoV-2, SARS, or MERS, have been classified as arboviruses capable of infecting a mosquito and transmitted through its bites. Therefore there is no reason to think that the tiger mosquito or the common mosquito can transmit SARS-CoV-2.


References:

Ciota AT, Kramer LD. 2010. Insights into Arbovirus evolution and adaptation from experimental studies. Viruses 2: 2594-2617

Coronavirus Study Group of the International Committee on Taxonomy of Viruses. 2020. The species Severe acute respiratory syndrome-related coronovirus: classifying 2019-nCoV and naming it SARS-CoV-2. Nature Microbiology 5: 536-544

Hanley KA, Weaver SC. 2008. Arbovirus Evolution. In: Origin and Evolution of Viruses. pp: 351-391

Iqbal MM. 1999. Can we get AIDS from mosquito bites? The Journal of the Louisiana State Medical Society 151: 429-433

Kuno G, Chang GJ. 2005. Biological transmission of Arboviruses: reexamination of the new insights into components, mechanisms, and unique traits as well as their evolutionary trends. Clinical Microbiology Reviews 2005: 608-637

Rückert C, Ebel GD. 2018. How do virus-mosquito interactions lead to viral emergence? Trends in Parasitology 34: 310-321

Wolf YI, Kazlauskas D, Iranzo J, Lucía-Sanz A, Kuhn JH, Krupovic M, Dolja VV, Koonin EV. 2018. Origins and evolution of the Global RNA Virome. American Society for Microbiology 9: e02329-18

Why mosquitoes appear in spring

Most of us like spring and summer months. We feel better when we have left behind the cold and the gray cloud accumulation of winter. Spring is the season of colors, nature is full of flowers and greenery. Life seems to manifest again after months in which it seems to have been dormant. With spring, mosquitoes also reappear. From where? How? What had happened to them during the winter?

For a long time it was believed that the origin of many living beings was found in dead matter and its putrefaction. A phenomenon that was known as spontaneous generation. Aristotle (384-322 BC) used this phenomenon to explain the sudden appearance of worms, mosquitoes, eels and mice, among other animals. They all reemerged from the most diverse materials. Mosquitoes poured out of the mud, just like that. The idea of ​​spontaneous generation, in which the inert can give rise to life, persisted for centuries, until an experiment dismantled it in the early seventeenth century.

Do mosquitoes appear by spontaneous generation?

It was the Medici court physician, Francesco Redi, who in 1668 published his book Experiences around the Generation of Insects, attacking the doctrine of spontaneous generation. Redi himself relates that the idea to carry out his experiments came to him after reading Homer’s Iliad, where it is described that Thetis, mother of Achilles, covers Patroclus’s corpse to protect it from worms and flies that “corrupt the bodies of men killed in battle.” Why cover the bodies if, according to Aristotle, worms and flies arose directly from decomposing bodies?

Fig. 1. Francesco Redi’s experiment to deny the theory of spontaneous generation. In the cans sealed with a stopper or gauze, no fly larvae appeared in the rotting meat, because the flies were unable to enter to lay their eggs. Source: Mosquito Alert (CC-BY-NC-2.0)

 

Motivated by doubt, Redi carried out a series of experiments in which she placed pieces of meat, both in uncapped jars, in gauze-covered jars, and in hermetically sealed jars (Fig. 1). With this elegant experiment, he demonstrated that spontaneous generation did not exist. Only fly larvae appeared in the uncovered jars where flies had been able to enter to lay the eggs. His experiment made it clear, life comes from life, not the inert. So what about mosquitoes in spring. How do they arise if there were no mosquitoes in winter?

In winter we don’t see them but they are still there

The answer is that mosquitoes never disappeared in winter. In one way or another they have been there. Latent, but present. Like seeds waiting for the good time to bloom. Mosquitoes, like the vast majority of insects, have a programmed latency program known as diapause.

Diapause not only occurs in winter in temperate zones, it also occurs in dry seasons in tropical zones. When the environmental conditions are bad for the development of the mosquitoes they go into diapause. Understanding that it promotes this state of latency is very important to understand the dynamics of the diseases they transmit, since sometimes this phase serves as a reservoir for pathogens that can re-emerge with the reactivation of mosquitoes.

While most mosquitoes diapause when conditions are poor, the yellow fever mosquito does not

Although one or another type of diapause has been described in most mosquitoes to survive adverse environmental conditions, some species, such as the yellow fever mosquito (Aedes aegypti), seem to lack it.

Diapause is a hormonally programmed latency stage in response to environmental conditions. It allows animals to advance in bad weather and “encapsulate” before the cold kills them. To predict the arrival of winter, mosquitoes consist of several mechanisms. On the one hand, the photoperiod, that is, the number of daylight hours. That the days become shorter is surely the most evident sign of the approach of winter. Much more than the change in temperature which is much more variable. But how does a mosquito know that time passes? With the help of an internal clock present in all animals. This internal clock regulates two different temporal systems. On the one hand the circadian cycles, which are the rhythms of activity of day and night, and another that would allow the identification of seasonal cycles, something like an internal calendar. The latter would be in charge of recognizing that the days become longer or shorter, and as a result, activate a hormonal and physiological response in the mosquito (Fig. 2).

 

Fig. 2. Mosquitoes can perceive the change of season thanks to having an internal clock with which to compare external light periods. The red curves represent the daily oscillations of the internal clock. The light and dark areas, the light and dark hours of each day. As winter approaches, daylight hours become shorter. This signal induces diapause in the mosquito. Source: Mosquito Alert (CC-BY-NC-2.0).

 

To the stimulation of the hours of light, the one of the temperature is added. Although more variable than light, temperature allows you to refine the animal’s response. Its effect when initiating diapause has been observed in the tiger mosquito (Aedes albopictus), in other Aedes species, as well as in the common mosquito (Culex pipiens).

Diapause can occur in embryos, larvae, or adults

The change in daylight hours, temperature, and humidity perceived by female mosquitoes causes their embryos to go into diapause. So it is with the tiger mosquito. It is not the adult mosquito that goes into diapause, but the embryos, which remain dormant during the winter in their eggs without hatching. These eggs are different from normal eggs. They are larger and contain a higher amount of lipids to protect and nourish the embryos for months. They have to hold on until spring returns.

The tiger mosquito survives winters in their embryonic phase, protected inside the eggs waiting for the return of good weather

The diapause of the common mosquito (Culex pipiens) is different. The change in daylight hours and temperature alter the behavior of their females. With the eminent arrival of winter, females stop feeding on blood, instead of being attracted to humans and other vertebrates, they are attracted to the nectar of flowers. They also start the search for a dark and humid place in which to take refuge during winter. The common mosquito does not spend the winter as an egg, as the tiger mosquito does, but as adult mosquitoes that will reactivate with the arrival of good weather.

The common mosquito overcomes winters in its adult phase, sheltered in dark and humid places that protect it from low temperatures

In other mosquito species, diapause does not occur during the embryonic phase, nor during the adult phase, but during the larval phase (Fig. 3). Whether in the embryonic, larval or adult phase, what is known is that diapause is something much more complex than a simple stop in the development of the animal. Diapause involves molecular, physiological, developmental and behavioral changes before and during the diapause, which affects the development and reproduction of animals after the diapause is over.

Fig. 3. Phylogenetic relationship of different genus of mosquitoes and the phases of the biological cycle in which diapause has been described for different species. The numbers refer to the number of species of the genus with diapause, for example, in Aedes mosquitoes, 18 species with embryonic diapause have been described, including the tiger mosquito, and 6 species with diapause in the larval phase, but no species with diapusa in the adult phase. In Anopheles and Culex mosquitoes it is the other way around, most species have diapause in the adult phase. Data from Denlinger & Armbruster 2005. Annual Review of Entomology 59: 73-93. Source: Mosquito Alert (CC-BY-NC-2.0).

Invasive mosquitoes adapt their diapause to new climatic conditions

The study of invasive species such as the tiger mosquito has shown that the timing of the diapause can evolve rapidly. During their expansion along different climatic gradients, mosquito populations have been adapting to the signals that trigger the diapause, advancing or delaying it. Not all populations respond the same amount of daily light hours. The answer depends on the latitude at which they are. This suggests that they will also be able to respond to climate change. Understanding how mosquitoes respond to environmental changes is essential to predict the spread of the diseases they transmit.

The molecular basis of the diapause of species of sanitary interest is increasingly known. Advances in genetics and changes in gene expression may lead to the development of new ways to control their populations, based on genetic or chemical disruption of diapause.

At the moment, we know that with the arrival of good weather, mosquitoes come out of their diapause state and are active again. They have never disappeared. They have always been here. Egg-shaped, larvae or adult sheltered in some dark corner, they have endured winter to return once again in spring. It is time to remember to take the necessary measures so that our home is not a place where they can raise. Avoid having containers that store standing water, and have the Mosquito Alert application prepared to report the presence of mosquitoes.


References

Armbruster PA. 2016. Photoperiodic diapause and the establishment of Aedes albopictus (Diptera: Culicidae) in North America. Journal of Medical Entomology 53: 1013-1023

Bova J, Soghigian J, Paulson S. 2019. The prediapause stage of Aedes japonicus japonicus and the evolution of embryonic diapause in Aedini. Insects 10: 222

Chang V, Meuti M. 2020.Circadian transcription factors differentially regulate features of the adult overwintering diapause in the Northern house mosquito, Culex pipiens. Insect Biochemistry and Molecular Biology: doi.org/10.1016/j.ibmb.2020.103365

Denlinger DL, Armbruster PA. 2014. Mosquito diapause. Annual Review of Entomology 59: 73-93

Lacour G, Vernichon F, Cadilhac N, Boyer S, Lagneau C, Hance T. 2014. When mothers anticipate: effects of the prediapause stage on embryo development time and of maternal photoperiod on eggs of a temperate and tropical strains of Aedes albopictus. Journal of Insect Physiology 71: 87-96

Lacour G, Chanaud L, L’Ambert G, Hance T. 2015. Seasonal synchronization of diapause phases in Aedes albopictus. PLoS One 10: e0145311

Medley KA, Westby KM, Jenkins DG. 2019. Rapid local adaptation to northern winters in the invasive Asian tiger mosquito Aedes albopictus: a moving target. Journal of Applied Ecology 56: 2518-2527

Robich RM, Denlinger DL. 2005. Diapause in the mosquito Culex pipiens evokes a metabolic switch from blood feeding to sugar gluttony. Proceedings of the Natural Academy of Science of the United States of America 103: 15912-15917

 

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