mosquito preferences

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

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

 

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