climate change

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

Bloodthirsty: the other reason why a mosquito bites

Mosquito females are the only ones that bite, they do it to acquire protein to the development of their eggs. Yes, our blood serves to give rise to a new generation of mosquitoes. But recent work suggests that blood can also act as a snack for mosquitoes in dry and warm periods.

The study, published in Scientific Reports, has found that mosquitoes exposed to dry environments, with a higher level of dehydration, are more aggressive than those that are well hydrated. Dehydrated mosquitoes venture more to land on a host, bite and feed on blood more often than the others. Scientists believe that during periods of drought the risk of disease transmission can increase. More bites more risk of transmission.

The idea that in the dry periods there may be more cases of infection seems contradictory given the great dependence of mosquitoes on water. Most of them lay their eggs in wetlands where the larvae develop. Thus, the number of mosquitoes depends on weather, increasing their number after rains, once there is stagnant water in abundance where they can reproduce. This relationship between climatic conditions and the number of mosquitoes has also been established among diseases transmitted by mosquitoes. One expects that more rain implicates more mosquitoes and more diseases transmitted. But the data don’t always confirm these expectations.

Droughts cause major epidemic episodes of West Nile fever

In the case of West Nile fever, it has been seen that epidemic episodes are greater in the years of drought. The authors believe that their observations may explain why sometimes epidemics occur during periods of drought.

To study how droughts and dehydration alter the physiology and behavior of mosquitoes, they designed an experiment that they tested on three mosquito species. The species evaluated were: (1) Culex pipiens, the common mosquito that could transmit the West Nile virus in the United States (2) Aedes aegypti or yellow fever mosquito, which can also transmit dengue, chikungunya and fever from Zika, and (3) Anopheles quadrimaculatus, a mosquito from the North American Atlantic coast capable of transmitting malaria.

Mosquito Alert

Fig. 1. Mosquitoes were offered an artificial host made with a collagen membrane, which contained chicken blood to study the biting rate at different levels of dehidration. Figure based on the original: Hagan et al. (2018) Scientific Reports 8: 6804. Source: Mosquit Alert (CC-BY-NC-2.0)

 

The researchers exposed hundreds of mosquitoes of each species to different temperature and humidity conditions to generate individuals with different levels of dehydration. In the experiment they included another factor, as some mosquitoes had access to water or nectar with which to hydrate while others had no access to any type of liquid. After some time in these conditions, its effect on the mosquito behavior was analyzed. For this, the mosquitoes were offered an artificial host made with a collagen membrane at a temperature of 37 °C, covered with artificial sweat to attract mosquitoes, which contained chicken blood (Fig. 1).

 

Thirsty mosquitoes’ bite more

When quantifying the times, they landed on the artificial host and fed on it; they saw that the number of bites was higher among dehydrated mosquitoes that also had no access to water or nectar. Only 10 percent of the mosquitoes with access to water landed and bite the host, while among the mosquitoes deprived of water was 30 percent. Mosquitoes in conditions that recreated a drought bite much more than the other mosquitoes (Fig. 2).

The sensors available to mosquitoes to detect a host, make it easier for them to detect a host than to locate a water point with which to hydrate when they are thirsty. The highest rates of West Nile virus transmission observed during droughts could be due to mosquitoes using blood to replace the water they lose.

Mosquito Alert

Fig. 2. Mosquitoes in condition that recreated a drough with higher levels fo dehidration bite much more than well hidrated mosquitoes. Figure based on the original: Hagan et al. (2018) Scientific Reports 8: 6804. Source: Mosquit Alert (CC-BY-NC-2.0)

 

Knowing the climatic conditions that alter mosquito bite behavior has practical applications as it can be incorporated into epidemiological mathematical models. The abundance of mosquitoes is an important factor to consider in these models, but studies like that one show that environmental factors alter mosquito-human interaction so that the risk of transmission can vary with weather.

The increase in bites to quench thirst is not the only mechanism that can explain the relationship between drought periods and epidemics. Other studies have found that during droughts there are fewer wetlands and resources, which helps animals and mosquitoes come into contact more easily, favoring disease transmission. In addition, in the few existing wetlands there is a greater concentration of nutrients necessary for larval development. In these temporary and ephemeral ponds the proliferation of mosquitoes is favored by the absence of large predators, which inhabit permanent aquatic environments. The periods of heat and drought not only affect predators of aquatic environments but also the potential predators of adult mosquitoes, allowing their populations to be larger.

In short, that periods of drought can favor the proliferation of some species of mosquitoes, and make that mosquitoes become more aggressive. The increase in temperatures caused by global warming could lead to more frequent and longer periods of drought in areas where mosquitoes are a threat to human health.

 

References:

Barnard DR, Dickerson CZ, Murugan K, Xue RD, Kline DL, Bernier UR. 2014. Measurement of landing mosquito density on humans. Acta Tropica 136: 58-67

Chase JM, Knight TM. 2003. Drought-induced mosquito outbreaks in wetlands. Ecology Letters 6: 1017-1024

Hagan RW, Didion EM, Rosselot AE, Holmes CJ, Siler SC, Rosenlade AJ, Herdershot JM, Elliot KSB, Jennings EC, Nine GA, Perez PL, Rizlallah AE, Watanabe M, Romick.Rosendale LE, Xiao Y, Rasgon JL, Benoit JB. 2018. Dehydration prompts increased activity and blood feeding by mosquitoes. Scientific Reports 8: 6804

Paull SH, Horton DE, Ashfaq M, Rastogi D, Kramer LD, Diffenbaugh NS, Kilpatrick AM. 2017. Drought and immunity determine the intensity of West Nile virus epidemics and climate change impacts. Proceedings of the Royal Society B 284: 20162078

Paz S. 2015. Climate Change impacts on West Nile virus transmission in a global context. Philosophical Transactions Royal Society B 370: 20130561

Paz S, Malkinson D, Green MS, Tsioni G, Papa A, Danis K, Sirbu A, Ceianu C, Katalin K, Ferenczi E, Zeller H, Semenza JC. 2013. Permissive summer temperatures of the 2010 European West Nile fever upsurge. PLoS One 8: e56398

Romano D, Stefanini C, Canale A, Benelli G. 2018. Artificial blood feeders for mosquitoes and ticks – where from, where to? Acta Tropica 183: 43-56

Shaman J, Day JF, Stieglitz M. 2005. Drought-induced amplification and epidemic transmission of West Nile virus in southern Florida. Journal of Medical Entomology 42: 134-141

Wang G, Minnis RB, Belant JL, Wax CL. 2010. Dry weather introduces outbreaks of human West Nile virus infection. BMC Infectious Diseases 10: 38

 

The tiger mosquito and diseases, at the exhibition “After the end of the world” at the CCCB

Last 14th November the ICREA researcher Frederic Bartumeus participated in a conversation organized by the CCCB “New climate, new diseases?”. Together with the researcher Xavier Rodó of the ISGlobal Institute, they highlighted how climate change could favor the emergence of new diseases and their vectors, such as insects. The conversation was moderated by the journalist Mercè Folch.

 

The two ICREA researchers spoke on the role of climate change in the emergence and spread of infectious diseases. Bartumeus explained how public participation through Mosquito Alert can contribute to the development of scientific studies to study the tiger mosquito. At the same time, these studies also serve to estimate the risk of transmission of the diseases that this invasive insect can transmit. The conversation ““Nou clima, noves malalties?” (new climate, new diseases) is an activity that belongs to the exhibition “Després de la fi del món” (after the end of the world) of the current planet irreversibly transformed.

View the whole video of the conversation by clicking on the image or here:

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