Culex pipiens

A mosquito could kill Alexander the Great

As he approached Babylon, Nearch, who had returned to the Euphrates by the great sea, said that some Chaldeans had spoken to him, urging him not to enter Babylon; but this one did not pay attention, but continued his march, and when he was already touching the walls, he saw many crows fluttering and pecking at each other, of which some fell in front of him.

This is how, in the late 1st century, Plutarch described the arrival of Alexander the Great in Babylon. This small fragment of volume V of his “Parallel Lives” took a few years ago to the epidemiologist, John Marr, and to the expert in infectious diseases, Charles Calisher, has announced that they had found the cause of death of the Macedonian conqueror.

After seizing the Macedonian throne, Alexander overthrew the Persian Empire and looked east, eventually invading much of India. It gave rise to the largest empire of the time, but it died suddenly in 323 BC. A. In the mesopotámica city of Babylon, located near the present Baghdad. His death has intrigued historians for years, the chroniclers of the time did not mention any of the endemic diseases in the region, making his death a mystery. Modern authors have suggested different types of poisonings, influenza, malaria and typhoid fever as possible causes. All of them based on the different documents that describe the illness that affected him for two weeks until his death.

A few years ago, Marr and Calisher came up with a new suggestion: Alexander the Great was the victim of encephalitis caused by the West Nile virus. A fever, which is now common in parts of Africa, Western Asia and the Middle East, although more and more cases are detected in Europe and the United States. The virus is mainly housed by birds, with mosquitoes transmitting it from one individual to another, or from one species to another, sometimes being able to transmit it to humans or horses and other equines.

Dead crows: bad omen or symptom of West Nile fever?

His death took place in late spring. Babylon, located on the Euphrates River, bordered on the east by a swamp. Birds and mosquitoes had to be abundant, as they are in other swampy areas.

Plutarch’s text is the one that gave the lead to both investigators, to suggest West Nile fever as the cause of their death. The scene of crows flying frantically, with collisions and individuals falling dead, led them to think of the West Nile virus (Fig. 1). Among the birds, the virus mainly affects corvids. The family of birds to which crows belong are particularly susceptible to the pathogen, with some of their species being responsible for their spread. The virus is transmitted from one bird to another and, from time to time, some mosquitoes can transmit it to people and horses (Fig. 2). Humans and horses can get sick, but they cannot transmit the virus, unlike birds. From the point of view of the virus, infecting a person is a dead end, unlike infecting birds.

Alejandro Magno virus Nilo Occidental mosquito muerte

Fig. 1. The scene of the crows at the entrance to Babylon is the one that suggests that these birds could be infected with the West Nile virus and end up infecting Alexander the Great.


In addition to the crows text, Marr and Calisher tested their idea using an online diagnostic program: GIDEON (Global Infectious Diseases and Epidemiology Network). When introducing the symptoms described in the texts of the time about Alexander’s death (respiratory infection, liver disorder, skin rashes, etc …) along with that of the birds, the response obtained by the program was only one: West Nile fever.

Obviously, not all authors have accepted this version. Some argue that symptoms of West Nile virus infection are generally mild, similar to the flu, and most people recover within a few days. The disease can be complicated for the elderly and people with a weakened immune system, requirements that a 32-year-old Alexander the Great who had conquered such a vast empire does not seem to meet.

But regardless of whether or not Alexander could have been weakened enough to complicate the infection, other researchers provided other data to doubt West Nile fever as the cause of his death.

Was there West Nile virus in Alexander’s time?

The authors, Marr and Calisher, assumed that the virus, endemic in the Middle East, had to circulate between the Tigris and the Euphrates, not only centuries, but millennia, and just as the birds were affected by the virus, Alejandro He was able to contract the disease from a mosquito bite.

ciclo del virus del Nilo Occidental

Fig. 2. West Nile virus cycle that is maintained by transmission between birds through mosquitoes, mainly Culex. Sometimes infected mosquitoes can transmit the virus to people and horses.


But this hypothesis does not seem to be fulfilled in view of genetic studies on these viruses. A study on the analysis of the time of divergence between different flaviviruses (the group to which the West Nile virus and dengue belongs), estimates that the virus, which supposedly killed Alexander, appeared 1043-1274 years ago, that is, more a thousand years after his death. Other work suggests that some of the modern lines of the virus emerged 300-400 years ago.

Viruses evolve quickly, it not only makes it difficult for us, based on the effects of current viruses, to infer what viruses or symptoms suffered from ancestors thousands of years ago. But also, that they can be dated correctly at what time some viruses diverged from each other. Substitution rates are reliable for the most recent divergence events, but they lose reliability as you go back in time. It is possible that in the Babylon of Alexander the Great 2,500 years ago, an ancient flavivirus was already circulating, but can it be said that it was the same as the current West Nile virus?

The death of Alexander the Great will remain a mystery unless one day his remains were found, another mystery as great as that of his death for archaeologists and historians. But the debate unleashed from the work of Marr and Calisher added an important point to the topic: attending to the details of the environment when diagnosing a disease, wondering what the climate, fauna and behavior were like, if there are descriptions of it. Today we know that our health is intimately linked with that of other animals and ecosystems.



Cunha BA. 2004. Alexander the Great and West Nile Virus Encephalitis. Emerging Infectious Diseases 10: 1328-1333

Galli M, Bernini F, Zehender G. 2004. Alexander the Great and West Nile Virus Encephalitis. Emerging Infectious Diseases 10: 1328-1333

Mackenzie JS, Gubler DJ, Petersen LR. 2004. Emerging flaviviruses: the spread and resurgence of Japanese encephalitis, West Nile and dengue viruses. Nature Medicine 10: S98-S109

Marr JS, Calisher CH. 2003. Alexander the Great and West Nile Virus Encephalitis. Emerging Infectious Diseases 9: 1599-1603

May FJ, Davis CT, Tesh RB, Barrett. 2011. Phylogeography of West Nile virus: from the Cradle of Evolution in Africa to Eurasia, Australia, and the Americas.

McMullen A, Albayrak H, May FJ, Davis CT, Beasley DWC, Barrett ADT. 2013. Molecular evolution of lineage 2 West Nile virus. Journal of General Virology 94: 318-325

Pearson H. 2003. West Nile Virus may have felled Alexander the Great. Nature news031124-11

Simmonds P, Aiewsakun P, Katzourakis A. 2018. Prisoners of war – host adaptation and its constraints on virus evolution. Nature Reviews Microbiology 17: 321-328

Who eats mosquitoes?

There is no summer without mosquitoes. There are many summer nights in which as soon we turn off the light we hear the unmistakable hum of the common mosquito. It is the announcement of a bite, taking over from species like the tiger mosquito that bite us during the day. Mosquitoes are not only annoying but have a great impact on the health and economy of societies by transmitting a large number of diseases. Malaria, dengue fever, yellow fever, Zika, chikungunya or West Nile fever are some of the diseases that affect millions of people in the world and cause more than half a million deaths each year.

From a human perspective, mosquitoes are nothing more than a plague that feed on our blood, but there are other animals in nature that do not see them that way, but as part of their diet. Although the idea of ​​using these mosquito predators to control their populations is attractive, in reality, they rarely help to control them effectively.

Mosquitoes have many natural enemies, but do they help us control them effectively?

Throughout their life cycle, mosquitoes are exposed to a large number of predators, those that prey on larvae, pupae or adults. The natural enemies of mosquitoes change from one habitat to another, from aquatic to terrestrial environments. Many groups of animals feed on mosquitoes, among which we find arachnids, crustaceans, fish, amphibians, birds and even mammals. Despite the large number of species that include them in their diets, it is very difficult to assess their true impact on mosquito populations.

Something understandable if we consider that there is no predator that completely eliminates its prey. If he did, he would be putting his own livelihood at risk. Predators and prey usually establish a dynamic balance between the two. To this we must add the enormous demographic capacity of mosquitoes that allows them to compensate for the losses suffered by predators. The growth of their populations is such that if we eliminate 50 of 100 mosquitoes we would not notice any change. Possibly we would not perceive changes in the abundance of mosquitoes if we were able to kill 90 of them, it is estimated that to perceive a change in the number of mosquitoes it is necessary to suppress more than 95% of them.


Predators of their larvae

In aquatic environments it is where they find a greater number of predators. Among them are mosquito fish, both Gambusia affinis and Gambusia holbrooki, both species native to North America. These fish have been introduced worldwide with the idea of ​​controlling mosquito pests. In part they did, at the beginning of the 20th century they were used in the Mediterranean rice fields where they managed to eliminate many mosquitoes from the Anopheles group, thus controlling malaria. However, studies on the diet of these fish carried out in Catalonia show that in environments of high diversity, mosquitoes are a small part of their diet and are therefore not very effective in their control. In fact, its great voracity and its reproductive potential have generated an ecological disaster by displacing native fish species and preying on native amphibians and insects, thus destroying the trophic networks of aquatic environments. Today, both species are included in the list of the 100 most harmful invasive alien species in the world and, in Spain, their possession, commercialization or manipulation by the Law of Protection of Natural Heritage and Biodiversity is prohibited.


Beyond fish, mosquito larvae and pupae find large predators among insects. Among them are the Notonectidae, popularly known as backswimmers, and the aquatic beetles (Ditiscidae). Larvae of dragonflies and damselflies also hunt mosquito larvae. In more eutrophized environments, copepods, small crustaceans that can occupy freshwater environments, include mosquito larvae in their diets. The use of copepods as a biological control worked in a region of Vietnam to reduce populations of the yellow fever mosquito, Aedes aegypti, and thus the transmission of dengue.

mosquito alert depredadores naturales mosquitos

Fig. 1. [1] Larva of Toxorhynchites sp. that predates on larvae of Aedes aegypti. [2] Fish, some species of fish reduce the number of mosquito larvae. [3] Heteroptera, such as notonectidae, are small predators of mosquito larvae. [4] Dragonfly larva, another predator of mosquito larvae in some environments. [5] Amphibian tadpoles, some species also prey on mosquito larvae. Source: Mosquito Alert CC-BY


Other works have studied the potential of amphibians as predators of mosquitoes, particularly frogs and toads. It has been seen that tadpoles of various species of Sri Lanka fed on the larvae of Aedes aegypti and were able to reduce their populations. However, similar experiments carried out in Thailand with local species did not produce satisfactory results. Even demonstrating its effectiveness, control with amphibians, as with fish, is impossible in urban and home environments where Aedes mosquitoes occupy small water spaces.

Mosquitoes eating mosquitoes

A highly researched group is the Toxorhynchites mosquito larvae, known as the elephant mosquito, large mosquito larvae that consume larvae from other mosquitoes. The good thing is that the Toxorhynchites are non-hematophagous mosquitoes, that is, they do not feed on blood and, on the other hand, they do kill mosquito larvae of sanitary interest such as the Aedes. Under laboratory conditions, carried out in the Philippines, it has been observed that Toxorhynchites can consume half of the mosquito larvae of Aedes aegypti, Aedes albopictus and Culex quinquefasciatus. But the continuous release of Toxorhynchites larvae in bamboo areas in Indonesia have failed to reduce Aedes aegypti populations.

In most of the cases cited, the experienced predatory species are tropical, exotic species for Europe, so it would be irrational and illegal to introduce them, however effective they might be: mosquito fish have already caused major ecological disasters to generate one again by introducing new exotic species. Here we see the biggest drawback of the so-called “biological control.” The effective species, generally, are not native species but species that we have to introduce, since the native species have been living with the mosquitoes for centuries or thousands of years without having extinguished them. A predator ending its prey would face a big problem. It would be as if we ate all the chickens on the farm without leaving layers … bread for today, hunger for tomorrow.

Much of the species that effectively prey on mosquitoes are tropical, exotic species in Europe, where it is illegal to introduce them


Swifts, swallows and bats: do they eat many mosquitoes?

When mosquitoes fly they also expose themselves to several predators, including dragonflies that feed on all kinds of flying insects not focusing on mosquitoes. Even so, it has been estimated that some dragonflies are capable of hunting between 30 and hundreds of daily mosquitoes. Everything and this ability to hunt, dragonflies can reduce their populations a bit but not solve, much less, the problem. We are talking, perhaps, of many thousands of mosquitoes flying in our garden.

Among the birds we also find mosquito predators. The species that eat the greatest number of mosquitoes are: the common house martin (Delichon urbica), the meadow pipit (Anthus pratensis), the European pied flycatcher (Ficedula hypoleuca), the swallows (Hirundo rustica) or the common swift (Apus apus).

If a swift or swallow had to feed only on mosquitoes, it would need to hunt 14,000 mosquitoes daily

It is quite common to think that having these birds near you frees you from mosquitoes, given their ability to hunt them, the reality is different. If a swift, common house martin or swallow had to feed only on mosquitoes it would require ingesting about 14,000 mosquitoes daily. The same amount of energy can be obtained by capturing a dozen beetles. Pursuing and hunting mosquitoes is a great expense of energy and time that is not very important to them, so that mosquitoes are occasional prey but not the basis of their diet.

Fig. 2. A spider male, Icius hamatus, capturing a tiger mosquito in a garden in Barcelona. Photographed sent to Mosquito Alert by Antonio Piñera. Source: Antonio Piñera CC-BY


Something similar happens with bats despite the popular belief that they are large predators of mosquitoes. The reality is very different and especially complex. Bat prey varies depending on the size of the bats themselves. Large species feed on large insects, while small species do on smaller insects, including some mosquitoes. The main and most important prey for all bats are moths. If you are a bat that is spending energy flying after the prey, what would you choose: a moth, equivalent to a 500-grilled steak, or a mosquito, which would become a 5-gr minihamburger.

 Spiders stalk mosquitoes in the shade

Beyond the active flight period, adult mosquitoes spend much of their time resting among the vegetation. While resting they are exposed to a large number of predators, where we find spiders among them the most effective. A large number of spider species include mosquitoes in their diets, both those that build cobwebs and those that don’t.

mosquito alert depredadores naturales mosquitos

Fig. 3. [1] Bats, some species can actively prey on adult mosquitoes. [2] Swallows and swifts are also natural predators of mosquitoes. [3] Geckos, also occasionally prey on mosquitoes. [4] Spiders prey on mosquitoes in their shelters. [5] Other types of reptiles can sometimes prey on mosquitoes. Source: Mosquito Alert CC-BY


Recent studies show that predators not only have a direct effect reducing the number of mosquitoes, but altering their behavior, is what biologists call: landscape of fear. Thus it has been seen that aquatic environments with a greater number of potential predators are avoided by females to deposit eggs.

The most effective method to reduce the number of mosquitoes at home is still to prevent them from having to reproduce

Mosquito populations depend on numerous factors, including landscape (urban or rural), abiotic or climatic factors such as rain or temperature, and biotics such as trophic networks with the predators that have been described. All these factors are in turn related, so that the landscape affects the species that could prey on mosquitoes. In urban environments, the natural enemies of mosquitoes often do not survive, making them a space free of enemies for mosquitoes.

Despite having a large number of animals capable of preying on mosquitoes, the most effective method to control them on our properties is still to avoid favorable habitats. Mainly, avoid providing small water points that can be used to reproduce. If you do not want to contribute to a new generation of mosquitoes, avoid the accumulation of water in your spaces, because in the water of your vase there is no predator that lives. Just mosquitoes.


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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.



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