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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

How a mosquito sneaks after it steal your blood

Imagine you sneak into a banquet. A huge one. Where there is plenty of food everywhere. You eat, eat and eat. You eat so much that without realizing it you have doubled or tripled your weight. You can’t anymore It’s time to sneak out, you think, but do you think you could do it? You have doubled your weight, you will be slow, you would move dragging the belly, the most possible is that you collapsed. But if you were a mosquito, things would be completely different. If you were a mosquito, you would take off without problems after the feast, and most important of all, you would fly without being detected.

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Genetics determine the likelihood of being bitten by mosquitos

It is already known that the mosquitos that bite us are almost always females (exceptions can be made for some species) since they need proteins and other nutrients in our blood in order to create eggs.  Is it true that mosquitos bite some people more than others, or is this just a myth? A mere suspicion of those who wake up covered in bites?

James Jordan (CC BY-ND 2.0)

James Jordan (CC BY-ND 2.0)

As it turns out, this may be backed up with scientific fact.  Recent studies have shown that there are people who are really just irresistible to these small vampires. It has been shown that the smell of the human body attracts mosquitos since sweat contains lactic acid, ammonia and other chemicals that emit aromas which are enticing to mosquitos; in other words, a sort of mosquito-attracting perfume.

Female mosquitos are able to sense such odors with olfactory systems in their antennas, and, once detected, if our odor pleases them there will be no escaping.  For this reason, if we go to the country or a forest or other places with a lot of mosquitos, it is recommendable to wear long pants and sleeves and have a good repellant on hand.

Over the past decades a number of theories have been debated: that mosquitos are attracted by chemical substances eliminated through the epidermis after drinking alcohol (especially beer), by dark clothing (especially black and red), high body temperature, floral or citric perfumes, CO2 emissions exhaled by those exercising in the outdoors, or women in the third trimester of pregnancy, etc. (however, this last theory is not relevant to the tiger mosquito or many mosquitos found outdoors). Some people are even convinced that mosquitos, given the choice, would choose someone with O- or B negative blood types over someone with A.

Foto: BigStock

Photo: BigStock

 

The truth is that none of the previously mentioned theories are based on scientific evidence which proves them, and it is even less likely that they could be applied to all mosquito species.  On the other hand, what has in fact been demonstrated is that mosquitos are attracted to the presence of certain bacteria on the skin, but only in special cases in which the microbial community is large and has very low diversity.

Over the past few years research has been advancing to discover the genetic components of the real influential factors, such as body smell, the removal of certain substances through sweat, and the presence of certain bacteria on the skin. These are differences which are detectable by mosquitos and come directly from our genes; meaning, unfortunately, that they will passed onto our children.

Of the 400 different chemical compounds excreted in human sweat, 85% have a genetic origin.

Researchers suggest that the development of genes lending to unappealing odors for mosquitos could be part of genetic evolution of natural defenses against these animals which have transmitted diseases for millions of years.

A study from 2015 published in PLOS ONE worked with nearly 40 pairs of twins: of these, 18 were identical twins sharing 100% of their genes, and 19 twin pairs shared as many genes as normal brothers or sisters of individual births. The results of the study showed that only some of the non-identical twins (one of the two individuals) were more likely to be bitten, while the mosquitos showed the same interest for identical twins, whether high or low.  It is now thought that such genetically-originating factors may hold the key.

The next step is to discover the combinations of genes responsible for the particular body odors in order to know the risk of being bitten. This could lead to the development of pharmaceuticals which increase the production of natural repellents by the body instead of always relying on applying artificial products on the skin. It is important to keep in mind that these small animals infect millions of people each year with dangerous diseases such as malaria or dengue, and the incidence of these is generally highest in countries with low access to health care.

Information sources:

G. Mandela Fernández-Grandon, Salvador A. Gezan, John A. L. Armour, John A. Pickett, James G. Logan. Heritability of Attractiveness to Mosquitoes. PLoSOne, 10 (2015), p. e0122716

El País (24/4/2015): https://elpais.com/elpais/2015/04/21/ciencia/1429613571_574034.html

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