COVID-19

Heat and regular rains, this is how the year has been with the most mosquitoes in Spain

2020 has been a year marked by the COVID-19 pandemic and an unusual year in terms of the number of tiger mosquitoes. In Spain, twice as many of these insects have been observed than last year, accumulating 1,798 notifications compared to 885 in 2019. The year of the pandemic has turned out to be the year with the most tiger mosquitoes registered by the Mosquito Alert project since 2014. “Densities of Tiger mosquitoes are always explained by two factors, the climate and human behavior, both are those that periodically provide them with water points, where they breed ”, explains Frederic Bartumeus, co-director of the project and ICREA researcher at CEAB-CSIC and CREAF. The 2020 mosquito season started with a warm and rainy spring that allowed them to proliferate during the months of May and June. Furthermore, according to Roger Eritja, entomologist at CREAF and the project, confinement could have played a relevant role in these data, “the ban on traveling to second homes during the months of March and April could affect the maintenance of their gardens, swimming pools and terraces, where the water that mosquitoes need to multiply could accumulate ”, he says.

Tropical summer

This explosion of mosquitoes in spring was followed by a summer that, for the sixth consecutive year, registered higher than normal temperatures, with intense and repeated episodes of rainfall in the Iberian peninsula. The Mediterranean is heating up. The summer is coming sooner, longer and more intense, conditions that can benefit the tiger mosquito when accompanied by regular rains, as has happened this year.

Fig 1. Part of the Mosquito Alert team (John Palmer, Frederic Bartumeus, Roger Eritja) at the Marimurtra Botanical Garden in Blanes where experiments are being carried out with tiger mosquito populations. Source: Mosquito Alert CC-BY

 

These weather conditions have two effects on mosquitoes. In summer, high temperatures allow the larvae to develop faster, shortening the time between generations, and leading to an exponential growth of their populations. This happens as long as they have places with water to reproduce. On the other hand, global warming, with a warm spring and autumn, could lengthen its seasonality allowing mosquitoes to be active for more days of the year.

Mushrooms, chestnuts and mosquitoes

Autumn is being warm and mosquitoes are still active, as evidenced by the more than 250 reports that the Mosquito Alert platform has received in the last two weeks from Spain, and more than 250 confirmed observations of the tiger mosquito between October and November. In 2019, in the same period, 116 tiger mosquitoes were notified, 9 of them in the month of November, until November 28, 2020 there have been 50 reports of tiger mosquitoes, this is five times more than in the previous year. During November there have been also reports of tiger mosquito activity from Italy, France, Hungary and Turkey. Beyond the temperatures, what most affects the abundance of mosquitoes is having availability to reproduce, that is, having water in the breeding places, either due to the presence of regular rains or due to human activity.

“After having modeled the data, considering the number of users with the application by region and their degree of participation, we see that 2020 slightly exceeds 2015, although their patterns have been very different”, comments John Palmer, UPF professor and co-director of the project. The weather conditions this year have been favorable to the mosquito continuously since spring, while in 2015 its populations exploded exponentially in summer. Understanding the climatic and social factors that determine the dynamics of their populations will allow predicting their abundance in the future.  

 

Study shows mosquitoes cannot transmit COVID-19

With the start of the mosquito season, many people have wondered if the COVID-19 virus, SARS-CoV-2, can be transmitted by mosquitoes. In a previous post, we explained that everything suggested that this was not possible. Arguments based on the knowledge that we have today of the transmission mechanisms of different viruses by mosquitoes. Now researchers have carried out an experiment to demonstrate whether mosquitoes can become infected with SARS-CoV-2 and thus be able to transmit it. The results are clear: mosquitoes are not infected with the virus and therefore cannot transmit it.

In the study, the authors artificially infected mosquitoes of different species: the tiger mosquito (Aedes albopictus), the yellow fever mosquito (Aedes aegypti) and Culex quinquefasciatus, the main transmitter of filiariasis in tropical and subtropical regions. None of the three species was infected with the virus.

Neither the tiger mosquito nor the yellow fever mosquito was infected with the COVID-19 virus.

To demonstrate this, they inoculated a high dose of the virus inside their bodies and studied how the virus progressed within mosquitoes over time. In fact, in the study, in addition to injecting them with a high dose of the virus, they have skipped the first two barriers that viruses must overcome before being able to infect a mosquito’s body: surviving and replicating in their digestive system, and overcoming the basal lamina that surrounds it, represented in the figure with the numbers 1 and 2 respectively.

 

Infección de un mosquito por virus

Fig. 1. 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)

 

For the study, researchers from the University of Kansas have inoculated the virus directly into the cavity of the mosquito’s body (Fig. 1, number 3), which is known as hemocoel, which contains the blood-like fluid of vertebrates from insects: hemolymph.

Looking for the presence of the virus in mosquitoes two hours after infecting them, one day, three days, seven days, ten days and fourteen days, they observed that the virus was only detectable during the first two hours after inoculation (Table with results from the original article Huang et al. 2020). At that time the virus concentration corresponded to the inoculated dose, that is, there was no evidence that the virus was replicating in the mosquito.

Species Inoculation Time
2 hoours 1 day 3 days 7 days 10 days 14 days
Aedes albopictus SARS-CoV-2 5/6 1/14 0/15 0/20 0/21 0/31
83,3% 7,1% 0,0% 0,0% 0,0% 0,0%
Aedes aegypti SARS-CoV-2 5/6 0/17 0/17 0/24 0/26 0/27
83,3% 0,0% 0,0% 0,0% 0,0% 0,0%
Culex quinquefasciatus SARS-CoV-2 3/3 0/17 0/17 0/25 0/28 0/25
100% 0,0% 0,0% 0,0% 0,0% 0,0%

 

A day after the injection with the virus, there was no trace of it in the mosquitoes. The table represents the number of mosquitoes where the virus was detected (for example, 5 out of 6 for Aedes albopictus at two hours, which represents 83.3% of individuals; or only in 1 of 14 mosquitoes analyzed after 24 hours). Beyond the first day, the virus disappeared in the three species studied. In none of the analyzed species did the virus progress, indicating that it is not possible to transmit it.

The study has not yet been reviewed by other specialists, but published as a preprint on the Research Square platform.

 


References:

Huang YS, Vanlandigham DL, Bilyeu AN, Sharp HM, Hettenbach SM, Higgs S. 2020. SARS.CoV-2 and mosquitoes: an extreme challenge. Preprint from Research Square: 10.21203/rs.3.rs-32261/v1

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

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

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