A few days ago, a video of the doctor Cesar Carballo in which he talked about the mosquito transmission of coronavirus it sowed a great controversy on social networks. On the one hand, by the people who were scared to think that this could be a new way of contagion. On the other, by those who criticized that he made such a statement without having scientific evidence available.
In fact, in the video, in which he answered a question from Carmen Porter, he clarified that he did not know scientifically. Therefore, it would have been easier to say that he did not know, without actually launching a doubtful yes. Yes, we must admit that, although this video has been released now, it is actually one of the early pandemic, when there was much more uncertainty with everything concerning the disease.
Now, in what situation are we currently? Could mosquitoes have been part of the SARS-CoV-2 transmission in all this time? The truth is that it seems highly unlikely. And now there are studies and evidence in this regard.
Studies on the transmission of coronavirus by mosquitoes
In order for a mosquito to transmit a virus through its bite, it is necessary that it it replicates in your cells. For this reason, there are numerous studies aimed at verifying whether this is possible with SARS-CoV-2, which causes COVID-19.
The studies are aimed at verifying if the virus can replicate in the mosquito
One of the first studies was published in July 2020, just four months after the declaration of a pandemic situation. Mosquitoes of three species were analyzed: Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus. The mosquito was introduced to all of them by intrathoracic inoculation for, some time later, to check if it had been able to replicate itself. And, interestingly, only particles of the still infectious virus were found in one of them, which was collected 24 hours later. However, the amount was the same as in the inoculation, so it did not appear to have been replicated.
Despite this study, some uncertainty remained. For this reason, in January 2021 a new study was carried out to verify if the transmission of coronavirus by mosquitoes. This time the two mosquito species that act as the most common viral vectors in Europe were used. The first was Aedes albopictus, who had also participated in the previous work. The second, Culex pipiens, is a mosquito known, for example, for the transmission of the West Nile virus.
Unlike in 2020, the virus was not inoculated to mosquitoes through the thorax. This time what would happen naturally was reproduced, feeding them blood from infected patients. Afterwards, the mosquitoes were collected on days 0, 3, 7 and 10 after inoculation. Again, they observed that the virus could not replicate, so it was impossible for them to transmit it through their bites.
And, as if all this were not enough, in July of this year another study was published, this time with Culicoides sonorensis, Culex tarsalis and Culex quinquefasciatus. They all are RNA virus vectors, as is SARS-CoV-2. But can you talk to them about the transmission of the coronavirus by mosquitoes? Again, it seems not.
As in the January study, the mosquitoes fed on blood from infected patients, although this time they were only collected at 10 days. Then, a PCR was done to detect the virus in your body. 85% of them tested positive. However, it should be noted that positive is not the same as infective. A positive PCR result indicates that viral RNA has been detected in host cells, in this case mosquitoes. But this virus may not be replicating and therefore cannot infect.
Therefore, we proceeded to homogenize samples mosquito. This is a technique that allows cell membranes to be gently broken, releasing their content. A content that they later cultivated in African green monkey cells. In this way, if there were viruses with infective capacity, these cells would be infected. But after three cycles of this procedure, no trace of the viruses was found. Therefore, it was again concluded that the transmission of coronavirus by mosquitoes is highly unlikely.
Centers for Disease Control and Prevention (CDC)
Maybe by contact?
In the January study on the spread of coronavirus by mosquitoes, there was a separate small experiment.
Today we know that contagion by contact is practically insignificant
And it is that, unlike C. pipiens, A. albopictus has a very particular way of biting, since it can make several meals in different guests in no time. Therefore, it was thought that perhaps, if not through bites, it could transmit the virus mechanically, by contact.
To verify this, feeders were placed with infected blood and, later, with uninfected blood. The virus was not transferred, so it did not seem that this type of contagion could occur.
But what about other insects? Last July, a new study was carried out, but this time based on flies, specifically the house fly (Musca domestica). The insects were not fed, but were collected directly from outdoor areas of two hospitals in the city of Shiraz, in Iran. It was at a time with a high number of COVID-19 cases, between May and June 2020.
They observed that when washing the bodies of the flies, the presence of viral RNA was detected in 75% of them. This meant that there were viral particles attached to his body. To see if they were also in his body, they proceeded to a homogenization, after which the virus was detected by PCR by 37%. This again would mean that there is RNA, but not necessarily that it is infective.
Therefore, the authors of this study consider that, in any case, it could be that contagion occurred mechanically, by contact. Flies are insects that tend to settle a lot in body secretions and, in risky places, such as hospitals, perhaps, based on their results, could be worthy of attention. However, it should be remembered that, to this day, we know that contact transmission is practically negligible compared to inhalation of droplets and aerosols. Based on the scientific evidence, it doesn’t seem a bit worrisome. So no, neither at the beginning of the pandemic nor now should we fear mosquitoes. Their bites are a nuisance and sometimes they carry disease, yes, but COVID-19 does not appear to be one of them.