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Tuesday, September 22, 2020

Explained: How we know mosquitoes cannot spread coronavirus

In clinching evidence, scientists demonstrate that SARS-CoV-2 is unable to replicate in mosquitoes even when they are injected with it

Written by Kabir Firaque , Edited by Explained Desk | Updated: July 20, 2020 8:25:46 am
The experiments were conducted on three common and widely distributed species of mosquitoes

So far, there was no evidence that mosquitoes can spread the novel coronavirus that causes Covid-19. Now, scientists from Kansas State University have provided evidence that mosquitoes indeed cannot. Their study is published in Nature Scientific Reports.

Was that not known already?

Until this study, there was no confirmation either way. “To date there has been no information nor evidence to suggest that the new coronavirus could be transmitted by mosquitoes,” the World Health Organization (WHO) says in its Mythbusters section on Covid-19, first published in March.

The WHO, nevertheless, has been definite that mosquitoes cannot transmit the virus. The new study refers to this as a presumption, and says it may be based on “various observations and facts extrapolated from other coronaviruses”. For example, the coronaviruses that cause SARS and MERS produce relatively low levels of virus in the blood, compared to the levels produced when mosquitoes are infected with dengue and yellow fever.

So, what has been proved now?

The researchers have demonstrated in lab experiments that SARS-CoV-2 cannot infect mosquitoes. It is the first such confirmation. “We did everything possible to get the mosquitoes infected even though the conditions that we used would not be found in nature,” Stephen Higgs, director of Kansas State University’s Biosecurity Research Institute, told The Indian Express by email.

The mosquitoes did not get infected even in these extreme conditions.

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What does it mean for a mosquito to get infected?

If replication takes place, there should be a sufficient level of virus in the blood to indicate this. “The more virus in the blood then the more likely the mosquito is to be infected,” Higgs said. “But not all types of mosquitoes are susceptible to infection. We do not know what makes some mosquitoes susceptible and others resistant.”

Take the examples of dengue and yellow fever. With SARS and MERS, the level of coronavirus does not reach the level of dengue virus or yellow fever virus that would be considered high enough to infect a mosquito.

Higgs cited the mosquitoes that spread dengue and chikungunya. These are transmitted by Aedes aegypti which is susceptible, but not by Anopheles species which are resistant. “You can look at it another way and say some viruses can infect some mosquitoes and others cannot  Dengue virus infects Aedes but not Culex. Again we do not fully understand this,” Higgs said.

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So, which mosquitoes are not infected with the new coronavirus?

The experiments were conducted on three common and widely distributed species of mosquitoes. Aedes aegypti, Aedes albopictus and Culex quinquefasciatus. The first two species are the vectors for dengue fever, chikungunya, Zika fever, and yellow fever. Culex quinquefasciatus spreads viruses that cause lymphatic filariasis and certain kinds of encephalitis.

All three species are present in India. The coronavirus was unable to replicate in any of them.

How were the experiments conducted?

In order to be infected, a mosquito must feed on blood with virus in it. However, “rather than feed mosquitoes on infected blood, we actually injected the virus into the mosquitoes”, Higgs said.

“The reason that we did this was that it is an extreme test of mosquito susceptibility to viruses. If the virus does not grow when injected then we can be confident that even if a mosquito fed on a person with lots of virus in their blood then they would not be infected,” he said.

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