Tissue In Human Eye Appears To Be Resistant To SARS-CoV-2

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A new study has investigated whether transmission of Sars-CoV-2, the virus that causes Covid-19, could be entering the body through tissue in the cornea.

Nearly a year into the pandemic, and we are constantly still learning about how Covid-19 spreads. Epidemiologists believe the main way the virus is transmitted is through respiratory droplets or small particles that are released by an infected person when they cough, sneeze, talk, or breathe. During the early days of the pandemic, however, governments around the world believed a large part of transmission came through touching a contaminated surface and then touching your own face.

The US Centers for Disease Control and Prevention (CDC) notes that droplets can "land on surfaces and objects and be transferred by touch. A person may get Covid-19 by touching the surface or object that has the virus on it and then touching their own mouth, nose, or eyes." This is still thought to be a known method of transmission, though the CDC now notes "Spread from touching surfaces is not thought to be the main way the virus spreads".

It's been suggested that as well as entering through the mouth and nose, the virus could make its way in through the eyes. Patients with the disease often get conjunctivitis, which could be caused by the virus or by some other mechanism, something a team of researchers at Washington University School of Medicine in St. Louis has sought to investigate.

Reporting in the journal Cell Reports, the team wanted to establish whether the eyes could be an entry point for Sars-CoV-2, given that Zika virus has previously been shown to be shed in tears. They exposed human (from donors) and mice corneas – the transparent front part of the eyes that cover the iris, pupil, and anterior chamber – to the new coronavirus, as well as Zika and herpes, to see if they could replicate there. 

Though herpes and Zika were able to reproduce in the cornea, Sars-CoV-2 was not.

"The cornea and conjunctiva are known to have receptors for the novel coronavirus, but in our studies, we found that the virus did not replicate in the cornea," senior author and ophthalmologist Rajendra S. Apte said in a statement. "Our data suggest that the novel coronavirus does not seem to be able to penetrate the cornea."

They found key substances in the eye that could inhibit or promote viral replication, including one called interferon lambda. It was found to prevent the efficient growth of both Zika and herpes, but there was no effect on Sars-CoV-2, mainly because it was unable to replicate in the cornea in the first place.

While welcome news, the team is keen to provide words of caution. They note that they are yet to determine if other tissues in and around the cornea like the tear ducts and conjunctiva are vulnerable to the coronavirus. It's also not clear if symptoms like conjunctivitis are caused by the virus or a secondary inflammation when the body's defenses are low. 

“Our findings do not prove that all corneas are resistant,” first author Professor Jonathan J. Miner said. “But every donor cornea we tested was resistant to the novel coronavirus. It’s still possible a subset of people may have corneas that support growth of the virus, but none of the corneas we studied supported growth of SARS-CoV-2.”

The team warned that as their findings are still preliminary people should still take every precaution against the virus, including using eyewear in hospitals, until we know more.

“We may learn that eye coverings are not necessary to protect against infection in the general community, but our studies really are just the beginning," Miner said. "We need larger clinical studies to help us better understand all the potential routes of SARS-CoV-2 transmission, including the eye.”

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