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Eyes may be a reservoir for Zika virus, say scientists, including one of Indian origin, who found evidence of the virus in eyes and tears of infected mice, a discovery that raises the possibility that the infection may spread through tears.
Zika virus causes mild disease in most adults but can cause brain damage and death in foetuses.
About a third of all babies infected in uterus with Zika show eye disease such as inflammation of the optic nerve, retinal damage or blindness after birth.
In adults, Zika can cause conjunctivitis — redness and itchiness of the eyes — and, in rare cases, a condition known as uveitis that can lead to permanent vision loss.
"Our study suggests that the eye could be a reservoir for Zika virus," said Michael S Diamond, a professor at Washington University School of Medicine.
"We need to consider whether people with Zika have infectious virus in their eyes and how long it actually persists," Diamond said.
To determine what effect Zika infection has on the eye, the researchers infected adult mice under the skin and found live virus in the eyes seven days later. The observations confirm that Zika is able to travel to the eye.
It is not yet known whether the virus typically makes that trip by crossing the blood-retina barrier that separates the eye from the bloodstream, travelling along the optic nerve that connects the brain and the eye, or some other route.
Eye infection raises the possibility that people could acquire Zika infection through contact with tears from infected people.
The researchers found that the tears of infected mice contained Zika's RNA — the genetic material from the virus — but not infectious virus, when tested 28 days after infection.
"Even though we didn't find live virus in mouse tears, that doesn't mean that it couldn't be infectious in humans," said Jonathan J Miner, from Washington University.
"There could be a window of time when tears are highly infectious and people are coming in contact with it and able to spread it," said Miner.
The eye is an immune privileged site, meaning the immune system is less active there to avoid accidentally damaging sensitive tissues responsible for vision in the process of fighting infection.
Consequently, infections sometimes persist in the eye after they have been cleared from the rest of the body.
"We are planning studies in people to find out whether infectious virus persists in the cornea or other compartments of the eye, because that would have implications for corneal transplantation," said Rajendra S Apte, professor at Washington University.
Even if human tears do not turn out to be infectious, the researchers' detection of live virus in the eye and viral RNA in tears still has practical benefits.
Human tears potentially could be tested for viral RNA or antibodies, a less painful way to diagnose recent Zika infection than drawing blood.
The research appears in the journal Cell Reports.
