The Zika virus can live in eyes, researchers said on Tuesday, after conducting experiments on mice that may explain why some patients develop ocular disease and in some cases become blind.
The study, published in the journal Cell Reports, studied the effects of Zika virus infection in the eyes of fetal, newborn and adult mice.
"Our study suggests that the eye could be a reservoir for Zika virus," said Michael Diamond, a professor at the Washington University School of Medicine in St. Louis who is one of the study's senior authors.
"We need to consider whether people with Zika have infectious virus in their eyes and how long it actually persists," he added, noting that patients could spread the infection through their tears.
The researchers are planning to expand their study to include humans infected with Zika virus.
Zika causes only mild symptoms such as fever and a rash for most people but pregnant women who catch it can give birth to babies with microcephaly, a deformation marked by abnormally small brains and heads.
One third of babies infected with Zika in utero have eye disease such as inflammation of the optic nerve, retinal damage or blindness, the researchers said.
In adults, Zika can cause conjunctivitis, also known as pink eye, and in some rare cases uveitis, a condition in which part of the eye wall becomes inflamed. It can lead to permanent vision loss.
In conducting the study, researchers infected mice under the skin, similar to the way a human would be infected by a mosquito.
They found live Zika virus in the rodents' eyes seven days later.
Infection in the eyes means it's possible people can become infected with Zika simply through touching contaminated tears.
It's still not clear how the Zika virus makes its way to the eyes. One possibility is that it crosses "the blood-retina barrier that separates the eye from the bloodstream, traveling along the optic nerve that connects the brain and the eye," researchers said in a statement.
Researchers found genetic material from Zika in the tears of infected mice 28 days after infection — though not the virus itself.
"Even though we didn't find live virus in mouse tears, that doesn't mean that it couldn't be infectious in humans," said lead author Jonathan Miner, who teaches medicine at 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."