Researchers have found that Zika virus can live in eyes and have identified genetic material from the virus in tears, according to a mouse study released Tuesday.
The study, published the U.S. journal Cell Reports, may help explain why some Zika patients develop eye disease including a condition known as uveitis which can lead to permanent vision loss.
"Our study suggests that the eye could be a reservoir for Zika virus," said Michael Diamond, professor of medicine at the Washington University School of Medicine in St. Louis and one of the study's senior authors.
Zika virus causes mild disease in most adults but can cause brain damage and death in fetuses.
About a third of all babies infected in utero 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, uveitis.
In the new study, the researchers infected adult mice under the skin, similar to the way humans are infected by mosquitoes, and found live virus in the eyes seven days later.
That means that Zika is able to travel to the eye, but it's not yet known whether the virus typically makes that trip by crossing the blood-retina barrier that separates the eye from the bloodstream, traveling along the optic nerve that connects the brain and the eye, or some other route.
Further research 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 lead author Jonathan Miner, an instructor in medicine at the Washington University School of Medicine in St. Louis.
"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."
The presence of Zika RNA so long after the virus is cleared is also concerning because it could mean that corneal tissue, a common tissue for transplant procedures, could harbor the virus.
The researchers now are planning complementary studies in human patients infected with the virus.
"We need to consider whether people with Zika have infectious virus in their eyes and how long it actually persists," said Diamond.
