The discovery from tests on mouse fetuses, newborns and adults may help explain why some people infected with Zika develop eye disease that puts them at risk for permanent vision loss. What isn't yet known is whether or not this Zika material in the eyes can transmit infection, and if it can do so in people, the researchers said.
Further studies on people infected with Zika are being planned, according to researchers at the Washington University School of Medicine in St. Louis. That's because findings from animal studies don't always turn out the same in people.
"Our study suggests that the eye could be a reservoir for Zika virus. We need to consider whether people with Zika have infectious virus in their eyes and how long it actually persists," study co-senior Dr. Michael Diamond, a professor of medicine, said in a university news release.
About one-third of babies infected with Zika in the womb have eye problems. These problems can include inflammation of the optic nerve, retinal damage or blindness after birth, the study authors noted.
In adults, Zika can cause conjunctivitis -- redness and itchiness of the eyes. In rare cases, adults with Zika infection can develop a condition called uveitis, which can lead to permanent vision loss, the study authors said.
"Even though we didn't find live virus in mouse tears, that doesn't mean that it couldn't be infectious in humans," said study lead author Dr. Jonathan Miner, an instructor in medicine. "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."
Samples of mouse tears were taken 28 days after infection, according to the researchers.
The immune system is less active in the eye in order to avoid accidental damage to sensitive tissues responsible for vision when the body is fighting an infection. This means infections sometimes persist in the eye after they have been eliminated from the rest of the body, the study authors explained.
Study co-senior author Dr. Rajendra Apte said the researchers "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." Apte is a professor of ophthalmology and visual science.
Even if further studies find that Zika cannot be transmitted through human tears, it may be possible to test tears for Zika genetic material or antibodies to offer a less painful way to diagnose recent Zika infection than the current method of taking blood samples, the researchers said.
Also, the eyes of mice could be used to test anti-Zika drugs, the authors suggested.
The study was published Tuesday in the journal Cell Reports.
The Zika virus, which is usually mosquito-borne but may also be sexually transmitted, doesn't pose a significant health threat to most people. But it can cause birth defects, including microcephaly, which leads to babies born with abnormally small heads and brains.
The vast majority of Zika infections have been in Latin America and the Caribbean. However, last month, two neighborhoods in the Miami area reported cases of locally acquired infection.
The U.S. Centers for Disease Control and Prevention has more on Zika.
This Q & A will tell you what you need to know about Zika.
To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.
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