Traces of Zika virus' RNA could be found in human tears, scientists said. Researchers from the Washington University School of Medicine in St. Louis have found that Zika virus can spread in the eyes of an infected person and that tears may also contain genetic material from the virus.

After conducting experiments on laboratory mice, the researchers were able to explain why some infected patients develop eye diseases, including a condition known as uveitis -- an inflammation of the eye affecting the middle layer of tissue in the eye wall or uvea.

Zika could also cause conjunctivitis in adults, which is characterized by redness and itchiness of the eyes. One-third of babies infected with Zika in utero were also found to have optic nerve inflammation, retinal damage or even blindness after birth.

"Our study suggests that the eye could be a reservoir for Zika virus," Michael Diamond, a professor of medicine and one of the study's senior author, said in a statement. "We need to consider whether people with Zika have infectious virus in their eyes and how long it actually persists."

In the published in Cell Reports, the scientists detail their findings on the effects of Zika virus in the eyes of mouse fetuses, newborns and adults. After injecting adult mice with Zika virus, the researchers found that the virus was able to travel to the mice's eyes and infect the irises, retinas and optic nerves of the animals. But the researchers are yet to find out what specific route the virus takes within the eye.

Since the immune system is less active inside the eyes, infections of the eye linger for several days - even months - and take some time to disappear from the eye even if the rest of the body is free from infections, the researchers said.

After 28 days, the researchers tested the infected mice and found that the animals' tears contained Zika's RNA -- the genetic material from the virus -- but not infectious virus. According to the researchers, the findings of the study raise the possibility that people could contract Zika through contact with the tears of an infected person.

The scientists cautioned that more studies are needed to determine if tears could spread the Zika virus in people. According to the researchers, even if the tears of infected patients are not infectious, the detection of the virus in the eye and the viral RNA in tears could be used in diagnosing Zika infection rather than drawing blood. Moreover, the experiment on lab mice could be used in testing anti-Zika drugs.

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