Daniele Santos holds her baby Juan Pedro, who has microcephaly, on May 30 in Recife, Brazil. (Mario Tama/Getty Images)

Researchers have found that the Zika virus can live in eyes, and research in mice may help explain why some Zika patients develop eye disease, including a condition that can lead to permanent vision loss.

In a study published Tuesday in Cell Reports, researchers from Washington University School of Medicine in St. Louis describe the effect of Zika virus infections in the eyes of mouse fetuses, newborns and adults.

The study suggests that the eye could be a reservoir for the virus. Eye infection raises the possibility that people could become infected with Zika through contact with tears from infected people, they said.

Zika virus is primarily spread from the bite of an infected mosquito. It is also spread through sex. Some researchers are also considering whether other bodily fluids may play a role.

Researchers have detected Zika virus in blood, urine, semen, saliva and breast milk. It has also been found in genital tract swabs and fluid inside the eye, health officials have said.

Researchers said bodily fluids, including tears, are one possibility for how an elderly Utah man may have spread the virus to his son. The elderly man died in June after contracting Zika from travel abroad, but his son did not leave the country and the mosquitoes known to spread the disease are not found in their area. Officials haven’t been able to determine how his caregiver son became infected. The son has since recovered.

“That one case illustrates the real conundrum,” said Rajendra Apte, an ophthalmology professor and retina specialist who is one of the study’s senior authors.

To determine the effects of Zika infection on the eye, Washington University scientists infected adult mice and found live virus in the eyes seven days later. They didn’t find virus in the eyes of baby mice eight days after they were born to infected mothers. But after infecting neonatal mice, they found evidence of Zika infection once they were born, Apte said.

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.

What researchers don’t know is whether in mice “there is a point where tears are actually infectious,” Apte said. He added that in people, “it could be a completely different story.”

Unlike other parts of the body, the eye is a place where the normal immune response is suppressed. It’s an evolutionary response to avoid inflammation of sensitive tissue “because if you have a lot of inflammation or scar tissue, it’s not good for vision,” he said. As a result, infections sometimes persist in the eye after they have been cleared from the rest of the body.

American doctor Ian Crozier treated patients in West Africa during the Ebola epidemic in 2014, and later became infected himself. The virus lurked in his eye months after he had been declared Ebola-free.

Researchers are planning studies to determine whether infectious Zika virus persists in human eyes, Apte said. They also want to know how long the virus stays in the eyes because of other public health implications.

Hepatitis C virus, a related virus, can infect the human cornea and is transmitted by corneal transplants, the study said. The study pointed out that eye banks may need to start testing corneas for Zika virus as well.

Read more:

Zika is spreading and remains global emergency, WHO declares

These eight countries in Asia and Africa are at highest risk for Zika outbreaks

U.S. running out of money to fight Zika domestically

Florida finds first local mosquitoes infected with Zika