The Zika virus can live in the eyes, and evidence of the virus has been found in the tears of animals. Scientists say it is not known whether the infection can be spread in humans through tears.
About one-third of babies born with the Zika virus have inflammation of the eyes, sometimes rendering them blind. In adults, Zika has been shown to cause eye infections such as uveitis and conjunctivitis in up to 15 percent of people who are symptomatic.
Researchers studied the eyes of a mouse model of Zika, and discovered that infection of the eye is widespread, according to Michael Diamond, a professor of molecular microbiology at Washington University School of Medicine in St. Louis, Missouri.
"There were many different parts of the eye — the cornea, the iris, the retina. All of those seemed to have virus associated with them, in particular cells. And, indeed, the fluid associated with the eye, as well," he reported.
Diamond and his colleagues found the viral RNA was still present in mouse tears 28 days after scientists infected them with Zika.
When investigators tried to infect a group of rodents that did not have Zika with the tears of infected mice, they were unable to do so. However, that does not mean human tears are not infectious. Scientists won't know for certain until human tears are studied, Diamond said.
His team is working with clinicians in Latin America, Puerto Rico and Florida who are treating Zika patients to get samples of human tears for testing.
"Only after doing that will we begin to get a handle as to whether the tears are carrying enough infectious virus to transmit it, or whether there's very small amounts and they're not really transmissible," Diamond said. However, he added, testing bodily fluids — including tears — could be a simple and painless way to diagnose Zika virus.
Investigators are looking for alternative routes of infection, through bodily fluids, because the mosquito-borne virus is spreading more quickly than epidemiologists predicted. It's known that a number of bodily fluids, including semen and saliva, can harbor the virus. Researchers are also investigating whether Zika can be transmitted by vaginal fluid.
The speed of disease transmission is often tracked by the amount of virus contained in the blood. Compared to other mosquito-borne illnesses, such as dengue fever, Diamond said there is relatively little Zika virus in human blood for a mosquito to transmit from person to person through a bite.
The eyes, Diamond said, are considered an "immune-privileged site," meaning the immune system is less active there, so as not to accidently damage delicate eye tissue. The fact that the eyes act as a reservoir for Zika, long after the infection has cleared the rest of the body, could have implications for the natural history of the disease, especially if it's discovered that transmission is possible through human tears.
The finding that eyes harbor the virus — published in the journal Cell Reports — could also have implications for eye tissue transplants from deceased donors to those whose eyesight is impaired.