A significant number of Zika patients experience problems with their eyes. Now scientists find that the Zika virus can infect every part of the eye in mice. Moreover, its RNA can be detected in teardrops, and samples from infected eyes appeared able to infect other mice. The scientists detailed their findings online September 6 in the journal Cell Reports.

Zika has provoked alarm worldwide for its apparent capacity to infect fetuses in pregnant mothers and cause birth defects. Previous research also found that up to 15 percent of adult Zika patients developed eye problems, such as conjunctivitis, commonly known as pinkeye, and uveitis, or inflammation of the uvea, where the iris and retina are located. Worse, newborns could develop inflammation of the optic nerve, atrophy of the retina and the underlying layer of blood vessels and nerves, and blindness.

To learn more about Zika’s effects on the eyes, scientists experimented with adult mice bred to be susceptible to the virus. Signs of eye disease appeared within seven days of injection with Zika. “Zika virus directly infected all compartments of the eye,” says study co-senior author Rajendra Apte, an immunologist, retinal surgeon, and molecular biologist at the Washington University School of Medicine in St. Louis.

All in all, the virus could infect the iris, cornea, retina, and optic nerve. Furthermore, the researchers found Zika RNA was abundant in mouse tears. “Tear fluid may be an important secretion for its diagnostic implications,” Apte says.

However, viral RNA found in mouse tears seven days after infection did not cause symptoms when injected into another rodent. The researchers would like to study mouse tears at several different time points “to see if virus in tears is infectious at any time point,” Apte says.

Further research is needed to see whether Zika virus can be found in human tears, and if so, whether such the disease might spread through contact via tears to fingers. If it can, there are clear public health implications. “You can’t tell people to not touch their faces,” Apte says, “so you have to practice other public health precautions, such as cleaning surfaces that patients touch, until we know more about what dose levels may be important in spreading the virus.”

The scientists also found that Zika RNA taken from inside mouse eyes, seven days after infection could cause symptoms when injected into another rodent. “This raises concern that corneal transplants may harbor passenger Zika virus and transmit the disease to corneal transplant recipients,” says Jerry Niederkorn, an immunologist at the University of Texas Southwestern Medical Center in Dallas, who did not take part in this research.

Corneal transplantation is the most common form of solid-organ transplantation in the world, with over 40,000 corneal transplants performed each year in the United States alone, according to Niederkorn. He notes that future research should examine whether Zika can spread via corneal transplants, and whether eye banks should screen donated corneas.

The researchers did not see eye defects in newborn mice after Zika infection during pregnancy. It remains uncertain whether the virus triggers neurodevelopmental changes that might lead to blindness or other serious eye problems in children born infected with Zika.

It also remains unclear how exactly Zika enters the eye. The researchers noted that possible routes of infection include transmission from the brain along the optic nerve, or by crossing the blood-retina barrier. Further research from this group will attempt to pin down the cellular mechanisms by which the virus enters cells.