A new study has revealed that material from the Zika virus is found in significant quantities in the tears of infected mice, raising the possibility that the disease is spreadable through human tears. Although the virus poses a minimal risk for neurological damage in adults, it is known to sometimes cause an inflamed condition in the eyes of the infected known as uveitis, which can lead to blindness if it isn’t treated.
Writing in the journal Cell Reports, the team notes that the tears of afflicted mice contained genetic material (RNA) produced by the virus nearly a month after the subjects were first infected. It is not yet known precisely how the virus originally got there, either by crossing the blood-retina barrier that segregates the eye from the bloodstream, or via the optic nerve that connects the brain and the eye.
“Our study suggests that the eye could be a reservoir for Zika virus,” said senior author Michael Diamond, a distinguished professor of medicine from the Washington University School of Medicine in St. Louis (WUSTL), in a statement. “We need to consider whether people with Zika have infectious virus in their eyes and how long it actually persists.”
Working out how long the eyes act as a Zika reservoir is vital for estimating possible transmission risks. As with many viruses, Zika is present in the bodily fluids (urine, blood) of those that have contracted it.
Curiously, it persists for far longer in the semen of infected males – up to 93 days after the onset of the illness – which means that even after the cold-like symptoms have stopped, certain men may still be infectious. This also makes Zika a sexually transmittable disease.
As actual samples of live virus itself were not found in the tears 28 days post-infection, it seems to show that in mice at least, the virus is not transmittable this way after four weeks or so. Whether it persists in human tears for the same amount of time, or whether it lingers in the eye for longer just like it does in semen, is currently unknown.
So far, the study only applies to mice, but it's likely the same applies to humans, too. Mirko Sobotta/Shutterstock
“Even though we didn't find live virus in mouse tears, that doesn't mean that it couldn't be infectious in humans,” lead author Jonathan Miner, an instructor in medicine at WUSTL, added. “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.”
The Zika virus is currently flaring up in Singapore and parts of the US thanks to the spread of infected Aedes aegypti mosquito and the close proximity of afflicted humans. Although for most, an infection is harmless and sometimes symptomless, it poses a major threat to unborn children.
As experimental studies have conclusively shown, the virus preferentially infects and destroys cells in the central nervous system, which leads to microcephaly in newborns and sometimes stillbirths. One recent paper suggested that over 1.65 million childbearing women within Central and South America are currently at risk from being infected by the virus – and up to 13 percent of them will suffer from pregnancies marred by neurological damage.
Arguably, the most significant threat to others is the damage it can do to their eyes. On occasion, an infection causes mild to severe inflammation in the eye, which at best causes sight problems and at worst renders a person blind.
In fact, when it comes to infections during pregnancy, one in three babies exhibit some degree of eye disease, including optical nerve inflammation, retinal damage, and possible blindness post-birth.
A pest control worker fumigates a school in Kuala Lumpur, Malaysia. The country reported its first locally transmitted Zika case on September 3 in a 61-year-old man who has since died. MOHD RASFAN/AFP/Getty Images