Could protecting the eyes be an extra layer of defence against COVID-19? Photo: Shutterstock.

The strategy of covering the mouth and nose to avoid being infected with coronavirus and other airborne diseases may neglect the largest and weakest point of entry to the human body – the eyes – specialists from UNSW Sydney and ANU suggest.

Human eyes have a combined surface area greater than the mouth and nose and provide a direct route to our internal respiratory systems via tear ducts connected to the nose.

According to UNSW ophthalmologist Professor Minas Coroneo AO and Infectious Diseases Physician & Microbiologist, Professor Peter Collignon AM from Australian National University (ANU), the focus on the mouth and nose as the primary sources of infection for COVID-19 may have left us vulnerable to the virus in droplets being transmitted to the respiratory system via our eyes.

In commentary published in The Lancet Microbe today, the duo argue that in parts of the world where there is widespread community infection, hygiene strategies and donning of masks have not been enough to stop the rampant spread. “Perhaps something major is missing in our approach,” they suggest.

Protecting the eyes, they argue, could be an extra layer of defence to supplement current best practice including social distancing, wearing of face masks, good hand hygiene and not touching one’s face. While more research is needed to warrant the wholesale adoption of eye-protection, they highlight past research and an observational study which suggest the eyes may play a role in bacterial and viral infection.

Evidence from history

Prof. Coroneo says a clue to the potential role of eyes in virus transmission was revealed more than 100 years ago by US Army Captain Kenneth Maxcy, an epidemiologist who conducted experiments to find out whether covering the mouth and nose was completely effective in avoiding airborne pathogens – in this case, the bacteria Serratia marcescens.

“In 1919, Maxcy recruited volunteers to test how airborne pathogens enter the body,” Prof. Coroneo says.

“While their mouths and noses were covered, he sprayed their faces with a bacterial marker to simulate a cough. This meant that the spray ended up making contact with their eyes and he was able to find the bacteria in the nasopharynx after 15 minutes.

“He concluded that the bacteria was able to infect people via the eyes.”

Professor Coroneo says Maxcy’s study is invaluable but would be difficult to replicate today because of ethical and safety considerations. Subsequent studies have been observed in animal models including ferrets and monkeys and have shown the same thing: that the eyes are vulnerable to airborne pathogens.

Protective glasses?

The commentary in the Lancet also points to an observational study reported by doctors working in China at the beginning of the pandemic who noted that people who wore glasses for eight or more hours a day were underrepresented among patients with confirmed COVID-19.

Like the authors, Professor Coroneo is cautious about inferring a causal relationship from this limited study, particularly given it was conducted at a time before masking and social distancing were common practice.

“But the point is, when exposure to the COVID-19 virus is common, front-line medical staff are still being infected, even when wearing three-layered surgical masks.”

‘Ocular tropic’

Professor Coroneo says this is likely because COVID-19 and other respiratory viruses such as influenza are ‘ocular tropic’, meaning they are attracted to the surface of the eye which is covered by three layers of tear film.

“This COVID-19 virus can sometimes remain suspended in the air for extended periods of time, in one form or another,” he says.

“So if you walk through a crowded, inadequately ventilated environment, contaminated by viral particles and they go anywhere near your eye, or even on the eyelid skin, they can be conveyed to the tear film and quickly transported via the nasolacrimal duct into your nose.

“But because coronavirus is considered a respiratory virus, all the focus has been on breathing it in via the mouth or nose, while ignoring this other potential pathway.”

The Lancet commentary calls for front-line medical staff to wear eye protection as a matter of course.

But the authors concede that improvements in design and more evidence are needed. Some eye protectors like face shields still let in air circulating close to the face, while hermetically sealed eye protectors are for short-term use, not eight-hour intensive care unit shifts. And there are also practical problems such as fogging, obstruction of view and discomfort.

“There is an urgent need to develop better eye protective strategies, based on the understanding of ocular interactions with the environment,” the authors conclude in their commentary.