More than half of coronavirus patients have the pathogen on the surface of one of their eyes, research suggests.
The coronavirus' effect on eyes was first brought to light when Dominic Cummings claimed the infection affected his vision. Boris Johnson's former chief adviser used this to justify driving to the beauty spot Barnard Castle to reportedly test his eyesight during the UK's first lockdown.
While experts were on the fence at the time, research has since suggested severely-ill coronavirus patients may endure "significant" eye abnormalities.
The coronavirus was identified on the "ocular surface" of 57% of the coronavirus patients. Whether tears could spread the infection "could not be determined", however.
How the coronavirus coats the eyes is unclear, however, airborne contamination "seems to be the most likely theory".
The coronavirus primarily spreads face to face when infected droplets are expelled via a patient coughing or sneezing.
Surfaces can also become contaminated with the virus, however, the extent to which this drives transmission has been debated.
There is also evidence the infection may spread via human waste.
"The viral communicability may be mediated by various body fluids, but insufficient information is available on the presence of the virus in human tears," the Italian medics wrote in the journal JAMA Ophthalmology.
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To learn more, the Italian medics analysed 91 coronavirus patients – average age 58 – who were admitted to intensive care at the Azienda Socio-Sanitaria Territoriale Sette-Laghi Hospital in Lombardy between 9 April and 5 May 2020.
Northern Italy was the epicentre of Europe's coronavirus outbreak early in the pandemic.
The results of the conjunctival swabs reveal the coronavirus was on the ocular surface of 52 (57%) of the patients, with "wide variability in the mean viral load from both eyes".
Just under a quarter (23%) of the patients had a "discrepancy" in swab results between the eyes, i.e. one tested positive and the other negative.
The coronavirus' presence on the eye's surface may be "variable", at times low or undetectable.
Patients may also cut the sampling short if it is uncomfortable, particularly when a specialist swabs the second eye, according to the medics.
Seventeen of the patients had a negative nasal swab for the coronavirus at the time of their intensive care admission, of whom 10 still tested positive for the infection via an ocular assessment.
These 10 patients had a "high viral load in tear samples".
"Because patients may have positive test results with a conjunctival swab and negative results with a nasopharyngeal swab, use of the slightly invasive conjunctival swab may be considered as a supplementary diagnostic test," wrote the medics.
"Samples of tears are easily provided by the patients, they do not require specialised training for collection and the procedure may not be uncomfortable."
Perhaps surprisingly, the medics observed "a low rate of ocular signs or symptoms in patients positive for the conjunctival swab".
The NHS states the main signs of a coronavirus infection are a fever, cough, or loss of taste or smell.
This list has been accused of being too limited, with the World Health Organization (WHO) stating conjunctivitis – or pink eye – is a "less common" coronavirus symptom.
Previous studies have failed to demonstrate positive coronavirus conjunctival swabs, which the Italian medics put down to "several critical issues in sampling and laboratory processes".
Their conjunctival swabs were carried out by the same specialist and processed in one laboratory.
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Although unclear how the coronavirus coats the surface of the eyes, the organs may be directly contaminated via airborne droplets.
Atmospheric particles can also act as "carriers" for viruses.
"By piggybacking, viruses adhere to atmospheric fine powders consisting of solid or liquid particles that are able to remain in the atmosphere for hours, days, or longer, especially in a non-windy and polluted climate like the Po Valley in Lombardy," wrote the Italian medics.
An individual may also touch a contaminated surface, before rubbing their eyes.
The virus could also diffuse into the eye from the bloodstream, which has been observed with the human immunodeficiency virus (HIV).
"Among the theories described herein, direct contagion from airborne droplets seems to be the most likely theory," wrote the medics.
The virus may also infect the rest of the body after entering via the eyes.
This may have occurred in the much-publicised case of Dr Li Wenliang, according to the Italian medics.
The ophthalmologist caught the coronavirus while operating on a patient in Wuhan, the Chinese city where the infection emerged. Among the first to warn about the dangers of the virus, Dr Wenliang later died of its complications.
"Although the infectivity of the viral material detected in the present study is unknown, these results support the use of eye protection for people working in environments where infection through the ocular route is feasible," wrote the Italian medics.
"Eye protection probably should be considered if viral material might exceed certain limits, especially in the absence of wind or indoor systems designed to clear the air."