ABSTRACT
The COVID-19 pandemic has piqued interest in spontaneous face-touch as a possible route of microbial infection, with eye-touch of particular importance since the ocular surface is a likely portal of human Coronavirus infection. Spontaneous face-touching is a poorly understood, ingrained habit for humans, who engage in this activity on average between 9 to 162 times per hour. Nearly half of spontaneous face-touches involve mucous membranes, and one third of those involve the eyes. The infective sequelae of self-touch are well documented in ophthalmological conditions such as infectious conjunctivitis, with risks for ocular surface disease beyond primary infection from pathogens such as human papillomavirus. Through tear film conveyance via the nasolacrimal duct, ocular surface pathogens may furthermore have access to the nasopharynx, oropharynx, and respiratory/gastrointestinal systems beyond. Ocular surface and face self-touch therefore represent a concerning possible method of not only local, but also systemic, self-inoculation. Conversely, microbial diversity in the mutualistic microbiome is being increasingly implicated as integral for developing immunity, and protecting against endocrinological and neurodegenerative disease, including those that affect the eye. Spontaneous face-touch brings the hands, the part of the body most in contact with the external world and with the highest temporal diversity, into direct contact with the body's multiple microbiomes. The authors hypothesise that spontaneous self-touch may represent an important mechanism by which the skin, ocular surface, gastrointestinal, and respiratory tracts maintains microbial diversity and prevents dysbiosis. It may be that whilst the eyes are at risk of infection through self-touch, they may paradoxically benefit through the acquisition of a mutualistic microbiome, protective not only for the eyes, but for the body as a whole.