Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 74

The epithelia that cover the exposed part of the sclera (the outer fibrocollagenous coat of the eyeball) and form the inner surfaces of the eyelids (conjunctivae) provide the route of entry for several viruses, including some adenovirus types, enterovirus 70, and herpes simplex virus. Every few seconds, the eyelid closes over the sclera, bathing it in secretions that wash away foreign particles. Like the saliva, tears that are routinely produced to keep the eye hydrated also contain small quantities of antibodies and lysozymes. Of interest, the chemical composition of tears differs, depending on whether they are “basal” tears produced constantly in the healthy eye, “psychic tears” produced in response to emotion or stress, or “reflex tears” produced in response to noxious irritants, such as tear gas or onion vapor. The concentration of antimicrobial molecules increases in reflex tears, but not psychic tears, underscoring the fact that host defenses are finely calibrated to respond to changes in the environment.

The primary function of tears is to wash away dust particles, viruses, and other microbes that land on the eye or under the eyelid. There is usually little opportunity for viral infection of the eye, unless it is injured by abrasion. Direct inoculation into the eye may occur during ophthalmologic procedures or from environmental contamination, such as improperly sanitized swimming pools and hot tubs. In most cases, viral reproduction is localized and results in inflammation of the conjunctiva, a condition called conjunctivitis or “pink eye.” Systemic spread of the virus from the eye is rare, although it does occur; paralytic illness after enterovirus 70 conjunctivitis is one ex ample. Herpesviruses, in particular herpes simplex virus type 1, can also infect the cornea, mainly at the site of a scratch or other injury, and immunocompromised individuals are at greater risk of retinal infection with cytomegalovirus. Such infections may lead to immune destruction of the cornea or the retina and eventual blindness. Inevitably, herpes simplex virus infection of the cornea is followed by spread of the virus to sensory neurons and then to neuronal cell bodies in sensory ganglia, where a latent infection is established. Injury to the eye that allows for viral entry need not be a major trauma: small dust particles or rubbing one’s eyes too aggressively may be sufficient to damage the protective layer and provide an opportunity for virus particles to access permissive cells.

While one may not normally think of eyelashes and eye brows as key components of host defenses, these well-placed patches of hair help to capture fomites that might invade the eye. An intriguing thought is that, as evolution progressed from apes to humans, dense hair was lost from all except a few parts of the body: on top of the head, in the pubic region, and around the eye. It is tempting to speculate that individuals who retained these patches of hair may have had an evolutionary advantage because they were more resistant to certain infections.