Viruses are exclusively intracellular parasites and are also the smallest infectious agents known [1]. Its mechanisms of disease are still not completely clear; however, there are direct factors that contribute to viral tropism: viral receptors in the host cell, specific cell line, and physical barriers that enable and/or inhibit infection. Once inside the cell, the virus may damage or destroy it through direct cytophatic effects, host antiviral immune responses, and/or transformations of the infected cells [2].
It is known that the eye is a site for viral infection that might appear in the intra- or extraocular space without visible systemic reverberation and affect multiple structures with variable manifestations [3]. Red eye, pain and blurred vision are one of the first clinical signs and symptoms of viral ocular impairment. Conjunctivitis is the topmost ocular infection in primary health care, given that 65-90% of cases are caused by adenovirus (AdV) and occasionally herpes simplex virus (HSV) or varicella zoster virus (VZV). Keratitis caused by HSV-1 is also commonly seen, and it is still the main cause of blindness succeeding infection in developed countries, presenting approximately 40 thousand new cases of visual impairment every year worldwide [4]. Viruses frequently associated with ocular or systemic complications are Epstein Barr virus (EBV), Measles morbilivirus and Paramyxovirus [5].
Viral diagnosis is made after clinical signs and symptoms and laboratory results that support medical hypotheses. There are a large variety of laboratory tests, but specificity and sensitivity change from one microorganism to another. The doctor in charge, based on clinical information and previous experiences, should decide between available options considering the patient’s singularities. Cell culture and analysis of genetic material with samples collected from blood, mucosa or secretions are the main methods. In the eye, the most commonly used collection method is conjunctival swabs, but Schirmer strips have also shown good results.
Schirmer test
The idea of collecting tears as a clinical test was first introduced by the German ophthalmologist Köster in 1900. The test consisted of the placement of filter paper on all extensions of the conjunctival sac while the nasal mucosa was stimulated to produce tearing caused by nasal irritation. The objective was to exhaust tear production to evaluate the function of lacrimal glands. Therefore, this test could take up to 90 minutes, becoming exhaustive and unviable for daily medical practice [6].
In 1903, Otto Schirmer, also a German ophthalmologist, shortened the size of the paper strips and quantified tear production for 5 minutes by three distinct methods: in method I, the patient remained blinking normally; in method II, the ocular surface was anesthetized with topical cocaine, and the nasal mucosa was stimulated; and in method III, the ocular surface was also anesthetized, but the patient kept looking for the sun during the test. These methods analyzed three tearing stimulation pathways: ocular and palpebral mucosa, nasal mucosa, and the retina [6, 7]. Since then, several modifications have been proposed on the Schirmer test; however, this test remains important in the quantification and standardization of tear volume. Currently, it is realized with a filter paper strip 60 mm long and 5 mm wide, which is inserted in the temporal side of the conjunctival sac. The patient’s eyes are closed, the strips are removed after 5 minutes, and the wet part is measured. Normal values are considered to be results above 15 mm, but those can vary according principally to medication use, age and chronic diseases [8-10].
Conjunctival swab
Conjunctival swabs are the most commonly used method for microbiological analysis because they permit the collection of cells and materials dispersed in the conjunctival sac instead of tears alone [1]. The method of collection is also very fast and simple: a swab with a cotton tip is gently passed in rotational movements on the conjunctival sac [11]. Topic anesthesia can be used to make the procedure more confortable, since there is no significant difference in the final result when samples are analyzed by polymerase chain reaction (PCR) methods [12]. Proxymetacaine 0.5% is recommended once, and commercially available eye drops show fewer bactericidal effects [13].
Current scenario
The World Health Organization (WHO) recently declared COVID-19 a pandemic threat, because of that, the academic community has concentrated all forces to solve the pandemic and give people’s normal lives back. The Chinese Hero and ophthalmologist Li Wenliang, MD, first reported the possibility of a novel catastrophic virus, SARS-CoV-2, and now, it is already known that, in some cases, ocular manifestations are one of the first symptoms, and consequently, the eye may contribute to the understanding of COVID-19 pathophysiology [14, 15]. Since then, several studies on ocular manifestations of SARS-CoV-2 have been published, but virus collection methods and associations with the ocular surface are not clearly stated. In this context, this study has the objective of raising scientific evidence that highlights the use of Schirmer strips and conjunctival swabs as a method of virus collection on the ocular surface.