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Using the polymerase chain reaction (PCR) technique several viruses have been demonstrated in tears; they include herpes simplex viruses 1 and 2, Epstein-Barr virus, varicella zoster virus, human herpes virus 6, hepatitis B and C viruses, measles virus, and adenovirus. Now researchers in Singapore have used PCR to detect the virus of severe acute respiratory syndrome (SARS-CoV) in the tears of patients.
Between 11 and 18 April 2003, 36 consecutive patients at the Tan Tock Seng Hospital with probable or suspected SARS according to WHO case definitions had conjunctival swabs taken from the inferior fornices of both eyes. Samples were taken in ice to the WHO network laboratory, viral section, at the Singapore General Hospital. RNA extraction was followed by conventional qualitative RT-PCR with two sets of primers targeted at conserved sites of the polymerase gene of the SARS coronavirus. A positive result with either set of primers was confirmed by repeating the process with both sets of primers. For a positive result to be reported there had to be positive results with both sets of primers or one set positive plus a positive PCR result from a different sample type from the same patient.
The 36 patients were mostly healthcare workers (19 female) and eight had probable and 28 suspected SARS. The eight probable cases (four men, mean age 62 years; four women, mean age 36 years) all proved to have positive SARS serology. PCR results were positive for SARS-CoV for the tears of three probable but no suspected SARS cases. The three were a female healthcare worker aged 30 who recovered and two elderly male patients aged 74 and 85 who both died. PCR was positive on stool samples from the two men but not from the woman. The positive tear sample had been obtained early in the illness (on days 3, 4, and 9 after the onset of fever) whereas the five probable cases with negative tear PCR results had had samples taken later in the illness (average 19 days).
PCR testing of tears may help in the early diagnosis of SARS. The authors of this paper suggest using microcapillary pipettes or Schirmer’s filter paper strips to collect tear samples. Health care workers, and in particular ophthalmologists, could be at risk of infection from tears and ophthalmic equipment could become contaminated. Meticulous countermeasures are needed to prevent the spread of infection.
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