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Molecular diagnosis may help management of ocular infection

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Thousands of people worldwide may benefit from a promising new method to confirm an infection that is a leading cause of blindness. Preliminary indications from a study between India and the United States are that PCR might be used to diagnose fungal keratitis of the cornea, which commonly affects agricultural workers in developing countries.

Results with PCR and conventional methods showed complete matching in most patients with suspected infection who were tested (22/30, 74%). Three samples (10%) were PCR positive, culture negative, but in two the patients were judged clinically to have fungal infection. Conjunctival swabs from the healthy eye in each patient were PCR negative, culture negative in all but five patients (17%), whose infected eye was positive for fungal infection.

PCR was very sensitive—down to minimal amounts of fungal material. Specificity tests showed cross reactivity with several filamentous fungi, but not with S cerevisciae, nor with a range of bacteria.

The researchers obtained scrapings from patients in India with presumed fungal keratitis of the cornea and a conjunctival swab from the other eye. The samples were treated with two rounds of amplification of part of the fungal 18S ribosome sequence. The first was of a segment common to Candida albicans, Fusarium oxysporum, and Aspergillus fumigatus. The second was nested amplification with three different primer pairs for variable sequences for each species. Sensitivity and specificity were determined against human fungal pathogens, including Candida, Aspergillus, other species of fungi imperfecti, Cryptococcus, S cerevisiae, and reference strains of a range of bacteria.