Ultrastructural examination of two cases of stromal microsporidial keratitis

Two cases with chronic stromal keratitis are described in immunocompetent hosts where the diagnosis was originally thought to be herpetic or adenoviral disease. Light microscopy and ultrastructural examination of corneal tissue by electron microscopy were performed following penetrating keratoplasty (case 1) and corneal biopsy (case 2). Specimens from both cases were analysed for viral identification by PCR. Two different species of Microsporidia were identified. Case 1 represents the fourth reported case of corneal stromal Vittaforma corneae where the spores measured 3.3 x 1.4 microm, arranged in characteristic linear groups of about four to eight. Each spore contained a diplokaryotic nucleus and a single row of ten polar tube coils. By contrast, case 2 is the first reported case of stromal keratitis caused by Trachipleistophora hominis. In this case, spores measured 4 x 2.4 microm, located typically within packets. In this species, the polar tube was arranged as a single row of about 10-13 profiles. Viral DNA could not be amplified by PCR. In conclusion, microsporidial stromal keratitis should be considered in culture-negative cases refractory to medical therapy. As microbiological culture techniques are unsuccessful, diagnosis may only be established following histopathological and ultrastructural examination of corneal tissue.