Microsporidial Keratitis

Microsporidia are small eukaryotic, spore forming obligate intracellular parasites belonging to the phylum Microsporidia. They were first recognised 100 years ago from the silk worm Bombyx mori as an etiological agent of the pebrine disease-a destructive disease in these worms. Though initially believed to be early diverging eukaryotes, they are currently known to be closely related to fungi. These infectious pathogens can exhibit diverse clinical manifestations such as ocular, sinus, renal, intestinal, pulmonary, and muscular diseases. Ocular microsporidiosis can present as stromal keratitis, scleritis, keratoconjuntivitis and endophthalmitis. High index of suspicion is required for the diagnosis of ocular microsporidiosis.

Etiology

Microsporidia are small unicellular parasites and are considered as true eukaryotes because they have a nucleus with a nuclear envelope, an intracytoplasmic membrane system, and chromosome separation on mitotic spindles. Joseph et al. reviewed the disease and detected four genera that infect humans Encephalitozoon , Nosema , Pleistophora and Enterocytozoon. Seven genera (Enterocytozoon species, Brachiola species, Encephalitozoon species, Pleistophora species, Nosema species, Vittaforma species and Trachipleistophora species) as well as unclassified microsporidia (collectively referred to as Microsporidium ) have been found to be associated with human disease involving immunocompromised patients. Nosema sp. and Nosema like organisms of the nontaxonomic group Microsporidium are the parasites involved in ocular microsporidiosis in immunocompetent individuals whereas in immunocompromised individuals Encephalitozoon hellem or other Encephalitozoon -like organisms were involved.

Pathogenesis

Life cycle of these organisms is completed within the human host. There is no role of an intermediate host or a vector in transmitting the developmental stages of microsporidia. The spore is the infective and the resistant stage. In microsporidial keratitis spores are believed to enter the eye either by direct contact or trauma. The spore has a unique means of inoculation of the infective sporoplasm. It extrudes the sporoplasm via the everting and unwinding polar filament into the host cell. Further after invading the cell the sporoplast division takes place by binary fission forming schizont with 2-6 nuclei, which split into unicellular meronts. The meronts secrete a rigid capsule thus attaining its final size measuring about 2.5 × 1.5 microns. The meronts further differentiates into sporonts and spores. The cell finally ruptures to continue the cycle and further destruction of the host tissue eventually occurs. The spores of the genus Encephalitozoon reside in a parasitophorus vacuole which is a membrane bound vacuole. Members belonging to other genera complete their intracellular life cycle in direct contact with the cell cytosol.