Abstract: Objective To abserve the clinical features, etiological findings of a case series of patients with microsporidial keratoconjunctivitis (MKC). Design Retrospective case series. Participants Thirty-four patients with MKC at Beijing Tongren Hospital, between September 2024 and August 2025. Methods All patients were diagnosed through clinical feature evaluation, etiological examinations, and molecular biological methods. Diagnostic procedures included corneal scraping cytology, metagenomic next-generation sequencing (mNGS), nested PCR, and in vivo confocal microscopy (IVCM). Treatment consisted of isolating infected animals, administering oral albendazole, and applying topical polyhexamethylene biguanide or voriconazole eye drops. Treatment outcomes and recurrence were assessed. Therapeutic efficacy was defined as the disappearance of patient symptoms and improvement of slit-lamp findings, including resolution of conjunctival hyperemia, negative fluorescein staining of the corneal epithelium, disappearance of raised lesions, and absorption of stromal infiltrates. Main Outcome Measures Clinical symptoms, ocular signs, in vivo confocal microscopy findings, corneal scraping cytology, pathogen culture and PCR detection results, metagenomic sequencing findings, disease recurrence rate, and cure rate. Results The 34 patients had a mean age of 33.6±16.6 years (range: 7~67 years), and 91.1% had a clear history of animal contact. Clinically, most presented with epithelial keratitis and conjunctivitis; four patients progressed to subepithelial or stromal keratitis. Etiological examinations revealed microsporidia structures in corneal scraping samples from 28 cases. Among the 34 patients, 33 were infected with Encephalitozoon hellem (EH) and 1 with Enterocytozoon bieneusi (EB). Using a comprehensive treatment strategy(animal isolation, oral albendazole, and topical ophthalmic medications), all patients achieved complete symptom resolution, persistent negative fluorescein staining, and remission of inflammatory signs, resulting in effective cure. However, 7 patients who had previously used corticosteroids or immunosuppressants, or had repeated animal exposure, experienced symptom or sign exacerbation after initial improvement, or persistent inflammatory findings. These cases showed recurrent or prolonged disease, with a recurrence/prolongation rate of 20.59%. Conclusion MKC may have specific sources and routes of transmission in mainland China, closely related to animal contact.

Key words: Microsporidia, Microsporidial keratoconjunctivitis