Environmental and Climatic Drivers of Microsporidial Keratoconjunctivitis in Athletes: Molecular Evidence from Outbreaks in Japan

<i>Vittaforma corneae</i> (<i>V. corneae</i>)-associated microsporidial keratoconjunctivitis (MKC) is increasingly recognized as an emerging infection affecting healthy individuals. However, the molecular links between environmental reservoirs and human diseases remain poorly understood. In this study, we examined the potential relationship between environmental factors and human MKC following 2 outbreaks in Nagasaki, Japan, involving 16 patients by integrating clinical, molecular, and environmental analyses. We collected ocular samples from affected patients and 16 soil and 11 water samples from related geographic areas. These samples were analyzed using nested PCR and DNA sequencing, targeting the <i>V. corneae</i> microsporidian ribosomal genes. Our molecular comparisons revealed a high degree of sequence similarity between clinical and environmental soil isolates, suggesting a potential soil-associated reservoir for the infection. Outbreaks of MKC occurred following periods characterized by high temperatures, high humidity, and increased rainfall. While we cannot definitively establish causality, these findings support the hypothesis that environmental exposure may contribute to microsporidial ocular infections. Temporary visual impairment among affected athletes disrupted their training and raised concerns among sports teams regarding environmentally driven ocular infections. Overall, our findings reveal a clear molecular and ecological pathway for MKC transmission in a non-endemic country. They underscore the importance of environmental surveillance in sports fields and other high-contact environments.