Imported Non-toxigenic Cutaneous Corynebacterium diphtheriae in Japan: Clinical and Public Health Responses

Corynebacterium diphtheriae (C. diphtheriae) is rarely detected in Japan, with no toxigenic cases reported since 2000. We describe a cutaneous infection in a 16-year-old Japanese female who developed ulcerative skin lesions after returning from Fiji. Initial Gram stain suggested impetigo; however, matrix-assisted laser desorption ionization–time-of-flight mass spectrometry identified C. diphtheriae. Following voluntary notification, temporary infection control and respiratory screening were implemented. The lesions resolved within two weeks. Polymerase chain reaction confirmed the absence of the tox gene, and infection control measures were discontinued. Whole-genome sequencing revealed a low-virulence lineage closely related to Oceania isolates. This case illustrates the diagnostic and public health challenges associated with detecting C. diphtheriae in non-endemic settings. Although non-toxigenic strains are not notifiable in Japan, their detection in returning travelers may warrant prompt recognition, immediate public health consultation, and temporary infection control until toxigenicity is excluded. Genomic analysis may help distinguish imported low-virulence strains from potential outbreak threats.