Detection and diphtheria toxin status of Corynebacterium ulcerans in canine and feline clinical specimens in Japan.

Corynebacterium ulcerans (C. ulcerans) is capable of producing diphtheria toxin and causing zoonotic diphtheria, with companion animals recognized as an important infection source. We evaluated the occurrence and diphtheria toxin status of C. ulcerans in canine and feline clinical specimens in Japan. Out of 244 Corynebacterium isolates, 18 were identified as C. ulcerans using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and C. ulcerans-specific PCR, including 1 from a dog and 17 from cats. The nasal cavity was the most common isolation site (11 of 18 C. ulcerans isolates). Real-time PCR results from DNA extracted from bacterial colonies showed high concordance with MALDI-TOF-MS findings. Direct application of real-time PCR to clinical samples without prior culture may be considered in the future to enable more rapid identification of C. ulcerans. The tox gene was detected by PCR in 11 feline-derived isolates, and 9 of these demonstrated diphtheria toxin production in a toxigenicity assay evaluating cytotoxicity in Vero cells. Multilocus sequence typing revealed that 15 of the 18 isolates belonged to the same sequence type (ST337). These findings suggest that toxigenic C. ulcerans is more frequently isolated from diseased cats than from dogs in Japan, and that these strains exhibit low genetic diversity. Detection of the tox gene by real-time PCR would be useful for preventing human diphtheria, although toxin gene presence does not necessarily correlate with actual toxin production.