Diphtheria is a potentially life-threatening infection in humans. The three species capable of causing diphtheria are: , and . Although UK cases are rare, recently there has been an increase in reported toxigenic C. ulceransassociated with companion animals. Potentially toxigenic corynebacteria are sent to the National Reference Laboratory, Public Health England (PHE), London, UK for species confirmation, determination of presence of the diphtheria toxin gene by real-time PCR, and confirmation of toxin expression by the Elek test. We reviewed submissions of between January 2006 and November 2019.

Fifty-three isolates of toxigenic were received, 29 from humans and 24 from animals. The most common animal hosts were dogs (15) and cats (7), but isolates were also received from a horse and a captive rhinoceros. Multi-locus sequence typing (MLST) data were derived from whole genome sequencing. In three human cases, was isolated from companion animals and in all three, typing data supported an epidemiological link between human and animal hosts. The sequence types (STs) for these linked isolates were ST331 (human and dog, human and cat) and ST551 (human and cat).

Although MLST data are limited, the finding of the same ST (ST331) from canine and feline sources indicates that strains can be carried by both hosts.

Management of diphtheria cases is an important public health issue. Typing data can support epidemiological linkage and identify possible sources, allowing the potential intervention and treatment of animals thus avoiding onward transmission.

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