Corynebacterium of the diphtheriae complex in companion animals: clinical and microbiological characterization of 64 cases from France

Objectives Corynebacteria of the diphtheriae complex (Cdc) can cause diphtheria in humans and have been reported from companion animals. We aimed to describe animal infection cases caused by Cdc isolates. Methods 18 308 animals (dogs, cats, horses and small mammals) with rhinitis, dermatitis, non-healing wounds and otitis were sampled in metropolitan France (August 2019 to August 2021). Data on symptoms, age, breed, and the administrative region of origin were collected. Cultured bacteria were analyzed for tox gene presence, for production of the diphtheria toxin, for antimicrobial susceptibility, and genotyped by multilocus sequence typing. Results C. ulcerans was identified in 51 cases, 24 of which were toxigenic. Rhinitis was the most frequent presentation (18/51). Eleven cases (6 cats, 4 dogs, 1 rat) were mono-infections. Large breed dogs, especially German Shepherds (9 of 28 dogs; p < 0.00001) were overrepresented. C. ulcerans isolates were susceptible to all tested antibiotics. tox- positive C. diphtheriae was identified in 2 horses. Last, 11 infections cases (9 dogs, 2 cats; mostly chronic otitis, and 2 sores) had tox -negative C. rouxii , a recently defined species. C. rouxii and C. diphtheriae isolates were susceptible to most antibiotics tested, and almost all of these infections were polymicrobial. Conclusions Monoinfections with C. ulcerans point towards a primary pathogenic potential to animals. C. ulcerans represents an important zoonotic risk, and C. rouxii may represent a novel zoonotic agent. This case series provides novel clinical and microbiological data on Cdc infections, and underlines the need for management of animals and their human contacts. Importance We report on the occurrence, clinical and microbiological characteristics of infections caused by members of the Corynebacteria of the diphtheriae complex (Cdc) in companion animals. This is the first study based on the systematic analysis of a very large animal cohort (18 308 samples), which provides data on the frequency of Cdc isolates in various types of clinical samples from animals. Awareness of this zoonotic bacterial group remains low among veterinarians and veterinary laboratories, among which it is often considered a commensal bacteria of animals. We suggest that in case of Cdc detection in animals, the veterinary laboratories should be encouraged to send the samples to a reference laboratory for analysis of the presence of the tox gene. This work is relevant to the development of guidelines in case of Cdc infections in animals, and underlines their public health relevance given the zoonotic transmission risk. ### Competing Interest Statement The authors declare that 2 authors (KM, GR) are employees of Cerba Vet, which performs diagnostic testing on a commercial basis.