Toxigenic Corynebacterium diphtheriae isolated from a donkey and the Public Health response

Durda Slavic 

Animal Health Laboratory, University of Guelph, Guelph, ON. 

AHL Newsletter 2024;28(4):20.

A skin lesion swab from a 35-year-old neutered male donkey was submitted for bacterial culture and susceptibility. The donkey had a 6-8 week history of ongoing skin lesions on all four limbs with purulent discharge and odour. On routine bacterial culture, large numbers of Streptococcus equi subsp. zooepidemicus, Pseudomonas aeruginosa and Corynebacterium diphtheriae were recovered. Unlike S. equi subsp. zooepidemicus and P. aeruginosa, isolation of C. diphtheriae in this case was unexpected. 

C. diphtheriae is a causative agent of diphtheria, a communicable disease in humans, and it is rarely isolated from animals. It produces a diphtheria toxin which is responsible for clinical disease, mainly respiratory and cutaneous forms. Isolation of toxin positive C. diphtheriae from humans must be reported to the local public health unit and to the Ministry of Health. The donkey’s isolate was tested at AHL for the presence of diphtheria toxin gene by qPCR. After the toxin gene was detected, this finding was reported directly to Ministry of Health resulting in contact tracing and an extensive public health investigation. As not all qPCR toxin positive isolates are toxin producers, toxin production was confirmed by a modified Elek test at the National Microbiology Laboratory in Winnipeg. Following public health investigation, no human contacts were culture positive for C. diphtheriae. The other animals on the premises were not tested, and the source of infection for the donkey remains undetermined. The donkey was treated with antimicrobials primarily targeting P. aeruginosa, and was doing well at last check. This was the first isolation of C. diphtheriae from an animal recorded at the AHL.

In addition to C. diphtheriae, C. ulcerans and C. pseudotuberculosis can carry the diphtheria toxin gene and cause diphtheria-like infections in humans and animals. Human cases of diphtheria-like disease caused by toxigenic C. ulcerans isolates have been increasing. While zoonotic infections were previously associated with livestock exposure and consumption of unpasteurized milk, recent reports of domestic pets being source of C. ulcerans infections are increasing, with cats, dogs, and horses being implicated. Animals carrying C. ulcerans are frequently asymptomatic, however, clinical infections are also reported. In contrast, toxin-producing isolates of C. pseudotuberculosis and clinical infections associated with them are extremely rare.  

Both C. ulcerans and C. pseudotuberculosis (e.g., from caseous lymphadenitis in small ruminants) are routinely cultured at AHL, with C. ulcerans being less frequently recovered than C. pseudotuberculosis.  No routine qPCR for detection of diphtheria toxin gene is being done on any of these isolates, as reporting of toxin positive C. ulcerans and/or C. pseudotuberculosis from animals is not mandatory. As a result, the prevalence of diphtheria toxin gene carrying C. ulcerans and C. pseudotuberculosis isolates among AHL submissions remains to be established.

Clinicians should be aware that C. ulcerans and C. pseudotuberculosis isolates may have a potential to carry and produce diphtheria toxin, and that toxin-producing isolates may have a significant public health impact.   

References

1. Bernard KA, et al. Increase in detection of Corynebacterium diphtheriae in Canada. CCDR 201;45:11.

2. Prygiel M, et al. New Corynebacterium species with the potential to produce diphtheria toxin. Pathogens 2022;11:1264.

3. Zendri F, et al. Case report: Toxigenic Corynebacterium ulcerans diphtheria-like infection in a horse in the United Kingdom. Front Vet Sci 2021;8:650238.