Notes from the Field: Toxigenic Corynebacterium ulcerans in Humans and Household Pets — Utah and Colorado, 2022–2023

Investigation and Outcomes

Utah Case Investigation

The Utah resident lived with a spouse, another housemate, three cats, and one dog. Health officials recommended wound covering, masking, and use of disinfectant on household surfaces. The patient’s spouse and the other housemate and the four pets (all asymptomatic) were tested; toxigenic C. ulcerans was isolated from the patient’s spouse and two cats. Whole genome sequencing (WGS) of isolates from these cats and the index patient found that the isolates were the same type.^¶ The index patient, the patient’s spouse, and the other housemate were treated empirically with penicillin (1), and the four pets were treated with amoxicillin and clavulanic acid. After treatment, C. ulcerans was isolated from the index patient and all pets but not from the spouse; the other housemate was not retested. Subsequently, antibiotic susceptibility results from the index patient’s initial isolate indicated that the organism was susceptible to erythromycin but moderately susceptible to penicillin, indicating a higher dose of penicillin would be necessary.** After the index patient, the patient’s spouse, the housemate, and the four pets were treated with erythromycin, testing for all persons and pets living in the house did not yield C. ulcerans.

Colorado Case Investigation

The Colorado patient reported close contact with a spouse, two dogs living at the same house, and a visiting family member and dog who were at the house for two nights; all human and animal contacts were asymptomatic. Health officials recommended that the patient stay home from work and wear a mask during activities outside the home. C. ulcerans was isolated from the patient and the visiting dog^†† but not from the visiting family member and the patient’s own two dogs. Isolates from the patient and visiting dog were of the same WGS type.^§§ The patient’s spouse declined testing but was treated empirically with erythromycin. During treatment, antibiotic susceptibility testing results for both the human and dog isolates indicated susceptibility to erythromycin. After treatment with erythromycin, follow-up testing for the patient and the visiting dog did not yield C. ulcerans.

Preliminary Conclusions and Actions

Toxigenic Corynebacterium diphtheriae infection is nationally notifiable; however, infection with toxigenic C. ulcerans is not. Illness caused by toxigenic C. ulcerans can mimic toxigenic C. diphtheriae infection and necessitates prompt identification, treatment, and control (2). In addition, surveillance and routine vaccination with diphtheria toxoid-containing vaccines are important to protect persons from severe toxin-mediated illness caused by toxigenic Corynebacterium spp. (3).

These Utah and Colorado cases represent the first reported U.S. cases of toxigenic C. ulcerans infection among humans with concurrent household pet colonization (4,5). C. ulcerans is believed to be zoonotic (5); human-to-human transmission has not been documented. Evidence from this investigation suggests that transmission of toxigenic C. ulcerans between humans and household pets occurred, although the direction of transmission could not be determined.

Although penicillin or erythromycin are recommended treatments for toxigenic Corynebacterium infections (1), this investigation suggests that treatment of both human and veterinary patients should be based on antibiotic susceptibility results. Health officials can utilize a One Health approach considering human, animal, and environmental health to control C. ulcerans transmission and infections.